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To enhance the liquid nitrogen freezing efficiency and improve the quality of frozen prepared Monopterus albus, the effect of variable temperature liquid nitrogen quick-freezing combined with composite cryoprotectant treatment on the quality of prepared M. albus was investigated. Deep-fried cured M. albus was treated with a novel composite cryoprotectant consisting of 4% xylooligosaccharides, 4% sorbitol and 0.3% sodium bicarbonate before precooling followed by liquid nitrogen freezing at variable temperatures (from -80 to -50 ℃). Internal temperature, moisture content, thawing loss, centrifugal loss, moisture state, thiobarbituric reactive substances (TBARS) value, acid value (AV), fluorescent compound content, and total volatile basic nitrogen (TVB-N) content were measured, and ice crystal morphology and microstructure were observed. The results indicated significantly lower thawing loss (3.80%), centrifugal loss (11.6%) and free water percentage (24.4%) under variable temperature liquid nitrogen quick-freezing compared with freezer freezing (P 0.05). These findings suggested that pre-cooling followed by variable temperature liquid nitrogen quick-freezing combined with the novel composite cryoprotectant could enhance the quality of prepared M. albus during frozen storage, which will offer theoretical support for the development of highly efficient freezing and preservation technology for prepared M. albus products.

To investigate the impact of liquid nitrogen quick freezing on the quality of prepared chicken chops during frozen storage, this study compared −18 ℃ air freezing with −80 ℃ and −100 ℃ liquid nitrogen quick freezing. Then the frozen samples were stored in a −18 ℃ refrigerator for 0~180 days. The evaluation criteria included sensory scores, color, water holding capacity, texture characteristics, electronic nose analysis, pH, oxidation indicators, and total bacterial count. The results revealed a significant decrease in sensory scores for all groups with the extension of frozen time (P0.05), and the count remained below 5 lg (CFU/g) during frozen storage. Measurement of relevant oxidation indicators revealed that liquid nitrogen quick freezing effectively delayed protein and lipid oxidation. On the 180th day of frozen storage, the malondialdehyde (MDA) content in the −18 ℃ air freezing group was 21.95% and 37.43% higher than that in the −80 ℃ and −100 ℃ liquid nitrogen quick freezing groups, respectively. The total sulfhydryl group (-SH) content was reduced by 26.58% and 44.09% compared to the other two groups, respectively. In summary, the prepared chicken chops by all three freezing methods remained safe for consumption after being frozen at −18 ℃ for 180 days. But the quality characteristics of the prepared chicken chops treated with liquid nitrogen quick freezing were significantly improved during frozen storage.

In order to study the influence of liquid nitrogen leaching treatment on the mechanical properties and failure modes of coal samples under uniaxial and triaxial loading, uniaxial and triaxial tests were carried out on coal samples with different liquid nitrogen leaching time by triaxial test device. The results show that whether liquid nitrogen leaching treatment is carried out has no obvious influence on the evolution trend of mechanical properties of coal samples during uniaxial loading and triaxial loading; with the increase of axial strain, the axial stress increases slowly at first, then increases linearly, and decreases rapidly after reaching the limit of coal samples. The peak axial stress of coal samples under uniaxial loading at different time of liquid nitrogen leaching is 4.28 MPa, 7.21 MPa, 2.32 MPa and 2.34 MPa respectively, and the peak axial stress under triaxial loading is 44.31 MPa, 25.73 MPa, 37.74 MPa and 39.78 MPa respectively. With the increase of liquid nitrogen leaching time, the peak axial stress of coal samples under uniaxial loading shows a trend of increasing first and then decreasing, and the peak axial stress under triaxial loading shows a trend of decreasing first and then increasing. The liquid nitrogen leaching coal sample is partially broken more thoroughly after uniaxial loading and triaxial loading, but the longer the liquid nitrogen leaching time is, the more the number and volume of large coal generated after the coal sample is broken.

Phyllostachys praecox shoots were frozen using liquid nitrogen at –60, –90 or –120 ℃ to an internal temperature of –18 ℃, or frozen at –90 ℃ to an internal temperature of –6, –12 or –18 ℃, vacuum-packed and stored in a freezer at –18 ℃ for 24 weeks. In order to analyze the effect of liquid nitrogen freezing on physiological and biochemical characteristics of P. praecox shoots during frozen storage, L-phenylalanine ammonialyase (PAL) and peroxidase (POD) activities, total phenolic content, relative electrical conductivity, and water state were measured and ice crystal structure and cell morphology were observed. The results showed that with increasing freezing time, the PAL and POD activities, total phenolic content, and peak area of free water in all six groups decreased significantly (P 0.05). The PAL and POD activities and relative conductivity of P. praecox shoots frozen at –6 ℃ were higher than those frozen at –12 and –18 ℃, and the size of ice crystals was smaller and the degree of cell damage was greater in P. praecox shoots frozen at –6 ℃ than at –12 and –18 ℃. The difference between P. praecox shoots frozen at –12 and –18 ℃ was not significant (P > 0.05). Collectively, these findings indicated that the most suitable liquid nitrogen freezing conditions of P. praecox shoots are –90 and –12 ℃ for freezing and internal temperature, respectively.

In order to study the effect of liquid nitrogen injection cooling on spontaneous combustion of coal, coal samples from the second and third panel areas of Tingnan Coal Mine were selected for thermal and physical property experiments. First, the coal sample was pretreated by LFA457 laser flash analyzer, and then the thermophysical property parameters of the coal sample were measured in the range of 30-300 ℃, and the change law of the thermophysical property parameters of the coal sample with temperature was obtained after liquid nitrogen injection and cooling, and the factors affecting the thermophysical properties of the coal sample were compared and analyzed. The results show that the thermal diffusivity decreases with the increase of temperature, and the specific heat capacity and thermal conductivity increase with the increase of temperature; in the range of 30-200 ℃, the average change rate of thermal property parameters of coal samples after liquid nitrogen injection treatment is higher than that of raw coal samples, indicating that liquid nitrogen injection treatment will increase the sensitivity of coal samples to temperature; in addition, at the same temperature, the thermal physical property parameters of coal samples after liquid nitrogen injection treatment increased to different degrees compared with the original coal samples, and the average growth rate of thermal diffusion coefficient was the highest, indicating that the thermal diffusion ability of coal samples after liquid nitrogen injection treatment was enhanced during the oxidation and heating process.

Biomass, as a renewable energy source, features wide availability, multiple functions, and carbon neutralization. The high-value resource utilization of biomass is conducive to reducing reliance on high-carbon fossil energy, ensuring national energy security, and curbing environmental pollution, aligning with the needs of China's "dual carbon" national strategy. Pyrolysis, as the foundation of biomass thermochemical conversion, is one of the best technologies to achieve a high-value utilization of biomass energy. However, it has some core issues such as low bio-oil yield and poor quality. To overcome these problems, a cryogenic technology was innovatively applied to the field of biomass pyrolysis, and a new biomass oil production method of "cryogenic + fast pyrolysis" was proposed to increase the yield and quality of pyrolytic bio-oil. Taking two types of crop straw as experimental research objects, the rapid freezing pretreatment method with liquid nitrogen was adopted to study the evolution mechanism of the product distribution and composition of biomass pyrolysis under cryogenic pretreatment. The experiments show that compared with conventional pyrolysis, the increase in the bio-oil yield after cryogenic pretreatment of biomass pyrolysis can reach 14.16% to 23.73%, and the cryogenic pretreatment temperature has a positive correlation with the bio-oil yield, and the maximum bio-oil yield (22.3% to 26.3%) is reached at −90 ℃, exceeding the bio-oil yield of the aluminum method by 8.50% to 22.71%. The cryogenic pretreatment has a significant transformation effect on the composition of the bio-oil, and the content of benzene series (BTEX) and phenolic substances in the bio-oil composition increases significantly, with the maximum increase of 43.61% and 12.45%, respectively. The cryogenic pretreatment of liquid nitrogen inhibited the release of CO2 and CO in biomass pyrolysis gas, and the proportion of components decreased by 7.2%−29.7%. In addition, the inhibition of the release of CO2 and CO in the biomass pyrolysis gas and the increase of CH4 and H2 by cryogenic pretreatment increase by 0.2% to 14.7%. Moreover, the increase in C content and the decrease in O content in the solid product under cryogenic pretreatment are conducive to improving the combustibility of biochar and reducing its risk of spontaneous combustion.

In order to study the influence of low temperature liquid nitrogen immersion treatment on the pore structure and fractal characteristics of middle grade coal in Huainan Mining Area, coal samples were treated with different liquid nitrogen immersion time, and the pore structure of coal was measured by mercury injection and liquid nitrogen adsorption method. The fractal theory was combined to analyze the development law and scale characteristics of coal pores under different immersion time from multiple angles. The results show that with the increase of liquid nitrogen immersion time, the total pore volume of coal increases from 198.089×10−3 cm3/g to 371.553×10−3 cm3/g, while the total specific surface area decreases from 4.984 m2/g to 4.496 m2/g, and the effect is significant. The adsorption pore decreases, the seepage pore increases, and the pore connectivity of the adsorption pore increases to form a larger level of pore, which gradually changes to the seepage pore. There was a negative linear correlation between D1 (fractal dimension of seepage pore) and D2 (fractal dimension of adsorption pore) and the immersion time of liquid nitrogen, and the influence of liquid nitrogen immersion on D1 was more significant than that of D2. D1 and D2 decreased gradually with the increase of liquid nitrogen immersion time, indicating that the structural complexity of internal pores decreased with the increase of liquid nitrogen immersion time, and the porosity and permeability of coal increased. Dc (comprehensive fractal dimension) decreases with the increase of average pore diameter and total pore volume of coal body, and increases with the increase of total specific surface area. With the increase of liquid nitrogen immersion time, the comprehensive fractal dimension of coal body decreases, the adsorption capacity of coal body is weakened, and the seepage capacity is enhanced, which is helpful to improve the coal-bed methane extraction effect of low permeability coal seam in Huainan Mining Area.

The glass transition temperature Tg’ of ‘Northcountry’ blueberries was determined according to its state diagram. Using Tg’ as the freezing end point, blueberries were frozen at −80 ℃ using an ultra-low temperature freezer (RF−80 ℃), or at −80, −100 or −120 ℃ by direct immersion in liquid nitrogen (LN−80 ℃, LN−100 ℃, and LN−120 ℃, respectively). Freezing curves, juice loss, hardness, nutrient contents, cell membrane integrity, enzyme activities and microstructure were measured to investigate the effects of liquid nitrogen quick freezing combined with Tg’ on the quality characteristics of blueberries. The results showed that the Tg’ of blueberries was −52.55 ℃; compared with RF-80 ℃, the quality of liquid nitrogen frozen blueberries was better maintained. The freezing time of liquid nitrogen quick freezing was significantly shortened, the frozen fruit lost less juice after thawing, and the hardness was better maintained. The contents of soluble solids, titratable acid, ascorbic acid and anthocyanin were closer to those of fresh fruit, the relative conductivity and malondialdehyde (MDA) content were lower, the integrity of the cell membrane was less broken, the activities of peroxidase (POD) and polyphenol oxidase (PPO) were lower, and the microstructural compactness and integrity were stronger. The lower the ambient temperature of liquid nitrogen quick freezing, the better the freezing effect, and LN-120 ℃ minimized the quality deterioration of frozen blueberries. In conclusion, liquid nitrogen quick freezing significantly increased the freezing rate, and its combined with Tg’ helped to maintain good quality of frozen blueberries. The results of this study can provide a theoretical basis for the selection of blueberry freezing conditions.

The pore structure measurement in coal is commonly analyzed by mercury intrusion method, low temperature liquid nitrogen method and nuclear magnetic resonance method. Due to the different test methods, the results cannot be used uniformly, and the matrix compression effect in mercury intrusion method will cause large errors. In order to solve this problem, three high-rank coal samples from Jincheng and Changzhi Mining Areas in Qinshui Basin were taken as examples, and used mercury injection, low-temperature liquid nitrogen adsorption and nuclear magnetic resonance relaxation methods to test the pore structure of coal samples respectively. Through the compressibility correction of mercury injection data and splicing with low-temperature liquid nitrogen data at the connection aperture, the pore structure of coal was characterized by joint characterization, and the pore size distribution characteristics of coal were compared and analyzed in combination with nuclear magnetic resonance. The results show that the pore volume corrected by mercury intrusion data is closer to that of low temperature liquid nitrogen, and the deviation value is 22.40%-38.51%; the pore structure of different coal samples is analyzed by the joint pore method, and the position of the joint pore is between 75-89 nm; the results show that the pore volume of coal samples is 0.001 36-0.004 58 cm3/g, and the pore volume ratio of different pore sizes is transition pore>macropore>mesopore>micropore; compared with the single test method, the pore size distribution of the combined pore method and the nuclear magnetic resonance method is closer, but there are also differences. The average distribution ratios of micropores, transition pores, mesopores and macropores characterized by the combined pore method are 8.68%, 45.58%, 20.54% and 25.20%, respectively. The average distribution ratios of micropores, transition pores, mesopores and macropores characterized by the nuclear magnetic resonance method are 10.64%, 64.21%, 14.23% and 10.92%, respectively. The reason for the difference may be mainly related to the pore structure of coal changed by mercury intrusion method and low temperature liquid nitrogen method; through the comparison of NMR results, it is found that the combined corrected mercury injection and low temperature liquid nitrogen data can improve the accuracy of the pore size distribution test results in coal.

This study proposes to explore the coal mechanics and seepage characteristics under the coupling of water-saturation, freeze-thaw and different temperatures. The original coal of 9105 working face in Wangzhuang Coal Mine was taken as the research object, and non-contact digital image processing was used to carry out the triaxial loading test of the coal rock under the conditions of liquid nitrogen freeze-thaw and microwave heat injection. Results showed that: ① in the original coal samples after freeze-thaw, water-saturated, and water-saturated freeze-thaw conditions and gradient temperature treatment, there is increase in the compressive strength, modulus of elasticity, Poisson's ratio, and shear strain value of the coal rock; ② the original coal rock in the compression density, elasticity, yield and destruction of the four stages of the stress concentration phenomenon shows no obvious developmental changes, but by water-saturated, water-saturated freeze-thaw, water-saturated freezing and thawing conditions of 100 ℃ and 150 ℃ temperature treatment, the local stress of the specimen concentrates in the destruction phase with obvious amplification; ③ compared with the initial permeability of the original coal rock, the initial permeability of the coal sample underwent a small increase of 1.44×10-3μm2 after the liquid nitrogen freeze-thaw treatment. After the water-saturated liquid nitrogen freeze-thaw, water-saturated liquid nitrogen freeze-thaw treatment at 50 ℃ and 100 ℃, there is more significant increase in the initial permeability of the coal rock, and after the water-saturated liquid nitrogen freeze-thaw treatment at 150 ℃, the initial permeability of the coal sample underwent a significantly higher increase and reaches 7.40×10-3μm2. The study provide references for increasing the permeability of gas in deep low-permeability coal beds, which could improve the efficiency of downhole gas extraction.

To study the energy dynamic change law of moisture-contained coal in the process of liquid nitrogen freezing, a self-developed acoustic emission (AE) experimental system for the whole process of liquid nitrogen frozen coal was utilized to analyze the characteristics and the change laws of AE energy dissipation in the whole process of liquid nitrogen freezing in coal with different moisture contents. The results shown that AE energy during liquid nitrogen freezing of coal was divided into steep, fluctuating and calm periods in the time domain. The primary and secondary peaks of energy were both positively linearly related to moisture content, and the primary and secondary energy peak of 5.96% moisture content were 1.66 and 2.26 times higher than those of dry coal. The cumulative energy of liquid nitrogen frozen coal, divided into three stages of steep increase, slow growth and stabilization versus time, was positively linearly related to moisture content, which of 5.96% moisture contained coal was 2.88 times higher than that of dry coal. The energy amplitude of different moisture content coals was mostly concentrated in the range of 40-50 dB, accounting for 94.39%-99.11% of the total, and decreased linearly with the increasing moisture content of coal. The time series of acoustic emission ringing counts in liquid nitrogen frozen coals had chaotic fractal characteristics, and the correlation dimensions of the steep increase, slow growth and stable stages were positively exponentially, linearly and linearly correlated with the moisture content, respectively. Furthermore, the correlation dimension in the steep increase stage of 5.96% moisture contained coal was 2.00 and 5.78 times higher than that of the slow growth and stable stage, respectively. The type of coal cracks produced by the liquid nitrogen freezing was mainly tensile, its proportion with the increasing moisture content was a negative exponential decrease, and the proportion of shear cracks positively linearly increased with the increasing moisture content. The increase of moisture in coal strengthened the freezing and expansion force generated by the water-ice phase transition during the liquid nitrogen freezing process, and the increase of energy dissipation contributed to the rapid development of pore-crack and the structural damage and plastic deformation of coal. However, the structural damage was difficult to detect in real time and can be inverted by AE energy.

Currently the law of heat and mass transfer and the analysis of coal damage in the process of injecting low-temperature liquid nitrogen (LN2) into the borehole of coal seam are the key problems to be solved urgently for the application of LN2 in the field of coal seam permeability enhancement. By using flowmeter, endoscope, thermocouple, mass balance and ultrasonic detector, the initial accumulation process and liquid level changes of LN2 during its injection into a coal borehole were visualized and analyzed. On this basis, the variation laws of the actual liquid level, net liquid level, and injection efficiency of LN2 in the borehole with time at different injection rates were investigated, and the coal body was ultrasonically detected for damage at three different times. It was found that the initial accumulation of LN2 will form an obvious Leidenfrost effect at the interface of the coal at the bottom of the borehole. With the continuous reduction of the coal temperature at the bottom of the borehole, the radiation heat transfer coefficient decreases, and then the thickness of the vapor film decreases. At different injection rates, the maximum increment of the net level in a 1 m borehole is about 35 cm, which means that when LN2 is injected into a long borehole, it should be injected segmentally, and the increase of injection time of LN2 will lead to the waste of LN2. The ratio of actual liquid level to net liquid level decreases first and then tends to be stable. The ratio of two liquid levels is similar in stable state. The change rule of LN2 injection efficiency with time at different rates is reduction-increase-stability-reduction. The injection efficiency of LN2 is higher at low injection rate. When the injection rate is increased, the nitrogen gas with phase change per unit volume of LN2 will have shorter heat exchange time in the borehole and lower discharge temperature outside the borehole, thus increasing the loss of LN2 and lowering the injection efficiency. Ultrasonic detection results show that as LN2 penetrates the fractures, the cold shock effect can freeze and shrink the fractures in both sides. As a result, the original fractures grow wider and their tips are stretched and deformed by the freezing and shrinking forces on both sides. Consequently, new fractures are formed.

In order to study the influence of liquid nitrogen cracking time on mechanical properties and permeability of coal samples, the independently developed WYS-800 triaxial gas seepage test device and acoustic emission detection system were used to conduct triaxial mechanical seepage tests on four groups of coal samples treated with different cracking time and collect acoustic emission signals. The mechanical properties and permeability of coal samples in triaxial mechanical seepage experiment were analyzed, and the characteristics of acoustic emission signals were described. Based on boiling heat transfer theory, one-dimensional cylinder heat conduction theory and thermal stress theory, the cracking mechanism was analyzed, and the thermal stress under different cracking time was calculated. The relationship between average temperature drop, average thermal stress, initial permeability and cracking time was revealed by data fitting. The results show that: ① Liquid nitrogen fracturing time have different influence on the mechanical properties of coal samples, the compressive strength and elastic modulus with the increase of the crack time shows the tendency of increase with the decrease of the first, the poisson's ratio are increased after the first decreases, coal sample triaxial loading when the axial stress and axial strain curve of the periodic evolution has obvious difference, associated with the change of mechanical parameters. ② The permeability of coal samples with different cracking time changes in u-shape during the triaxial loading process. The initial permeability, minimum permeability and maximum permeability increase with the increase of cracking time, and the increase rate is 119.05%, 437.5% and 146.49% when the cracking time is 30min. Acoustic emission signals are not active in compaction and elastic stages, and are mainly generated in yield and failure stages. The peak value of acoustic emission ringing count of coal samples after cracking is generated near the failure point, which is more than 20000 times. ③ The film boiling heat transfer coefficient between the coal sample and liquid nitrogen is 570.4 W/(m2·K). The average temperature drop of the coal sample is related to the cracking time, and the average thermal stress and initial permeability play a leading role. The average temperature drop of the coal sample can reach 213.63 K and the average thermal stress can reach 5.85 MPa after 30 min of cracking. ④ There is a linear relationship between initial permeability and average temperature drop, and a negative exponential relationship between initial permeability and cracking time of coal samples after liquid nitrogen treatment. Changing the parameter value can be extended to other similar coal samples or actual production evaluation.

The range of the cold shock of low-temperature liquid nitrogen on coal, the variation law of thermal stress in coal caused by temperature difference and the mechanical action mechanism of coal damage are the key issues to be studied urgently in the application of liquid nitrogen in the field of coal and rock permeability enhancement. Using the non-contact full-field strain measuring device, infrared temperature distribution measuring device and the thermocouple temperature measuring device, the variation laws of the coal temperature field and strain field around the borehole with time are measured after the liquid nitrogen cold shock on the borehole of coal samples in the open environment. By establishing a thermal-stress model including elastic modulus, Poisson’s ratio, thermal expansion coefficient, temperature difference of measuring points and radius parameters of measuring points, the experiments are designed to measure the parameters of models and the parameters are input into the models for solution. The thermal stress at different positions in coal and rock and its variation law with cold shock distance and time are calculated, and the above laws of three types of coals with different metamorphic degrees are compared. The results show that in the process of liquid nitrogen cold shock, the temperature at each measuring point of three types of coal samples all shows a variation law of rapid decrease-slow decrease-stability, and the thermal stress at each measuring point has experienced three stages: rapid growth-slow growth-zero growth. The closer to the cold shock borehole, the greater the temperature difference between adjacent measuring points, and the greater the thermal stress. After 1 800 s of liquid nitrogen cold shock, the thermal stress at each measuring point of three kinds of coal samples basically reached a stable stage, and the thermal stress at 1 cm from the cold shock drilling hole was ≥ 1.84 MPa. The cold shock of liquid nitrogen can cause four kinds of deformation in the coal, and then damage the coal. Among them, the type II, III, IV deformation caused by “pressure-induced tension” and the compound deformation of three are the fundamental reasons for the formation of complex cracks in the coal. However, although the range of cooling and deformation of different coals is different, the overall range of action is limited, and it can only produce damage around the borehole. Therefore, the liquid nitrogen cold shock technology is suggested to be applied in combination with fracturing technologies, and the damage around the borehole after cold shock can effectively reduce the initiation pressure and make the fracturing fracture more complicated.

The permeability of coal reservoir is generally low in China, how to effectively improve the permeability of coal reservoir is a key and difficult point of coalbed methane exploitation, Liquid nitrogen fracturing technology as a kind waterless fracturing technology has received extensive attention in recent years. In order to reveal the influence of liquid nitrogen freezing and freeze-thaw on the mechanical properties of coal, the temperature distribution of coal samples was monitored by infrared thermal imaging technology, and uniaxial compression and acoustic emission tests were performed on the coal samples after the liquid nitrogen freezing and freezethaw, the P-wave velocity, porosity, acoustic emission and energy evolution characteristics of coal samples before and after the freezing and freeze thaw were analyzed. The research result showed that: ①After 360 min freezing and 12 freeze-thaw cycles, the P-wave velocity of coal samples decreased by 58.2% and 64.7%, respectively. The P-wave velocity does not decrease significantly during the initial freezing and freeze-thaw cycle stages, the velocity gradually decreases with the increase of freezing time and freeze-thaw cycles. ②The temperature of the coal sample gradually decreases with increase of freezing times. The surface temperature of the coal sample drops below -60°C after the liquid nitrogen frozen for 180s, the temperature distribution fluctuations at the center of the coal sample occurs due to the different thermal conduction coefficient of the coal particles. ③After liquid nitrogen freezing and freezethaw, the elastic modulus of coal sample decreases exponentially, while the porosity gradually increases. The increase in porosity of the coal sample after liquid nitrogen freeze-thaw is greater than that after liquid nitrogen freezing. ④The acoustic emission activity of coal samples during uniaxial loading is divided into development phase, active phase and severe phase, the maximum acoustic emission ringing count and cumulative acoustic emission ringing count of coal samples increase with the increase of freezing time and freeze-thaw cycles. ⑤Liquid nitrogen freezing and freeze-thaw will weaken the energy storage limit of coal sample, resulting in the reduction of the total energy, elastic energy and dissipated energy at the peak point during the uniaxial loading process.

With the development of coal seam fracturing technology, promote the gas extraction by liquid nitrogen cold soaking fracturing coal has attracted widespread attention. In order to study the effect of liquid nitrogen cold soaking on the microstructures of different coals (lean coal, fat coal and anthracite) before and after liquid nitrogen cold soaking, the scanning electron microscopy (SEM), pore size distribution instrument and physical adsorption instrument were jointly characterized. The influence of liquid nitrogen cold soaking on the distribution of pore volume and specific surface area of different coal samples was comparatively analyzed. The results show that the thermal stress generated during the liquid nitrogen cold soaking destroys the microstructure of the coal or causes the initiation of micro-cracks, After liquid nitrogen cold soaking, the total pore volume and specific surface area of coal samples increase. The total pore volume growth rate of fat coal is the lowest, followed by anthracite, and the lean coal’s is the highest. The volume of micropores, micropores, meso-pores and macropores/fractures in the coal samples increase. cold leaching of liquid nitrogen causes macropores to pass through and form macropores/fissures in coal samples, resulting in the reduction of pore volume of macropores. After liquid nitrogen cold soaking, the pore specific surface area of each coal sample concentrate in the range of 10 − 100 nm, and has obvious peak characteristics. Liquid nitrogen cold soaking can increase the adsorption capacity of lean coal, fat coal and anthracite coal samples, and the adsorption capacity difference is the largest in the middle high pressure region (0.4 < p/p0 < 1.0). Liquid nitrogen cold soaking can effectively transform the internal microstructure of different coals. The research results are helpful to reveal the spatial expansion and connectivity of macroscopic and microscopic pore fractures in coal reservoirs during liquid nitrogen cracking.

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The thermal fracture of rock under cryogenic impact is caused by the instantaneous huge thermal stress caused by rapid temperature change. The law of heat transfer inside rock determines the magnitude and action form of thermal stress. In order to study the heat transfer law of rocks under cryogenic impact, the internal temperature fields of granite, sandstone, and limestone under cryogenic impact in liquid nitrogen environment at different times were measured, and the influence effect of rock characteristics on heat transfer law was discussed. The results show that: 1) The temperature change process of rock during cryogenic impact can be divided into three stages: slow cooling, rapid cooling, and temperature flattening; 2) The evolution law of temperature and temperature gradient of different types of rock is roughly the same, but its distribution is different owing to different rock characteristics; 3) The heat transfer and deformation of rock under cryogenic impact are affected by the mineral composition and structural characteristics of the rock itself, and they together determine the final degree of rock fracture.

Copyright of Science & Technology of Food Industry is the property of Science & Technology of Food Industry Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

采用高效液氮加注技术对葵花籽油进行灌装，并与气氮加注、气液氮混合加注技术进行对比，研究其对葵花籽油品质和产品包装的影响。结果表明：3种充氮方式样品的酸值和过氧化值变化趋势基本一致；液氮加注样品常温储藏5个月后酸值（KOH）和过氧化值分别为0.07 mg/g和1.45 mmol/kg，与常温储藏相比，液氮加注样品45 ℃储藏5个月后酸值无差别，过氧化值稍有降低，为130 mmol/kg；液氮加注和气液氮混合加注样品初始及储藏过程中的过氧化值比气氮加注样品的稍高；常温下3种充氮方式样品的初始残氧值相差不大，但储藏过程中液氮加注和气液氮混合加注样品的残氧值明显低于气氮加注样品；常温下液氮加注样品瓶内存在正压，其满瓶抗压能力优于气氮加注和气液氮混合加注样品。High efficiency liquid nitrogen filling technology was used to fill sunflower seed oil, and it was compared with gas nitrogen filling and gas-liquid nitrogen mixed filling technology. The effects of nitrogen filling method on quality and product packaging of sunflower seed oil were studied. The results showed that the change trends of acid value and peroxide value of the samples with three nitrogen filling methods were basically the same. The acid value and peroxide value of liquid nitrogen filling samples were 0.07 mgKOH/g and 1.45 mmol/kg respectively after five months storage at room temperature. Compared with room temperature storage, the acid value of liquid nitrogen filling samples after five months storge at 45 ℃ had little difference, while the peroxide value decreased slightly to 1.30 mmol/kg. The peroxide values of liquid nitrogen filling samples and gas-liquid nitrogen mixed filling samples during initial and storage were slightly higher than that of gas nitrogen filling samples. There was little difference in initial residual oxygen values of three nitrogen filling samples, but the residual oxygen values of liquid nitrogen filling samples and gas-liquid nitrogen mixed filling samples during storage were significantly lower than that of gas nitrogen filling samples. There was positive pressure in the bottle of liquid nitrogen filling sample, and its full bottle compression capacity was better than that of gas nitrogen and gas-liquid nitrogen mixed filling samples.

To investigate the effect of freeze-thawing of liquid nitrogen on the pore structure and adsorption behavior of coal, the pore structure of coal samples before and after freeze-thawing was characterized by low-temperature nitrogen and CO2 adsorption method, and methane isothermal adsorption experiments were carried out under different freeze-thawing times. The results show that the hysteresis index HI of coal samples decreased after freezing and thawing, and the connectivity of the pore system was improved, which is conducive to gas migration in coal. The pores between 2 nm and 100 nm increased to varying degrees after freezing and thawing, and some pores evolved into pores smaller than their own pore size or even less than 2 nm. The microporous pore size distribution has changed, with an increase in microporous pore volume and adsorption volume. With the increase of the number of liquid nitrogen treatments, the effect of liquid nitrogen treatment on the final adsorption amount of methane kept weakening, and after reaching four treatments, the effect of liquid nitrogen treatment on the final adsorption amount of coal body almost reached saturation. The Langmuir parameters VL and pL increased after the coal samples were treated with liquid nitrogen at different times, the larger the pL, the more favorable it was for the extraction of CBM.

[Objective] As an ultra-low-temperature medium, liquid nitrogen offers the advantages of low cost, fast freezing, and high freezing strength, which contribute to its wide use in urban underground engineering, tunnel construction, water plugging, mine construction, and production. In addition, liquid nitrogen freeze-thaw can promote the development of coal rock fractures and further increase the permeability of gas-bearing coal rock reservoirs. Therefore, it is highly important for freezing-applied construction and coalbed methane extraction engineering in cold regions to study the evolution of the mechanical properties of coal rock mass under liquid nitrogen freezing and freeze-thaw conditions. [Methods] In this study, experiments were conducted on the mechanical properties of coal under liquid nitrogen freezing and freeze-thaw conditions using a liquid nitrogen freezing coal rock experimental device. Using liquid nitrogen freezing time and liquid nitrogen freeze-thaw frequency as single-factor variables, the stress-strain curve was used to quantitatively analyze the changes in the mechanical parameters, including the compressive strength and elastic modulus of coal. The mechanisms of enhancing the mechanical strength of frozen coal and deteriorating the mechanical strength of the freeze-thaw coal were discussed based on the micropore and crack angles. [Results] The experimental results on the mechanical properties of coal under liquid nitrogen freezing and freeze-thaw conditions showed the following: 1) liquid nitrogen freezing has a positive effect on the mechanical strength of coal. The elastic modulus of the coal sample after 80 minutes of liquid nitrogen freezing is 3.67 GPa, while the elastic modulus of the original coal sample is 1.40 GPa, with an increase of 165.69%. Liquid nitrogen freeze-thaw deteriorates the mechanical strength of coal. After 15 cycles of liquid nitrogen freeze-thaw, the elastic modulus of the coal sample is 0.52 GPa, while the original coal sample has an elastic modulus of 1.50 GPa, i.e., a decrease of 65.33%. 2) The formation of porous ice has three positive effects on enhancing the mechanical strength of coal. Porous ice has a certain mechanical strength, and the mechanical properties of ice continue to increase with the decrease in temperature. The volume expansion effect of the ice-water phase transition increases the contact area between ice and coal matrix particles, revealing the cementing role of pore ice formation. After the formation of pore ice, the length of pore cracks decreases, further decreasing the stress concentration at the crack tip, which hinders the development of the pore structure of the loaded coal. 3) The damage and deterioration of the coal body caused by liquid nitrogen freeze-thaw primarily contain three aspects. In liquid nitrogen freezing and thawing, the shrinkage expansion cycle of the middling coal matrix will eventually increase the porosity of the coal sample, damaging the interior of the coal. The volume expansion caused by the phase change of pore water and the formation of pore ice will promote the expansion and connectivity of primary fractures, and thermal stress will facilitate the formation of new pores inside the coal body, further damaging and destroying the coal structure. Freezing coal samples can reduce the stress concentration due to short cracks, while freeze-thaw coal samples can increase the stress concentration due to the increased length of internal cracks in the coal. [Conclusions] This experiment uses a combination of macro and microanalysis methods, with comparative and in-depth mechanism analysis as the research tools, to cultivate students' flexible and innovative thinking to adapt to the changes in scientific research and disciplinary development. In terms of experimental organization, allowing students to participate directly in experiments requiring hands-on practice, data processing, and result analysis can greatly stimulate their learning enthusiasm and improve their subjective initiative and innovative thinking ability. [ABSTRACT FROM AUTHOR]

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Hydraulic fracturing technology is widely used in fracturing coal seam and improving efficiency of coalbed methane drainage, but due to large water consumption, environment pollution by fracturing fluid and other problems, it is difficult to be used in water-deficient areas where are rich in coalbed methane. Based on conventional liquid nitrogen fracturing technology, a cyclic liquid nitrogen fracturing technology is proposed through altering the single injection mode of liquid nitrogen, and application of the technology in enhancing coalbed methane drainage is prospected. The technology uses cyclic injection mode of liquid nitrogen, which can make liquid nitrogen to fulfill micro-crack space continuously, so as to form complex crack networks. The principle of fracturing coal-rock of the technology is coupling mechanism of cyclic cumulative damage, low temperature fracturing and gas fracturing: ① During cyclic injection process of liquid nitrogen, different deformation responses appear among grains with different collocation, and accumulated damages among grains could weaken coal body strength of pressure resistance in local area to some degree, so as to generate new cracks and extend the primary cracks. ② Cold shock of liquid nitrogen to coal-rock leads to break of inner structure of coal-rock, so a large number of micro-cracks are generated and continuously extended with cyclic injection of liquid nitrogen. Meanwhile, water-ice phase transition caused by low temperature property of liquid nitrogen could further damage inner structure of coal-rock. ③ The generated high-pressure gas by vaporization of liquid nitrogen could make coal-rock volume to repeatedly extend and shrink, which further enhances growth and connection. Cyclic liquid nitrogen fracturing technology, which takes liquid nitrogen as fracturing fluid, greatly relieves dependence on water resources, and has no pollution to environment. The technology can be applied in coalbed methane drainage.

[Objectives] This paper aimed to explore the effects of liquid nitrogen quick freezing and immersion chilling and freezing on the eating quality and volatile flavors of soft-boiled chicken. [Methods] The soft-boiled chickens were treated by air blast freezing(AF) with -18 ℃, liquid nitrogen quick freezing (NF) and immersion chilling freezing (ICF), and were stored in a freezer at -18 ℃ after the core temperature of the samples reached -18 ℃. After one month storage, the soft-boiled chickens were thawed to explore the effect of different freezing methods on the eating quality of soft-boiled chicken. [Results] The freezing rate of NF and ICF treatments was about 60 times and 15 times higher than that of AF treatment respectively; the thawing losses of NF and ICF treatments were 3.82% and 3.16%, respectively, and the thawing losses of NF and ICF treatments were reduced by more than 30% compared with that of AF treatment. NF and ICF treatments were effective in maintaining color and delaying the decrease of pH value during the freezing process. The MDA content of NF and ICF treatments was 0.55 and 0.44 mg·kg-1. The flavor contents after freezing and thawing were different from those of freshly cooked soft-boiled chicken. The sensory scores of NF and ICF treatments were significantly higher than those of AF treatment. [Conclusions] Both NF and ICF treatments could prevent the quality deterioration caused by blast freezing. [ABSTRACT FROM AUTHOR]

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BACKGROUND: Vulnerable plaque is the pathological basis of acute cardiac and cerebrovascular ischemic events. Establishing a suitable animal vulnerable plaque model is of great significance for guiding the prevention and treatment of cardiovascular and cerebrovascular diseases. However, there is no well-established method for constructing the vulnerable plaque model. OBJECTIVE: To investigate whether immunostimulation can cooperate with liquid nitrogen freezing to establish an atherosclerotic vulnerable plaque model in rats, aiming to replicate the widely applicable atherosclerotic model. METHODS: Forty male Sprague-Dawley rats were randomly divided into four groups after 1 week of adaptive feeding, namely, control group, high-fat group, liquid nitrogen group, immunostimulation + liquid nitrogen group. After 2 weeks of conventional diet and special high-fat diet, respectively, the rats in the control group and the other groups were given a single dose of vitamin D3 injection through the abdominal cavity. Rats in the control and high-fat diet groups were subjected to sham operation with no immunostimulation and liquid nitrogen freezing, and given routine diet and special high-fat diet for 10 weeks respectively. Rats in the liquid nitrogen group and immunostimulation + liquid nitrogen group were given liquid nitrogen freezing, followed by 10 weeks of high-fat diet. During this period, the rats in the immunostimulation + liquid nitrogen group were given subcutaneous injection of bovine serum albumin (3 times per week, for 3 weeks) and intraperitoneal injection of ovalbumin (3 days interval, 5 times in total). At 10 weeks after modeling, blood samples were taken to detect blood lipid index, inflammation and oxidative stress injury index, and heme oxygenase-1 (HO-1) protein expression in rats. The target blood vessel segments of each group were executed for hematoxylin-eosin staining, and the pathological changes of atherosclerotic plaques were observed under light microscopy. The study protocol was approved by the Animal Ethics Committee of Southwest Medical University, with an approval No. IACUC:20170917006. RESULTS AND CONCLUSION: Compared with the control group, there were no significant changes in cholesterol level, inflammatory and oxidative stress index, HO-1 protein expression and histopathological examination in the high-fat group (P > 0.05). Compared with the control and high-fat groups, the levels of serum lipids, inflammatory and oxidative stress indicators and HO-1 protein expression in the liquid nitrogen and immunostimulation + liquid nitrogen groups were significantly increased (P < 0.05). Histopathological examination also indicated that there were obvious plaques, and the increasing trend in the immunostimulation + liquid nitrogen group was more significant (P < 0.05). Compared with the liquid nitrogen group, the levels of serum lipids, inflammatory and oxidative stress indexes and HO-1 protein expression in the immunostimulation + liquid nitrogen group were significantly increased (P < 0.05), and histopathological examination also showed more obvious atherosclerotic plaques. Therefore, immunostimulation could accelerate the formation of rat common carotid atherosclerotic vulnerable plaque model in cooperation with liquid nitrogen freezing injury, which lays a foundation for the intervention experiment of atherosclerotic disease. [ABSTRACT FROM AUTHOR]

The available extraction methods for the determination of Br and I in geological samples, include semifusion, aqua ammoniae closed digestion and pyrohydrolysis. The sample digestion and extraction of Br and I are the major error sources, due to the low content and volatization of Br and I. A pyrohydrolysis method with liquid nitrogen trap has been developed and used to extract Br and I in geological samples using liquid nitrogen condensation absorption replacing alkaline solution absorption. Br and I are measured by Inductively Coupled Plasma-Mass Spectrometry. The detection limits of Br and I are 0.06 μg/g and 0.01 μg/g, respectively, calculated as 10 times standard deviation using dilution times of 50. These detection limits are lower than those of traditional pyrohydrolysis and semifusion methods and slightly higher than those of aqua ammoniae closed digestion method. The precision is 6.4%-21.0% RSD. The proposed method reduces the use of alkaline solution, lowers the blank and dilution times, and improves the precision. This method is much simpler than the semifusion method and can serve as a complementary method for aqua ammoniae closed digestion method. For soils and stream sediments, aqua ammoniae closed digestion method is preferred. For rocks and samples that cannot be completely digested by aqua ammoniae closed digestion method, the proposed method may be more suitable.

In order to research fracturing mechanism of coal subjected to liquid nitrogen cooling, submersion tests of liquid nitrogen for natural coal samples are carried out. A laser microscope is employed to observe the extension of native crack and propagation of cracks of coal samples after thermal shock. Then the fracture mechanics theory is used to analyze the extension mechanism of the crack during thermal shock. The main conclusions include: 1) Liquid nitrogen cooling causes temperature tensile stress and the stress concentration. When the stress intensity factor is more than the fracture toughness of coal native crack, the native crack will extend. 2) The temperature tensile stress makes the particle clusters with low tensile strength around the native crack fail and break off, and the destruction of the structure of the coal sample causes microcrack initiation. 3) There is a great effect of cooling time on the stress intensity factor. With the increasing of the temperature gradient and the cooling time, the stress intensity factor increases, with more susceptibility for coal to rupture.

To address the high rock strength and low drilling rate issues in deep oil/gas and geothermal exploitation, we performed mechanical property tests on three kinds of rock samples (granite, shale and sandstone) subjected to liquid nitrogen (LN2) cooling and conducted rock-breaking experiments using LN2 jet. Rock-breaking characteristics and mechanisms of LN2 jet, heat transfer features between LN2 and rock and thermal stress evolution in rock were analyzed. A novel high-pressure LN2 jet assisted drilling method was proposed accordingly. The study results show that LN2 thermal shock can significantly reduce uniaxial compression strength and elastic modulus of rock. Rock damage and corresponding mechanical deterioration become more pronounced with increasing rock temperature. The LN2 jet has merits of high rock-breaking efficiency and low threshold rock-breaking pressure. Rock failure under LN2 jet impact is characterized by large volume breakage and the rock-breaking performance becomes more significant with increase of rock temperature. Under the impact of LN2 jet, the damage of granite is the most remarkable among the three rock samples. Thus, this method works better for high temperature granite formations. It has a good application prospect in speeding up drilling rate in deep hot dry rock geothermal reservoirs. Key words: liquid nitrogen jet, cold shock, jet impact, rock-breaking experiment, assisted drilling, drilling rate improvement

Objective To explore the reliability of liquid nitrogen cryotherapy for infant lip mucosa venous malformations.Methods A retrospective review of 57 pediatric patients with venous malformations involving the lips who underwent liquid nitrogen cryotherapy was enrolled in. Among them, 35 patients were males and 22 patients were females; the age of the patients ranged from 3 to 21 months (average 7.4 months). Photographs were taken preoperatively, immediately postoperatively and one month postoperatively. Following up period was 2-16 months and the outcome was assessed.Results 71.9% cases were completely effective, 19.3% cases were mostly effective, 8.8% cases were partially effective. No cases showed the anesthesia complications.Conclusion Liquid nitrogen cryotherapy is an effective, simple and safe treatment for lip mucous venous malformations.

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Objective In order to reduce the equipment investment and energy loss of extracting helium from natural gas by using a single low-temperature method, a new natural gas helium extraction combined with LNG production process is provided. Methods Based on the data of low helium feed gas from a domestic gas field, the characteristics and adaptability of this new process have been simulated and analyzed, and the refrigeration cycle of the mixed refrigerant has been optimized by genetic algorithm. Results（1） Characteristic analysis shows that the lower the temperature before throttling and the higher the throttling pressure, the better the concentration effect of the flash tank on helium gas. At the same time, nitrogen products with a purity of 99.9 mol% can be produced, and liquid nitrogen products can be produced by itself;（2） The new process has strong adaptability to changes in nitrogen and helium content in the feed gas, as well as the pressure of the feed gas. The higher the feed gas pressure and the lower the nitrogen and helium content, the lower the compressor shaft power;（3） Through analysis of characteristics and adaptability, as well as optimization of mixed refrigerant refrigeration process parameters, it can be obtained that the temperature before throttling in the flash tank is-150 ℃, the throttling pressure is 350 kPa, the pressure of the helium extraction tower is 3 200 kPa, the number of plates in both the denitrification and helium extraction towers is 12,the optimal boost pressures for the first and second stages of the mixed refrigerant compressor are 1 341 kPa and 2 767 kPa, respectively, and the circulation rate of the mixed refrigerant is 1 722.67 kmol/h, the total compressor shaft power is 3 765 kW, the cooling water consumption is 6 474 t/d, the comprehensive energy consumption and unit energy consumption are 1 097 216.00 MJ/d and 3.66 MJ/m~3, respectively. Conclusions The helium recovery rate is more than 98%, and the crude helium purity is higher than 70 mol% for the new process. It can produce refrigerants by itself and obtain high-purity nitrogen and liquid nitrogen products, and has good adaptability to the gas quality conditions of the feed gas [ABSTRACT FROM AUTHOR]

In order to solve the problems of high crack ratio and liquid nitrogen loss, a three-chamber liquid nitrogen quickfreezer was designed based on food freezing curve. The three chambers are precooling chamber (4.2 m, -70 °C), crystallization chamber (5.8 m, -100 °C) and cryogenic chamber (5.0 m, -120 °C). Compared to single-chamber liquid nitrogen quick-freezer (15.0 m, -100 °C), the quick freezer decreased the freezing time by 10.0% and crack ratio by 40.4% of pre-packaging prepared crayfish, and the loss of liquid nitrogen by 15.0%. Besides, it significantly improved texture (hardness, elasticity, cohesion, resilience, chewability) and color (L*, b*), reduced volatile basic nitrogen, increased water content, salt soluble protein and sensory quality (color and texture). These results provide a reference for the improvement of liquid nitrogen quick-freezing technology and equipment. [ABSTRACT FROM AUTHOR]

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Spray cooling characteristics of TG6.5 solid cone nozzle are studied by building a liquid nitrogen spray cooling experimental bench. The relationships between the superheat and the heat flux as well as the superheat and the heat transfer coefficient near the critical point are analyzed. Then the influence of the spray area and the spray flow rate on the heat transfer effect is discussed. Results show that, in the process of heat transfer, with the increase of the heat flux, the change of superheat can be divided into three stages: a slow increase, an obvious increase and a sharp increase. Under the influence of bubbles in nucleate boiling, the superheat at the maximum heat transfer coefficient is less than that at the critical heat flux. Increasing the spray flow can significantly improve the heat transfer effect. Increasing the heat sink area will reduce the impact frequency and intensity of droplet particles in the unit area, so that the heat transfer effect will decrease. [ABSTRACT FROM AUTHOR]

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In this study, the effects of different freezing methods and temperatures (freezer at −20 and −50 ℃, and liquid nitrogen at −50 and −80 ℃) on the water-holding capacity and structural properties of myofibrillar proteins from the hind leg muscle of Rana nigromaculata were investigated. The results showed that the time required for hind legs frozen in freezer at −20 and −50 ℃ and with liquid nitrogen at −50 and −80 ℃ to pass through the zone of maximum ice crystal formation temperature (−1 to −5 ℃) were 91.5, 57.0, 1.5 and 0.5 min, respectively. Compared with the freezer freezing groups, the liquid nitrogen freezing groups exhibited reduced thawing loss, increased content of immobilized water, and decreased content of free water. Compared with fresh samples, the water-holding capacity, solubility, total sulfhydryl content and endogenous fluorescence intensity of myofibrillar proteins decreased significantly after freezing, and the particle size, surface hydrophobicity and relative content of random coil showed an increasing trend. In the −50 ℃ liquid nitrogen freezing group, the water-holding capacity, protein solubility, total sulfhydryl content and endogenous fluorescence intensity decreased by 8.10%, 9.57%, 10.50% and 67.36%, respectively, and the volume average particle size, surface hydrophobicity and random coil content increased by 100.05%, 82.24% and 13.65%, respectively; these changes were significantly lower than those in the other freezing groups (P < 0.05). The degree of myofibrillar protein denaturation in the −80 ℃ liquid nitrogen freezing group was significantly lower than that in the −20 ℃ freezer freezing group, but higher than that in the −50 ℃ liquid nitrogen freezing group (P < 0.05). According to the results of scanning electron microscopy (SEM), muscle fibers were more tightly and regularly arranged in the −50 ℃ freezer freezing and −50 ℃ liquid nitrogen freezing groups, while those in the −80 ℃ liquid nitrogen freezing group were arranged regularly but slightly cracked, and those frozen in the −20 ℃ freezer freezing group were obviously broken. These findings indicate that liquid nitrogen freezing can slow down the migration of water in the hind leg muscle of R. nigromaculata and effectively inhibit the cold denaturation of myofibrillar proteins, the effect being more pronounced at −50 ℃ than at −80 ℃.

In order to explore the changes and distributions of the temperature of PDMS sample being impacted and cooled by the liquid nitrogen jet during the cryogenic micro-abrasive air jet machining the PDMS microchannels, this paper proposed a systematic research method of temperature that composed of temperature measurement experiments, Beck's inverse estimation method fitting and APDL temperature field simulation. The errors between the final simulation results and the temperature measurement ones are only 1.735%. The simulations find that it will take 5.747 s for the surfaces to reach the obvious embrittlement temperature -147 °C. PDMS may not reach a machinable embrittlement state at the moment of contact with liquid nitrogen, so it is necessary to pay attention to the pre-cooling time before machining. The simulation data was used to fit the cooling rate of PDMS to -147 °C, and the cooling rate was used as a reference to predict the pre-cooling time required in the actual machining. By comparing the cooling rate, it is found that the cooling rate in the depth range of 0~100 µm from the surface of the PDMS specimen is 22.054% faster than that in the depth range of 0~1500 µm, and the cooling rate of the aluminum alloy workbench is about 22.311% higher than that of the adiabatic workbench. [ABSTRACT FROM AUTHOR]

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Liquid nitrogen cooling auxiliary system is applied in the high temperature superconducting power transmission technique to reach the superconducting critical temperature of the cable. The cable can realize high density currents transmission in a low-cost, efficient and environmental-friendly way. Note that, the cable pipeline will be undergoing an extremely low temperature freezing and thawing problems. The ultrasonic tests, uniaxial compressive strength tests and nuclear magnetic resonance tests were performed on C25 concrete with different freezing and thawing cycles. The results show that the decrease in ultrasonic wave speed and the deterioration of compressive strength of concrete are caused by the freeze-thaw cycles. The ultrasonic wave speed and uniaxial compressive strength of concrete show a similar trend with the increasing freeze-thaw cycles. After four freeze-thaw cycles, the ultrasonic wave velocity reduced by 13.6%. While the uniaxial compressive strength decreased over 28%. Moreover, NMR tests revealed the changes in the micropores of C25 concrete under different freeze-thaw cycles. Test results show that the porosity increased with the increasing of freeze thaw cycles. The decrease of ultrasonic wave velocity and peak strength of concrete can be attributed to the increasing total porosity and the rising ratio of larger capillary pores to smaller ones caused by the freezing and thawing cycles. [ABSTRACT FROM AUTHOR]

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