Search

Palaeodrainage basin, as an important component of the source-to-sink system, contains critical information on provenance and palaeoenvironment. Previous studies indicate that the scaling relationships of source-to-sink system components generally follow power laws, and channel-belt thickness represents a reliable first-order proxy for the drainage area. In this study, a database of borehole cores and geophysical well logs of the Jurassic coal measures from Saishiteng area in the northern Qaidam Basin was used to reconstruct the palaeogeography, and to identify single-story channel-belts. Three palaeochannels, namely, River A, River B and River C, were identified which were persistent throughout the Dameigou and Shimengou Formations during the Middle Jurassic. The mean channel-belt thicknesses of River A, River B and River C in the Dameigou Formation were 9.8 m, 8.9 m and 7.9 m, respectively, and those in the Shimengou Formation were 7.4 m, 6.2 m and 5.4 m, respectively. We estimate the drainage area of three major rivers by using scaling relationships between drainage area and channel-belt thickness. The drainage areas of River A, River B and River C in the Dameigou Formation were 63.0 × 103 km2, 50.1 × 103 km2 and 37.7 × 103 km2, respectively, and those in the Shimengou Formation were 32.3 × 103 km2, 21.2 × 103 km2 and 15.3 × 103 km2, respectively. The drainage basin lengths of River A, River B and River C in the Dameigou Formation were 300.4 km, 239 km and 180.2 km, respectively, and those in the Shimengou Formation were 154.3 km, 101.3 km and 73.1 km, respectively. For both the Dameigou and Shimengou Formations, River A showed the largest scale, followed by River B and River C succeedingly, which was mainly determined by the stretch direction of provenance in the southern Qilian Mountains. The variations of channel-belt thickness, drainage area and drainage basin length between Dameigou and Shimengou Formations are the response of source-to-sink system to the transformation from extension to compression depression during the Middle Jurassic in the northern Qaidam Basin.

Acidithiobacillus caldus (A. caldus) is a common bioleaching bacterium that possesses a sophisticated and highly efficient inorganic sulfur compound metabolism network. Thiosulfate, a central intermediate in the sulfur metabolism network of A. caldus and other sulfur-oxidizing microorganisms, can be metabolized via the tetrathionate intermediate (S4I) pathway catalyzed by thiosulfate:quinol oxidoreductase (Tqo or DoxDA) and tetrathionate hydrolase (TetH). In A. caldus, there is an additional two-component system called RsrS-RsrR. Since rsrS and rsrR are arranged as an operon with doxDA and tetH in the genome, we suggest that the regulation of the S4I pathway may occur via the RsrS-RsrR system. To examine the regulatory role of the two-component system RsrS-RsrR on the S4I pathway, ΔrsrR and ΔrsrS strains were constructed in A. caldus using a newly developed markerless gene knockout method. Transcriptional analysis of the tetH cluster in the wild type and mutant strains revealed positive regulation of the S4I pathway by the RsrS-RsrR system. A 19 bp inverted repeat sequence (IRS, AACACCTGTTACACCTGTT) located upstream of the tetH promoter was identified as the binding site for RsrR by using electrophoretic mobility shift assays (EMSAs) in vitro and promoter-probe vectors in vivo. In addition, ΔrsrR, and ΔrsrS strains cultivated in K2S4O6-medium exhibited significant growth differences when compared with the wild type. Transcriptional analysis indicated that the absence of rsrS or rsrR had different effects on the expression of genes involved in sulfur metabolism and signaling systems. Finally, a model of tetrathionate sensing by RsrS, signal transduction via RsrR, and transcriptional activation of tetH-doxDA was proposed to provide insights toward the understanding of sulfur metabolism in A. caldus. This study also provided a powerful genetic tool for studies in A. caldus.

Toughening mechanisms and flank wear behavior of a TiC whisker toughening Al2O3-based ceramic cutting tool composite were investigated. The results showed that the wear behavior of the tool composite was greatly influenced by the whisker toughening mechanisms, of which the toughening effects on the composite can increase with increasing cutting temperature. As a result, the tool composite could still possess enough high fracture toughness and flexure strength at high cutting temperature, which resulted in an interesting phenomenon that the wear resistance of the tool composite at higher cutting speed was higher than that of itself at lower cutting speed.

A TiC whisker toughening Al2O3 composite ceramic tool material was developed by an in situ carbothermal reduction synthesis technology. The influence of different synthesis temperatures (1480°C and 1700°C) and holding times (90min, 60min and 30min) on the mechanical properties of the composite was investigated. The results showed that either the lower synthesis temperature (1480°C) or the higher synthesis temperature (1700°C) was necessary to the improvement on the fracture toughness of the composite. However, when an Al2O3-TiO2 eutectic system was formed during the higher temperature synthesis process, grain coarsening of the Al2O3 matrix became much serious, which resulted in obvious decrease in the flexure strength of the composite. Therefore, the carbothermal reduction synthesis process should be divided into two parts including the lower temperature synthesis process for increasing the yield of the TiC whisker and the subsequent higher temperature synthesis process for increasing the carbon content in the TiC whisker, and and the lower temperature synthesis process should be held for enough time (e.g. 60min) to avoid an incomplete carbothermal reduction of TiO2.

A TiC whisker toughening Al2O3-based ceramic cutting tool composite named ATW30 was developed by a two-step in situ synthesis technology. The fracture toughness, flexure strength and Vickers hardness of the composite were 7.63MPa•m1/2, 800MPa and 19.5GPa, respectively. Compared to a (W, Ti)C particle strengthening Al2O3-based ceramic cutting tool composite named SG4, ATW30 was with lower flexure strength and Vickers hardness but much higher flexure strength, and therefore obviously higher wear resistance at either lower or higher cutting speed. The dominant wear mechanism on the flank face of ATW30 is abrasive wear at lower cutting speed and adhesive wear at higher cutting speed. Because of the appearance of the adhesive wear, the wear resistance of ATW30 at higher cutting speed is lower than that of itself at lower cutting speed.

TiC whiskers have been synthesized via a carbothermal reduction technique in an α-Al 2 O 3 matrix within a temperature range of 1380–1580 °C in an argon atmosphere. The raw materials consist of TiO 2 , carbon, nickel and NaCl. Various mixing procedures and reaction temperatures were used. The yield of the whiskers is mainly dependent on the mixing procedures and the morphology of the synthesized composite powders is mainly dependent on the synthesis temperatures. The majority of the synthesized whiskers display an ideal aspect ratio of 10–30 with a diameter of 1–3 μm. No significant influence on the growth of the TiC whiskers by the present of the Al 2 O 3 matrix powder can be noted.

The composite materials, 3Y-TZP added with TiN or/and Al2O3 particles, were fabricated by hot-pressing techniques. The effect of TiN and Al2O3 on the low temperature degradation of 3Y-TZP was investigated. It is shown that both TiN and Al2O3 can inhibit t-m transformation of 3Y-TZP and prevent the transformation from propagating into the specimen interiors when aged at a low temperature. The inhibition to the low temperature degradation of 3Y-TZP by Al2O3 is more sensitive to the aged temperature than by TiN. Compared with Al2O3, TiN is more available to inhibit t-m transformation on aged surfaces. The degradation resistance of 3Y-TZP codoped by TiN and Al2O3 is stronger than that of 3Y-TZP dispersed only by TiN or Al2O3.

An advanced ceramic cutting tool material was developed by means of micro-scale SiC particle cooperating with nano-scale SiC particle dispersion. With the optimal dispersing and fabricating technology, this multi-scale nanocomposite may get both higher flexural strength and fracture toughness than that of the single-scale composite. The improved mechanical properties may be mainly attributed to the inter/intragranular microstructure with a lot of micro-scale SiC particles located on the grain boundary and a few nano-scale SiC particles located in the matrix grain. Because of the thermal expansion mismatch between SiC and Al2O3 resulting in the compressive stress on the SiC/Al2O3 interface, the interface bonding strength between Al2O3 and SiC was reinforced, which can compel the crack propagating into the relatively weak matrix when meeting the SiC particle on the boundary; while the alumina grain boundary is not the same strong as the SiC/Al2O3 interface and the Al2O3 grain, therefore the crack propagates sometimes along the Al2O3 grain boundaries and sometimes through the grains, when reaching to the nano-scale SiC particle inside the matrix, the crack was pinned and then deflected to the sub-grainboundaries. These coexisting transgranular and intergranular fracture mode induced by micro-scale and nano-scale SiC and the fining of matrix grain derived from the nano-scale SiC resulted in the remarkable strengthening and toughening effect.

An effect of nano-scale TiN grains on the mechanical properties and microstructure of Si3N4 based ceramic tool materials is investigated at the different sintering temperature. Compared to monolithic Si3N4 ceramic tool materials, the sintering temperature is decreased and mechanical properties is enhanced when only one percent of nano-scale TiN in term of mass is added into the Si3N4 matrix. The optimum mechanical properties are achieved when Si3N4/TiN nanocomposites tool materials were sintered at the sintering conditions of 1650, 30MPa and holding time of 40min. The flexural strength, fracture toughness and hardness are 1018.2MPa, 8.62MPa⋅m1/2 and 14.58GPa respectively. SEM micrographs indicate that microstructure is composed of the elongated and equiaxed β-Si3N4 grains, and some nano-scale TiN grains are enveloped into matrix grains.

For resolving the dispersing problem of whiskers and fabricating cutting tool materials with excellent properties, an in situ growth technology is used to directly synthesize TiC x N 1− x whiskers in α-Al 2 O 3 matrix by a carbothermal reduction process at a temperature range of 1250–1550 °C. The raw materials are consisted of TiO 2 , carbon, nickel, and NaCl. Various molar ratios from 1:3, 1:4, 1:5 to 1:7 of TiO 2 :C are experimentally used. For the molar ratio 1:3, a few whiskers can be found only at the synthesis temperature of 1550 °C. For the other molar ratios, large amount of whiskers can be observed at the whole synthesis temperature range. The highest yield of whisker is observed when the synthesis temperature is 1250 °C and the molar ratio of TiO 2 :C is 1:4. The compound AlO appears at 1250 °C and AlN instead at 1550 °C. The majority of the synthesized whiskers display an ideal aspect ratio of 10–30 with a diameter of 1–3 μm. No obviously influence on the whiskers growth by the present of α-Al 2 O 3 matrix powder can be noted.

An in situ growth TiC whiskers toughening Al2O3 ceramic matrix composite has been prepared based on a carbothermal reduction process by two separate steps such as in situ synthesizing of the TiC whiskers in alumina matrix powder and hot pressed sintering of the composite, respectively. The TiC content in the composite is 40% in terms of volume. The experimental results show that the composite is with a flexural strength of 780.9 MPa and a fracture toughness of 7.27 MPa m1/2 and such a preparation and fabrication process for in situ growth whiskers toughened ceramics can greatly improve the mechanical property of Al2O3 matrix composite.