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High-performance multifunctional materials for water splitting driven by low voltage are crucial for hydrogen evolution reaction (HER), but developing such materials is challenging. Herein, a simple strategy was designed to build a MoS2/Co9S8/MoC@CNT-N (MCM@CNT-N) heterostructure with a large number of interfaces. Regarding the HER, the synthesized MCM@CNT-N heterostructure catalyst showed high efficiency and stable electrocatalytic performance, with a low overpotential of 174.2 mV and a small Tafel slope of 84.7 mV dec−1 at a current density of 10 mA cm−2 in 0.5 M H2SO4. In addition to the function of heterojunctions, the excellent activity is also attributed to the introduction of Co and N atoms and the formation of carbon nanotubes. This work provides a new approach to build efficient and low-cost electrocatalysts for electrochemical reactions.

In this report, a pulsed electro-acoustic (PEA) device is modified to test the charge profile in epoxy resin at 77 K (-196°C) and successfully obtains the charge signal. A piezoelectric material suitable for cryogenic application is chosen for the measurement. In the experiment, the polarization and depolarization tests are operated with different voltage supply procedures. After initial wave recovery, the waveforms are discernable enough to identify the charge information. It is found that the charges are much less active at the low temperature with both slow dissipating rate and low injection quantity, comparing to the case at 293 K.

It still is a challenge to create a superior and easily coupled bifunctional electrocatalyst for water splitting impelled by a low voltage. In this work, the controlled growth of Co2P NAs on the surface of a MXene (Ti3C2Tx)-modified self-supporting electrode is demonstrated as a competent and reliable bifunctional electrocatalyst for efficient water splitting. The heterointerface in Co2P@Ti3C2Tx with an optimized adsorption free energy of H*, H2O, and better conductivity can give enhanced HER (hydrogen evolution reaction) activity, with a low overpotential (42 mV) at 10 mA cm-2. Additionally, the OER (oxygen evolution reaction) activity has also been similarly strengthened by the synergy of Co2P and MXene with an overpotential of 267 mV to arrive at 10 mA cm-2. Furthermore, the excellent bifunctional electrode (Co2P@Ti3C2Tx∥Co2P@Ti3C2Tx) exhibits efficient engineering water-splitting performance (1.46 V@10 mA cm-2) in alkaline solution. This simple design can propose a promising approach to exploit precious-metal-free catalysts for energy conversion.

The development of a system to measure simultaneously the current and space charge in a dielectric is described. The results obtained for low density polyethylene samples show that the relationship between space charge accumulation and the time-dependent current is not straightforward. In accord with previous work on space charge limited currents, the transient currents initially decreased, but after a while an increment or peak appeared. Additionally, the generation and extraction of space charge packets caused an impulse-like current peak such as previously observed. By comparing the electric field and current at the anode, it was found that the increment and the peak of the anode current are determined by injected negative carriers reaching the anode rather than the increase of electric field at the anode. It is shown that the dynamical evolution of the current can be divided into three stages similar to transient space charge limited current theory.

As an interface between the different insulating polymers exists in the joints of High Voltage Direct Current (HVDC) cables, the differences in the field-and-temperature dependence of the conductivities of the materials is regarded as a problem leading to interface charge accumulation and field distortion. In this report, using blocking materials to prevent charge injection from one or both sides of the electrodes, the contributions of charge injection and ionization in low-density polyethylene (LDPE), cross-linked polyethylene with nanoparticles (nano-XLPE) and ethylene propylene diene monomer (EPDM) are distinguished and evaluated based on the results from pulsed electro-acoustic (PEA) method. It is found that the contributions of charge injection and ionization behave differently in different materials. The mechanism behind the field-and-temperature dependences of the conductivities of the three materials is explained successfully. This study helps to clarify the charge transfer mechanism, and thus supports the quantitative calculation of the space charge and electric field distribution in cable joints.

Preparation of high-activity and earth-abundant bifunctional catalysts for efficient electrochemical water splitting are crucial and challenging. Herein, Co-doped Ni3N nanosheets loaded on nickel foam (Co-Ni3N) were synthesized. The as-prepared Co-Ni3N exhibits excellent catalytic activity toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline media. Density functional theory (DFT) calculation reveals that Co-Ni3N with redistribution of electrons not only can facilitate the HER kinetics but also can regulate intermediates adsorption energies for OER. Specifically, the Co-Ni3N exhibits high efficiency and stable catalytic activity, with an overpotential of only 30 and 270 mV at a current density of 10 mA cm-2 for the HER and OER in 1 M KOH, respectively. This work provides strong evidence to the merit of Co doping to improve the innate electrochemical performance in bifunctional catalysts, which might have a common impact in many similar metal-metal nitride electrocatalysts.

Residual charges produced in previous partial discharges (PDs) are considered as one aspect of memory effects that affect subsequent discharge behavior. In this paper, we review past studies about memory effects induced by residual charges during successive PDs. At first, surface charge dynamics for cavity and needle-to-dielectric discharges are presented. Subsequently, three methods used to unravel the memory effects are described, e.g. conditional distribution, pulse sequence analysis (PSA) and chopped PD (CPD). Then effects of residual charges on PD inception, discharge propagation and collective behavior are summarized. In addition, contributions of residual charges to the transitions between discharge modes and discharge patterns are presented. At last, some problems relevant to the memory effects are discussed, followed by suggestions for future work.

The effect of yttrium on the reaction of tungsten oxide with CO was studied for the preparing various morphologies and size of tungsten carbide. The reaction rate, phase composition, particles size and morphology, surface area of samples were studied by TG (thermogravimetry), XRD (X-ray diffraction), SEM (scanning electron microscopy) and BET (Brunauer–Emmett–Teller), respectively. It was found that the reaction route was following WO3 → W18O49 → WO2 → WC at lower temperature, while at higher temperature, an intermediate phase W was formed and then carbonized to WC. During the reaction of pure WO3 or mixtures (1 wt.% Y(NO3)3 + WO3) with CO at lower temperature, the final products almost maintained the large irregular block shape. With the increase of temperature, the large WC particles were degraded and the obtained sub-particles became much more dispersed. After the addition of yttrium nitrate, the rod-like sub-particles were transformed into more stable polyhedral-shaped sub-particles with a smaller size. Furthermore, the mechanism of effect of yttrium on prepared tungsten carbides were analyzed in detail.

In this research, the equivalence of space charge for coaxial cables and plane samples are analyzed through simulation. Bipolar charge transport models of coaxial cables and plane samples are established with the same material-dependent parameters, the bulk charge and total space charge distributions are simulated and comparatively researched under isothermal and temperature gradient conditions. The presented results indicate that the space charge behavior shows a basic equivalence in terms of charge variation laws and distribution. In addition, few discrepancies in the heterocharge amount and charge waveform amplitude are observed and mainly ascribed to the sample geometry, mobility gradient distribution and the spatial resolution of the measuring system, respectively. Investigations of space charge characteristics with a plane pulsed electro-acoustic measurement system may be accepted as a practice for providing the space charge characteristics of coaxial cable insulation, unless it is necessary to investigate the cable space charge distribution under actual operating conditions.

Epoxy resin is widely deployed as a high voltage electrical insulation material when compounded with inorganic fillers. However, in the laboratory, the filler prevents visual observation of the long-term degradation known as electrical treeing. To date therefore, much laboratory testing has been conducted on unrepresentative unfilled materials. Here, the impact of micro-sized fillers on the treeing phenomenon in an epoxy system has been explored. Sub-micrometre resolution 3D reconstructions of electrical trees are reported from X-ray computed tomography (XCT) using an advanced ‘pink beam’ synchrotron light source imaging system. The role of filler particles between 1 and 10 μm in size on tree channel propagation is reported. In highly filled materials (30% by weight) a radical change in tree growth behavior is seen, leading to bush tree rather than branch tree growth. The dielectric breakdown time at constant stress was also found to increase as the square root of the filler level. The change in geometry of tree growth may explain the extended life of filled materials in high voltage applications.

Electrical trees are widely grown in laboratories from metallic needle electrodes to enable the degradation process to proceed rapidly at voltages of ∼10 kV AC. Here it is shown that cone-shaped air gaps may form at a needle tip, even when they are not visible optically. Using X-ray imaging techniques, samples with and without air gaps up to 55 μm long have been characterized. The presence of air gaps led to partial discharge (PD) patterns resembling void-discharges. Prior to tree initiation, discharge characteristics evolved with increased voltage increments and increased phase lag behind the applied voltage. These changes to PD characteristics reflect a change in the physical state of the air gap. Although samples with air gaps generally initiated trees quicker than those with no gap, PD activities did not appear to play a decisive role in tree initiation times. However, initial voids were found to have major impact on the subsequent tree growth. In samples with persistent pre-tree air-gap PDs, trees grew larger branchy structures with higher PD magnitudes. In addition, the PDs became more asymmetric between positive and negative half-cycles than occurred in samples with no air gaps. It is shown that understanding the interface between the needle and polymer at a micron scale is critical to interpretation of laboratory tree growth experiments.

This paper reports observations of electrical tree growth by X-ray computed tomography (XCT) in multiple steps. Growth between times of observation has been obtained by subtraction of an image from the subsequent one. Partial discharge (PD) signals were also recorded during tree growth. It is determined that not all PDs support tree extension. By comparing the energy released by PD and the vaporization energy of newly grown tree channels, it is found that only the PDs that reach the tree tips can contribute to the growth of tree length. In such cases, the efficiency of energy conversion was very high, whereas PD resulting in increases in tree channel diameter had a much lower conversion efficiency. Nano XCT images reveal many finer branches along the trunk of the main tree and in the front of the main channel tree tips. It seems that those finer branches led to tree growth, but were not directly associated with local PD. The new imaging techniques have shown great value in resolving the relationship between partial discharge activity and electrical tree growth, providing a platform for deeper understanding of the tree growth process, and interpretation of partial discharge measurements.

The mobility of positive and negative charge carriers has been separately measured independently of carrier concentration in low-density polyethylene (LDPE) and its nanocomposites using space charge measurements in a bilayer system. It was found that the carrier mobility of positive charge carriers was larger than that of negative charge carriers for both LDPE and its nanocomposites and that the carrier mobility was one to two orders of magnitude less in the nanocomposite than unfilled LDPE under the same conditions. The mobility is interpreted in terms of a field-assisted hopping model whose estimated activation energies were verified by measurements at different temperatures. The form of the charge traps and carrier transport is interpreted in terms of known features and electron and hole traps and the effect of nano-particles upon these features is discussed.

It still remains a great challenge to develop alternative electrocatalysts with low cost, high efficiency and good stability for the hydrogen evolution reaction (HER). In this study, effectively conductive rGO (reduced graphene oxide) was used as the support both to promote charge transfer and to refine the particle size of WC, to realize efficient and stable HER performance. The facile in situ method can make highly dispersed WC nanoparticles firmly anchor onto the rGO substrate. Benefiting from its unique morphology and strong synergistic effect, the designed WC@rGO achieves a giant improvement in HER activity in both acidic and alkaline solutions compared with pristine WC. The overpotentials of WC@rGO are 2.82 and 2.30 times smaller than those of pristine WC to achieve 10 mA cm−2 in acid and alkaline solutions. More specifically, the Pt-modified WC electrocatalyst with negligible accumulation of Pt (4 wt%) even exhibits superior electrocatalytic performance (η10 and Tafel slope of 54 mV and 21 mV dec−1 in acid solution and 61 mV and 28 mV dec−1 in alkaline solution) to commercial 10 wt% Pt@C. The method reported in this study enables eco-friendly preparation of cheap, stable and effective HER electrodes.

Tungsten, one of the refractory metals, has lots of excellent properties, making tungsten powders become a choice of raw material for many applications. This work reported a simple method (the salt-assisted hydrogen reduction of WO3 powders) for preparing tungsten powders with desired morphologies and size. By adjusting the amount of additive (Y(NO3)3) and reduction temperature, metallic tungsten particles with different morphologies and size were obtained. It was found that both temperature and additive had apparent effects on the morphology and size of metallic W particles, and the large parallelepiped-shaped powders disappeared and large irregular blocky-shaped powders were degraded under the influence of additive. With the increase of temperature and amount of salt, particles became more dispersed and the size increased at higher temperatures, and the shape of particles was changed from spherical to polyhedral as well. On the contrary, the size of sub-particles became smaller as increasing amount of Y(NO3)3 at lower temperatures. Based on the results, the reaction and refining mechanisms for preparing tungsten were proposed as well.

Density functional theory (DFT) calculation indicators (ΔG, densities of state, D-band and bader charge) are commonly used to predict and analyze the hydrogen evolution reaction (HER) activity of catalysts, and most studies discuss only one or few of these indicators’ impact on catalysis, but still no report has comprehensively evaluated the influence of all these indicators on catalytic performance. Herein, foreseen by comprehensive consideration first, we report transition metal doped Ni3N nanosheets combined on Ni foam for utra-efficient alkaline hydrogen evolution. For dual transition metals doped Ni3N, Co,V-Ni3N exhibits remarkable HER performance with a significantly low overpotential of only 10 mV in alkaline electrolyte and 41 mV in alkaline seawater electrolyte at 10 mA cm− 2; while for single transition metal doped Ni3N, V-Ni3N exhibits the best performance with an overpotential of 15 mV and a Tafel slope of 37 mV dec− 1. Our work highlights the importance of comprehensive evaluation of DFT calculation indexes, and opens up a new method for the rational design of efficient and low-cost catalysts.

In this investigation, the Li2CO3-assisted hydrogen reduction method was proposed to produce different morphologies and size of metallic tungsten particles. The effects of temperature and content of Li2CO3 additive were both considered. The experimental results showed that both the temperature and additive had great effects on the morphology and size of produced tungsten particles. As the increases of temperature and fraction of Li2CO3, the morphology of tungsten particles was changed from spherical to polyhedral; while the size of these particles was increased; and the obtained products became more and more dispersed. The addition of Li2CO3 also affected the reduction rate dramatically. It was found that the Li2CO3 additive retarded the initial reduction reaction, but it could obviously improve the later reduction (WO2 to W) at a high temperature, furthermore, the higher fraction of Li2CO3 made this promotion more remarkable. Based on above results, the reduction mechanism was proposed: without additive, the reduction entirely obeys pseudomorphic transformation mechanism at low temperature, and obeys CVT mechanism at high temperature; with additive, the reduction follows a mechanism of mixture of nucleation, pseudomorphic transformation and CVT at low temperature, while at high temperature, the sublimation of WO3, nucleation and CVT mechanisms all play important roles. Keywords: Li2CO3, Nucleation, Reduction, Morphology and size, Tungsten

The effect of DC bias on electrical tree growth and partial discharge (PD) characteristics in epoxy resin (Araldite LY ® 5052/Aradur® HY 5052 supplied by Huntsman) is investigated using three waveforms: AC, AC with positive DC bias, and AC with negative DC bias. Needle-plane samples are used. Tree growth is shown to be accelerated by the combined effect of DC and 50 Hz AC, beyond the AC growth rate. Positive and negative DC biased tests result in 62% and 54% reductions in average time to breakdown, respectively. Different tree structures and stages of development are associated with different partial discharge characteristics, with thick dark tree branches associated with high PD magnitudes, whereas fine tree channel growth is linked with PD magnitudes below 1 pC. AC tests showed five distinct stages of tree growth compared to four stages seen in DC biased tests. In particular, trees growing in the ‘reverse direction’ from the planar to the point electrode, which is observed in the latter stages of AC tests, is not seen in the DC biased tests. It is concluded that for composite voltages, the AC component is the essential driver of tree growth but the DC component can accelerate propagation. AC noise may therefore compromise the reliability of insulation in HVDC networks.

The reduction of metal oxides by hydrogen is a very significant reaction in metallurgical industries. Particularly, hydrogen reduction is one of the effective methods to reduce CO2 emission in iron and steel industry. The mathematical modeling of H2 reduction process would help to define the operating parameters and to get a better control of the reduction process. In this study, a new kinetic model regarding hydrogen reduction of metal oxides controlled by external gas diffusion has been proposed. The model has incorporated numbers of parameters such as temperature, size of the metal oxides, diffusion coefficient constant, hydrogen partial pressure, density of metal oxides and time. Furthermore, the shape of metal oxides (spherical, cylindrical, lamellar particles) was considered as well, and the models for the reduction of metal oxides with different shapes were combined to a formula. Also, the developed model can be applied to describe the reduction kinetics under both isothermal and non-isothermal conditions. Because the model is in the analytic function, therefore, it is convenient for usage and theoretical analysis. Application of the developed model to practical systems validated that its predicted values agreed very well with the experimental data.

Electric tree growth is a key ageing mechanism leading to breakdown of high voltage electrical insulation. Partial discharges (PDs) are invariably associated with electrical tree inception and propagation. In turn, the physical structure of an electrical tree influences the characteristics of partial discharge activity. Interpretation of PD patterns is therefore central to developing an understanding of the tree propagation process, and also to the use of PD patterns as an asset management tool. Our previous research indicates that the phase resolved PD (PRPD) patterns and pulse sequence analysis (PSA) patterns evolve with tree propagation. A method was proposed to estimate the point-on-wave inception and extinction voltages of PDs in tree channels within each power cycle. It was shown that the evolution of PD patterns is a consequence of changes to PD inception and extinction voltages as a tree develops. This paper provides a deterministic model of partial discharge in tree channels. Simulations of PDs in a straight non-conductive tree channel are based on experimental PD inception, extinction and residual voltages. The quantitative simulations reproduce almost all the characteristics of observed PRPD and PSA patterns. It is concluded that PD events are determined by five key parameters: tree structure, applied voltage, PD inception voltage, PD extinction voltage and PD residual voltage. Key parameters estimated by the method, and the models proposed explain PD activity in non-conductive trees. It is suggested that the PSA and PRPD patterns should be discussed together to fully understand the PD events. This model forms a platform for generating robust information for asset managers using PD measurements from high voltage equipment in service.

In the present study, a new and simple method to prepare tungsten and tungsten carbides powders was proposed by using WO3 powders as the raw material and CH3OH vapor as the reduction and carbonization agent. Both thermodynamic and experimental studies were carried out, and based on the results, the reaction routes were obtained. It was found that at low and moderate temperatures, the final product of WC was formed via intermediates of W18O49, WO2, partial metallic W and W2C; while at high temperature, WO3 was first reduced to WO2 with the intermediate phase of W18O49, and then reduced to W. The morphology of final products almost retained the same shape as that of raw materials WO3 at low temperature, however, the large particles with blocky shape disappeared and the products were all small W particles with polyhedral shape at high temperature. Furthermore, the reaction mechanisms were proposed: the pseudomorphic transformation mechanism dominated the reaction at low temperature, but at high temperature, the reduction obeyed the chemical vapor transport mechanism, which is beneficial for grain refining of final product.

Space charge accumulation is an important issue for HVDC cable design. The space charge accumulation are determined by the charge transport property which can be monitored by the space charge and conduction current measurement. In this paper, based on PEA system and three-electrode system, the modified combined measurement system was developed to measure the space charge and current in LDPE samples simultaneously. The results show that the current gradually decreased with the accumulation of homocharges; the generation and movement of space charge packet resulted in the current peaks; the reduction of homocharge accumulation and the increase of heterocharge tends to increase the current.

EPDM, as a high-quality insulation material, is widely used in cable joints, but it bears a large compression force in the working environment, which may affect its insulation performance. We study the effect of pressure on charge transport in EPDM by calculating the change in the key parameter of carrier mobility in EPDM at different pressures. In this paper, by designing a unipolar carrier blocking experiment and combining improved PEA measurement equipment, the mobility of electrons in EPDM at six pressures of 0.2 MPa-1.2 MPa (interval 0.2 MPa) is calculated. In addition, the mobility data is analyzed in-depth through the hopping conduction model. It was found that the mobility in EPDM at different pressures did not change significantly, and the electronic transition distance and energy required for hopping after fitting the hopping conduction model also did not change much under different pressures. It is indicated that under normal working conditions, pressures will not affect the electrical performance of EPDM.

The recycling of high-titanium-bearing blast furnace slag (TiO2 > 23 wt.%) is an urgent problem that has attracted global research attention. To achieve high-efficiency, low-consumption, clean, and high value-added recycling utilization, a new process is proposed herein, which entails reacting a CH4-H2-N2 gas mixture with the titanium-bearing blast furnace slag to initially produce Ti(C, N, O) at a low temperature, whereafter the product is purified and chlorinated. The effects of Fe2O3, urea, and sawdust on the reduction and carbonitriding characteristics are also investigated. The results indicate that the Fe2O3 additive can promote the formation of Ti(C, N, O), while urea and sawdust are instrumental for enhancing gas diffusion in solid powders. The proposed novel recycling process was assessed and it evinced many advantages and great feasibility.

Methane reduction has attracted substantial interest in recent years because an abundant amount of natural gas has been found and methane possesses a strong reduction ability. However, due to the complexity of the reaction, the reductive-kinetics model has not been developed very well. This work reported a new mathematical model for methane reduction. The model is in a form of explicit functions incorporating many parameters to increase its precision. Particularly, it considers the comparison of methane cracking rate and reaction rate. Both the gas diffusion in the product layer and chemical reaction controlled-kinetics formulae were deduced by considering three kinds of shapes (spherical, cylindrical and lamellar) of particles. Also, by employing two parameters (shape coefficient Sc and equivalent diameter d0), the formulae for the same reduction mechanism could be unified to one formula, which was easier to use. The simulation of the model also considers both isothermal and non-isothermal processes of methane reduction. Furthermore, it can describe the reduction of oxides of varied-valence metals. The kinetics of reduction of metal oxides by methane agrees with the results obtained in the practical system.

Tungsten carbides, which can be widely used as cutting and drilling tools, chemical catalysts and aerospace coatings, have attracted widespread attentions. The present work reported a simple method to prepare tungsten carbides by using WO2 as the raw materials and CO as the reduction and carbonization agent. It was found that at lower and moderate temperatures, all the final product was WC, while at higher temperatures, the final products were W2C or W. The reduction and carbonization mechanism was also studied theoretically and experimentally. WO2 was reduced and carburized to WC in one step at lower temperatures; WO2 was first reduced and carburized to W2C and then to WC at moderate temperatures; while at higher temperatures, WO2 was first reduced to metallic W and then carbonized to WC or W2C, but with the temperature increasing, the carbonization became difficult and required much higher CO content.

Molybdenum hemicarbide (Mo2C) can be widely used in the fields of steel, metal ceramics, and catalyst of hydrogen-involved reactions. This work reported a new and simple method for preparing molybdenum hemicarbide by reducing and carbonizing MoO2 with methanol, and both thermodynamic and experimental studies were carried out. Based on the results, the reduction and carbonization routes are obtained: MoO2 was reduced and carburized directly to Mo2C in one step at lower temperatures, and first reduced to metallic Mo and then carburized to Mo2C at higher temperatures. The reduction mechanisms at low and high temperatures were proposed as well: The pseudomorphic transformation mechanism dominated the reaction at low temperatures, but at high temperatures, the reaction obeyed the chemical vapor transport mechanism.

The effect of temperature gradient on space charge behavior is necessary to investigate for HVDC insulation. In this paper, space charge distributions in neat epoxy resin (EP) and EP/SiO 2 nanocomposites (NC) were measured under different DC stresses and temperature gradients. We found that different temperature conditions applied to the electrodes had a great impact on space charge distributions: Only homocharge accumulated near anode at isothermal conditions, on the contrast, at temperature gradient of 60 °C, negative charge injected from cathode (high temperature side) and accumulated in the bulk, heterocharge appeared near anode (low temperature side). Moreover, SiO 2 nano-fillers added to NC could suppress the space charge accumulation significantly, and the pattern of space charge distribution in NC at temperature gradient of 60 °C also shows differently from that of EP. Numerical simulation based on the bipolar charge transport model was employed to study the experimental results. It shows that under temperature gradient, charge extraction plays an important role in heterocharge accumulation near the low temperature side. Moreover, it indicates that unlike the apparently measured conductivity, the charge mobility of NC does not increase rapidly with temperature in the range from 20 °C to 80 °C.

XLPE and its nanocomposites have been subjected to a progressive step voltage until failure occurred, and the results analyzed in terms of an inverse power law ageing expression. The step-stress tests have been conducted at three temperature 20°C,40°C and 60°C, and the exponent n of the inverse power law ageing expression has been estimated for the two materials by a new method, which fully considers the contribution to the failure of the whole sequence of voltage steps, including ageing processes at the lower voltages, the final step and the process of voltage increase. It was found that the value of exponent n increases slightly with increasing temperature for both XLPE and its nano-composites. More importantly the value of n for the nano-composites was found to be substantially lower than that for XLPE at all temperatures. A complementary experiment in which the space charge distribution was measured under the same step-stress protocol as the breakdown tests has also been carried out and the results of both experiments are discussed.

The impact of air gaps between a metallic needle tip and the polymer on electrical tree growth has been studied in low density polyethylene (LDPE). Gaps of $115\ \boldsymbol{\mu} \mathbf{m},\ 212\ \boldsymbol{\mu} \mathbf{m}$ and $1037 \boldsymbol{\mu} \mathbf{m}$ have been experimentally investigated. Corona discharges of different types were measured within those air gaps. Accelerated treeing to breakdown was found in the sample with a $115\ \boldsymbol{\mu} \mathbf{m}$ air gap. In this case the streamer coronas were slightly higher in magnitude than the maximum partial discharge (PD) during the tree growth in the no-gap sample. In contrast, lower corona discharge magnitudes were associated with larger air gaps, and electrical trees gradually stopped propagating to the counter electrodes and developed into pine-branch tree structures. This work highlights the differences between electrical trees that effectively grew from a metallic electrode in direct contact with the polymer and a point of corona source.

Partial discharge (PD) characteristics are inherently linked to associated electrical tree growth characteristics. This paper employs both phase-resolved patterns and pulse-sequence analysis (PSA) to study PDs in a short branch-tree grown from a metallic needle tip. It is found that the shape of PSA dV-dV patterns vary with applied voltage; in particular the values of characteristic voltage difference increase with applied voltage. At 10 kV the value of characteristic voltage difference decreases with tree growth, the average number of PDs per cycle increases, the PD magnitudes decrease, and the characteristic wing-like phase-resolved pattern evolves into a turtle-like pattern. This paper proposes a physical model relating PD activity to tree development, considering space charge accumulation inside the tree channel. All key features of experimental results are explained by the model. It is shown that the characteristic voltage differences are determined by the local PD inception and extinction electric fields. These key parameters determining the PD events can be calculated giving a clear physical interpretation of PD measurements, which is invaluable to those using PD as a condition monitoring or asset management tool.

Titanium oxycarbide can be used as a soluble anode to produce sponge titanium in USTB or MER molten salt electrolysis process. However, the key factor of these two processes is to prepare oxycarbide with appropriate C/O ratio and at a low cost as well. This work reported a simple method of preparing titanium oxycarbide from TiO2 in C-H2 system. Based on the prediction of thermodynamic calculations, titanium oxycarbide was prepared successfully at 1400 °C with the molar ratio of TiO2 to carbon exceeding 1:2, and the ideal electrolytic anode (TiC0.5O0.5) was produced at 1400 °C (with a ratio of 1:3 of TiO2 to carbon). Number of parameters (carbon content, temperature, sample's mass, additive (Fe element), and with ball milling treatment) were considered and discussed in the current study. Furthermore, this method can reduce not only the reaction temperature and time, but also the emission of carbon dioxide.

The effects of charge carrier injection and intrinsic ionic carriers on the conduction process in superclean low-density polyethylene and cross-linked polyethylene (LDPE and XLPE) and the associated development of space charge are investigated. The carrier mobility is measured directly by space charge measurements in a double- and triple-layer structure. It is shown that the difference between superclean LDPE and XLPE in the field-dependence of the current density results from the dominance of ions in XLPE introduced by cross-linking as compared to injected carriers in the case of superclean LDPE.

The present study investigated the reduction and carbonization behavior of perovskite-geikielite by using CH4–H2 gas mixture, which aimed to recycle the titanium resource in titanium-bearing blast furnace slag. Thermodynamic phase equilibrium analysis in the CaTiO3–CH4–H2, MgTiO3–CH4–H2 and CaTiO3–MgTiO3–CH4–H2 systems was performed by FactSage. The analysis indicated that CaTiO3 could be ultimately reduced and carbonized to TiCxOy by CH4–H2 gas mixture at temperature above 1300 °C, while the temperature for reduction of MgTiO3 was above 1200 °C. MgTiO3 is found to be reduced prior to CaTiO3 in the CaTiO3–MgTiO3–CH4–H2 systems. The reduction experiments of perovskite-geikielite were carried out in a temperature range of 1300 °C to 1450 °C in a flowing CH4–H2 atmosphere. Experimental results showed that perovskite-geikielite could be reduced completely to TiCxOy at 1400 °C and above after 8 h reduction, while geikielite was reduced prior to perovskite. The reduction and carbonization extent of samples increased with increasing time and temperature when in a low temperature range (below 1400 °C). However, a much higher temperature (above 1400 °C) would hinder further carbonization of TiCxOy for the production of deposited carbon. Addition of iron oxides to sample strongly facilitated the reduction reaction, and even promoted the reduction of titanium to be completed at only 1300 °C after 8 h reduction. The gas-solid reaction provided a low temperature method to reduce and carbonize perovskite and geikielite, and thus it provided a possible way to use titanium-bearing blast furnace slag.

Electrical tree growth through polymeric insulation is a key degradation process in HV cables. Partial discharges (PDs) are considered to be the energy conversion path for the tree growth. Several methods have been employed to study PDs in electrical tree channels, including the phase resolved PD (PRPD) pattern and pulse sequence analysis (PSA). Our recent work proposed a new method of estimating the PD inception and extinction voltages in non-conductive tree channel based on the PD signals. Here a typical non-conductive tree in polyethylene is considered. Both PRPD and PSA have been employed. It is found that after the PD inception, the PD inception and extinction voltages decrease with the tree growth, and become stable before breakdown. It is also found that the decrease of PD inception and extinction voltages causes an increase in the average number of PDs per cycle, and the shrinking of the dV-dV PSA plot. In turn, the average number of PDs per cycle, and the dV-dV plot can be used to monitor the change of PD inception and extinction voltage during the tree growth.

Tungsten carbides, which can be widely used for cutting and drilling tools, chemical catalyst and aerospace coatings, have attracted widespread attentions. However, the cost of conventional processes to produce tungsten carbides can be very high, therefore, there is a permanent effort to synthesize WC powder at low temperature to minimize the production cost. Some novel processing techniques have been developed to partially solve this problem. This paper reviewed the current research trends in preparation of WC containing spark plasma sintering, combustion synthesis, sol-gel and in situ carburization method, chemical vapor reaction synthesis and the spray conversion process, etc. The present review also discussed the potential applications, the feasibility, the advantages and disadvantages of industrialization.

The movement of space charge packets has been reported in polyethylene and epoxy resin under high electric fields by several groups, and has been characterized by the speed of propagation. A number of explanations of the formation and transport of slow space charge packets has been proposed. However, the mechanisms creating fast space charge packets are not clear. Dissado et al have suggested that the formation of fast charge packets is due to discontinuous electromechanical compression and charge injection at the electrode-insulation interface, and the transport of charge packets is related to corresponding relaxation processes. In that model, charges travel in packets because of group polarization. This paper gives an alternative model considering the reduction of charge carrier activation energy due to charge density triggered polymer chain movement and subsequent chain relaxation times. It is shown that the generation and transport of fast charge pulses are readily simulated by a bipolar charge transport model with three additional parameters: reduced activation energy, a charge density threshold, and a polymer chain relaxation time.

Electrical trees are degraded paths in polymeric insulation and are the mechanism of electrical failure of high voltage insulation systems. Previous studies have confirmed the application of X-ray Computed Tomography (XCT) imaging of electrical trees using phase contrast enhancement. This work evaluates the feasibility of X-ray imaging of electrical trees to measure growth at several stages of growth in the same treeing sample. The impact of x-ray dose on the epoxy resin studied is determined experimentally. An example of multi-stage tree imaging using laboratory micro-XCT and reconstructed renderings is provided. Based on the three-dimensional tree models, characteristics such as the degraded volume and surface area can be quantified. Furthermore, a comparison between two stages is presented to show the growth of the electrical tree in a fixed period of time. Insight into the treeing characteristics is also discussed.

As the temperature gradient always exists in the insulation of power equipment, the influence of temperature gradient on oil-impregnated paper insulation is studied by experiments and simulation. It's found that in the both single-layer and double-layer structures, the heterocharges always appear near the lower temperature electrode, and increase with the enhancement of temperature gradient and field stress, resulting in more considerable field distortion. However, there are more heterocharges in the single-layer samples while the field distortion is more severe in double-layer samples. Numerical simulations were carried out to investigate the mechanism of temperature gradient effect. It is indicated that the space charge accumulation is mainly due to the temperature dependence of the charge injection and conduction. The behavior of interface charges can be well described by assuming an energy barrier for charge transportation at the interface.

Two mechanisms have been proposed to explain the suppression of space charge in polyethylene by the addition of nano-fillers, i.e. an interface change that reduces charge injection and a bulk modification that affects charge migration and recombination. The relative importance of each mechanism in Low Density Polyethylene (LDPE) nanocomposites is investigated by the measurement of space charge in samples of double-layer and triple-layer structures. The experimental results show that the nano-fillers reduce charge injection, but do not entirely eliminate the space charge. Bulk changes also play an important role in space charge suppression. The space charge near the interface between the unfilled LDPE and nanocomposites shows a surface charge at 20 °C, but at 60 °C a bipolar blocking phenomenon is observed in some circumstances. It is shown that these space charge distributions can be reproduced using a band theory approach with the assumption that the introduction of deep traps by nano-fillers raises the Fermi level of the nanocomposite towards the conduction level.

The effect on space charge accumulation of the introduction of nano-filler into polyethylene is usually studied using thin 100 μm samples in electric fields reaching 100 kV/mm. However, the insulation thickness in an actual HVDC cable is usually some centimeters. Thus there is a great thickness difference between the experiment sample and the insulation application. Here we investigate the thickness dependence of space charge accumulation in unfilled and LDPE doped with nano-SiO2 over a range of temperatures in a field of 50 kV/mm. It is found that the space charge in LDPE/SiO2 nanocomposites exhibits marked thickness dependence whereas in the unfilled LDPE it shows very little thickness dependence. A simulation based on the bipolar injection model and Takada's trapping potential model has been carried out in order to explain the differences. It is found that when the simulation included the effects of charge injection, extraction, trapping, recombination, mobility and diffusion, it is able to explain the heterocharge formation in the unfilled LDPE, and the thickness dependence in the two kinds of materials.

In order to restrict the field distortion in the insulation of high voltage direct current (HVDC) cable under temperature gradient, this paper proposes a nano-filler, the addition of which can restrict the space charge accumulation of low density polyethylene (LDPE) under temperature gradient. To reveal the mechanism of the nannocomposite, the trap depth distribution of the pure LDPE and LDPE nannocomposites under different temperatures were tested based on the Pulsed Electro-Acoustic (PEA) method. The parameters of trap depth distribution were achieved by PEA method under different temperatures(20°C,40°C,60°C and 80°C), and the trap depth distribution under different temperatures were all calibrated to that of 20°C. The results indicated that the trap depths of LDPE decrease with increasing temperature, whereas the trap depths of LDPE nanocomposites changes inobviously with temperature variation. It is considered that, the result contributes to the restriction of space charge accumulation under temperature gradient.

Simultaneous imaging of electrical tree growth and partial discharge measurement have allowed 5 stages of tree growth to be identified. Partial discharge (PD) characteristics have been collected from tree growth in needle-plane epoxy resin samples under 15 kV peak AC voltage, and have been shown to behave differently at different stages of tree growth. Initial slow growth is accompanied by large discharges (about 10 pC). Thereafter, growth of fine channels is associated with smaller PDs (about 0.3 pC). It was observed that the tree propagation is further driven by even smaller PDs below the equipment sensitivity. However, when the tree approaches the counter electrode, the PD magnitude increases again and a more substantial tree grows with a low propagation rate. Reverse tree channels then occur from the planar electrode towards the needle, together with higher magnitude PDs characterized by thick and dark branches before breakdown.

The aging parameter n can reflect the aging property of material to some degree and determine the test voltage of the type test and prequalification test of HVDC cables. However, the aging parameter has rarely been studied systematically, especially under temperature gradient. In this paper, step-stress test was carried out to estimate the ageing parameter n of two materials, XLPE and its nanocomposites under temperature gradient. A new method based on the cumulative damage has been used to obtain the ageing parameters, which can fully take the consideration of the voltage procedure and avoid the errors brought by the approximation. Aging parameters of XLPE and its nanocomposite under temperature gradient ΔT =0, 20, 40°C are 13, 15.4, 17.7 and 8.6, 9.4, 10, respectively. The aging parameter n reduces when nano-fillers are introduced into XLPE. As XLPE nonocomposites can depress space charge availably, the larger aging parameter n of XLPE may be explained in terms of space charge effect and thus more severe field distortion at higher DC voltage. It is also suggested that the electrostatic force induced by space charge can contribute to material aging.

Space charge will accumulate within polyethylene when stressed under DC high voltage, and a temperature gradient will form throughout the insulation when a HVDC cable is in service. Previous research has revealed that temperature gradient can enhance hetero charge accumulation at the low temperature end, but the high temperature only helps a small quantity of homo charge accumulate, and the mechanism was explained for hetero charge formation due to ionization of impurities within the insulation and blocked extraction of injected carriers. However there is no direct evidence to distinguish whether the hetero charge is injected carriers or the ions. This paper intends to investigate and explain the influence of temperature gradient onto hetero charge formation. It was concluded that temperature gradient across the sample thickness could affect the space charge through its influence to the mobility of carriers, and the temperature difference between two electrodes could also influence the entire space charge distribution through its influence to the charge injection and extraction. The space charge distribution in cross-linked polyethylene (XLPE) samples with different thickness under different temperature differences and temperature gradient was investigated. The results show that both the temperature difference between the electrodes and the temperature gradient across the thickness can enhance the hetero charge accumulation.

have not yet been clarified. The experimental results for PE under a temperature gradient show clear evidence of heterocharge next to the electrodes. This feature has been simulated previously using a bipolar injection model that includes blocked extraction. A direct comparison with PEA experiments however is difficult because the finite spatial resolution of the measurement causes the bulk space charge to overlap with that of the electrodes leaving the experimental values as a mixture of the two quantities that are generated separately in the simulation. Here we allow for the finite spatial resolution of the experimental technique by broadening the simulated charge density values using a Gaussian function. This allows us to compare the simulation and the measurements more effectively. The space charge distribution in PE is simulated by including a temperature dependent charge injection and charge transportation. Under these conditions it is found that the simulation results give a detailed match to the experimental results. They also show that both the temperature dependent charge injection and temperature dependent charge transportation are of importance in determining the space charge distribution in PE under a temperature gradient.

Polyethylene (PE) and oil impregnated paper are widely used as insulation materials in high voltage power equipment however, at high voltages the space charge inside the insulating material can distort the electric field distribution and thus greatly influence aging and degradation. Moreover, there is usually a temperature gradient across the insulation when the equipment is in service, and this can also influence the space charge accumulation. In order to study the effect of temperature gradient systematically a PEA system suitable for measurement under a temperature gradient has been developed and appropriate corrections for the measurement error introduced by the temperature gradient implemented. Separate determinations of the space charge distributions of PE and oil-impregnated paper at different temperatures and voltages have been made. The results for both materials showed some similar characteristics. Obvious heterocharge accumulates at the low temperature side and the higher the temperature difference between the two electrodes or the higher the applied voltage, the more the heterocharge accumulates. As a result the electric field is greatly distorted and the maximum field always appears at the low temperature side. The effects of temperature gradient on the space charge distribution are discussed in detail, and the differences between the two materials are identified. The conductivity of both materials has also been investigated, and the space charge induced by the conductivity gradient produced by a temperature gradient in both materials has also been evaluated. The results show that in oil-impregnated paper the space charge induced by a conductivity gradient plays a great part in space charge formation, while in LDPE the conductivity gradient contributes much less to the space charge. These differences are discussed.