碳化硅纳米线的电子发射特性

以聚碳硅烷为原料,通过1 200 ℃高温裂解工艺制备了碳化硅纳米线,并采用碳化硅纳米线作为高功率微波源用阴极材料,进行了电子发射实验。结果表明：与天鹅绒阴极材料相比,碳化硅纳米线具有更高的电子发射电流密度,在115 kV外加激励脉冲高压下,电子发射密度为23.7 kA/cm2,而天鹅绒材料为14.0 kA/cm2,并具有更好的电子发射品质及更长的使用寿命。因此碳化硅纳米线作为高功率微波源用阴极,具有很好的应用潜力。     

On the role of vacancies in pore formation in the course of anodizing of silicon carbide

Experimental data on the preparation of stoichiometric nanoporous silicon carbide are analyzed. Theoretical calculations are performed under the assumption that nanopores are formed through the vacancy diffusion mechanism. The results obtained confirm the hypothesis that the formation of pores with a steadystate radius of several tens of nanometers in silicon carbide can be associated with the diffusion and clustering of vacancies. The experimental data indicating that the proposed mechanism of formation of nanoporous silicon carbide correlates with the existing model of formation of porous silicon carbide with a fiber structure are discussed. This correlation can be revealed by assuming that nanopores are formed at the first stage with subsequent transformation of the nanoporous structure into a fiber structure due to the dissolution of the material in an electrolyte.     

Microscopical study of the formation of adiabatic shear bands in 4340 steel during dynamic loading

In this study, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electron probe microanalyser were used to analyse the changes in microstructure of AISI 4340 steel specimens caused by impact at high strain rates and large strains. The structures of the steel prior to dynamic deformation and after dynamic deformation were examined to understand on a microscale level, the mechanism of formation of adiabatic shear bands (ASBs). The study also includes the structural changes that occur during post-deformation annealing processes which may relate to understanding of the mechanism of formation of ASBs. Prior to deformation, the tempered steel specimens consisted of lenticular laths of α-ferrite with precipitated platelet and spherical M₃C carbides. After impact, the structure inside the shear band was characterized by refined and recrystallized grains immersed in dense dislocation structures. In addition, residual carbide particles were observed inside the shear bands due to deformation induced carbide dissolution. Regions away from the shear bands developed ‘knitted’ dislocation walls, evolving gradually into sub-boundaries and highly misoriented grain boundaries at increasing strains, leading to grain refinement of the ferrite. After impact, annealing the shear bands at 350?°C resulted in an increase in hardness regardless of the heat treatment before impact, amount of deformation and the time of annealing. This is because of the occurrence of extensive reprecipitation of dissolved carbides that existed in the steel structure prior to deformation. It is concluded that dynamic recovery/recrystallization, development of dislocation structures and carbide dissolution all contribute simultaneously to the formation of ASBs in quench-hardened steels.     

Production of hard metal-like wear protection coatings by CO2 laser cladding

Protective coatings with hard metal-like wear properties could be obtained by laser beam surfacing of powder mixtures consisting of coarse-grained tungsten carbide and a nickel or cobalt hard alloy. The microstructure of the clad composite layers was investigated by metallography and SEM. The four coating systems produced were found to differ strongly in the degree of partial carbide dissolution and the formation of new hard phases. This also influences the hardness and wear behaviour.     

Spinodal decomposition of tungsten-containing phases in functional coatings obtained via high-energy implantation processes

We have studied structural and phase transformations in tungsten-containing functional coatings of carbon steels obtained during the high-energy processes of implanting tungsten carbide micropowders by the method of complex pulse electromechanical processing and micropowders of tungsten by technology of directed energy of explosion based on the effect of superdeep penetration of solid particles (Usherenko effect). It has been shown that, during thermomechanical action, intensive steel austenization occurs in the deformation zone with the dissolution of tungsten carbide powder, the carbidization of tungsten powder, and the subsequent formation of composite gradient structures as a result of the decay of supercooled austenite supersaturated by tungsten according to the diffusion mechanism and the mechanism of spinodal decomposition. Separate zones of tungsten-containing phases of the alloy are in the liquid-phase state, as well as undergo spinodal decomposition with the formation of highly disperse carbide phases of globular morphology.     

Mössbauer study of Fe powder mechanically alloyed by power ultrasonics

The Mössbauer and transmission electron microscopy (TEM) analysis of Fe-powder and Fe₄₆C₅₄-powder blend, mechanically milled by high power ultrasonics (USM) in He environment for 20–75 hours, have been carried out. As shown, the USM results in effective grinding of initial polycrystalline iron particles up to formation of single crystalline state, dissolution of carbon in iron particles, synthesis of carbides and possibly penetration of Fe atoms into graphite. Annealing of processed Fe₄₆C₅₄ powder causes carbide reaction.     

Diffusion phenomena on rubbing surfaces in relation to wear resistance

Conclusions When steel parts are working under heavy-duty conditions, there is always a possibility of the formation of carbides on their rubbing surfaces as a result of decomposition of lubricants and subsequent reaction of carbon with the metal. This process is inevitably associated with local seizing and mutual material transfer. As a result, the carbide phase formed during friction betwen two steels of a different composition may differ from the adjacent material in respect to its content of carbide-forming elements. When strong carbide-forming elements are present in a steel with a friction-induced surface carbide layer, heating such a steel to relatively low temperatures leads to localized decarburization which, under certain circumstances, may reduce the wear resistance of steel. In a simple case of a binary alloy, the rate of decarburization depends on the affinity of the alloying element to carbon.When this affinity in high, the decarburization rate may be slow because of a reduced intensity of the diffusion flux (due to reduced solubility and diffusion coefficients, and because of carbide formation in the contact zone), while elements with a lower affinity to carbon produce a more intense decarburization.The above considerations do not apply to high-alloy steels in which the extent of carbide-forming elements is so high that practically all the carbon diffusing to the contact interface combines with these elements to form carbides. In this case the reduction in the coefficient of diffusion of carbon in steel does not play any substantial part because its diffusion path is relatively short. It is to be expected that in this case the decarburization rate will increase with increasing tendency of the alloying elements to carbide formation.It should be pointed out that the dissolution of surface carbide layers takes place also in cases when the carbide layers are produced as a result of friction between similar alloy steels because a large proportion of the carbide-forming elements is dissolved in ferrite (i.e., is not combined in carbides), while the carbide layers contain large quantities of cementite. This means that the decarburization rate is determined in the first place by the chemical composition of the steel and not of the carbide layer.Facts reported in this article will assist, when necessary, in a rational selection of the friction pair components with a view to reducing or eliminating the decarburization of the surface layers. Needless to say, experimental studies (using the above described methods) will have to be carried out in each specific case. It is also evident that data of this kind are not sufficient to make recommendations about selecting materials for any given friction pair, since all the other factors determining the wear resistance must be taken into account.     

A review of recent developments in the surface modification of LiMn2O4 as cathode material of power lithium-ion battery

LiMn₂O₄ (LMO) is a very attractive choice as cathode material for power lithium-ion batteries due to its economical and environmental advantages. However, LiMn₂O₄ in the 4-V region suffers from a poor cycling behavior. Recent research results confirm that modification by coating is an important method to achieve improved electrochemical performance of LMO, and the latest progress was reviewed in the paper. The surface treatment of LMO by coating oxides and nonoxide systems could decrease the surface area to retard the side reactions between the electrode and electrolyte and to further diminish the Mn dissolution during cycling test. At present, LiMn₂O₄ is the mainstreaming cathode material of power lithium-ion battery, and, especially the modified LMO, is the trend of development of power lithium-ion battery cathode material in the long term.     

Formation of NaCl-type Cr carbide by carbon ion implantation

A new NaCl-type simple structured fcc chromium carbide was observed after 50 keV carbon-ion implantation into pure Cr thin films. The lattice constant was 0.403 nm determined by electron diffraction. TEM in situ annealing showed that the new phase was stable up to a temperature around 300°C. Auger spectra confirmed the formation of the carbide. The formation energy of the carbide calculated by Miedema's empirical method was –7 cal/mol.     

Structural studies of iron in vitrified toxic wastes

The formation of iron carbides by reactive milling of α-Fe and C powders is reported. The products formed were analyzed by Mössbauer spectroscopy and X-ray diffraction. It was found that iron carbide phases start forming after an incubation period of about 3 h depending on the ball-to-powder weight ratio (BPR). Carbide amounts increased with increasing milling time while α-Fe content decreased. Energy transfer increased with increasing BPR and high BPR resulted in an increase in the reaction rate. Although it was not possible to selectively synthesise a specific Fe _x C phase, samples containing predominantly one type of carbide phase, either Hägg carbide or cementite, were successfully prepared. The formation of the different iron carbide phases is discussed within the context of the Fe–C phase diagram for non-equilibrium processes.     

Synthesis and elevated temperature performance of a polypyrrole-sulfur-multi-walled carbon nanotube composite cathode for lithium sulfur batteries

A sulfur-multi-walled carbon nanotube composite (S/MWCNT) was prepared using a two-step procedure of liquid-phase infiltration and melt diffusion. Polypyrrole (PPy) conductive polymer was coated on the surface of the as-prepared S/MWCNT composite by in situ polymerization of pyrrole monomer to obtain PPy/S/MWCNT composite. The composite materials were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). The electrochemical performance of the as-prepared cathode material was investigated at 25, 40, and 70 °C at various rates. It was found that temperature has dual effects on the performance of Li/S cells. Increasing the temperature, on one hand, facilitates the lithium ion transport through the cathode and, on the other hand, leads to faster dissolution of active material into the electrolyte. The PPy coating can effectively trap polysulfides in its porous structure, even at elevated temperatures, leading to the improvement of the discharge capacity, the cycle stability, and the coulombic efficiency. The electrochemical impedance spectroscopy (EIS) results reveal that the PPy coating reduces the formation of passive layer on the cathode surface, even at high temperatures, resulting in a better elevated temperature performance. A high reversible capacity of 945 mAh g^?1 was maintained after 50 cycles for the PPy/S/MWCNT composite at 70 °C at a rate of 0.5 C.     

碳纳米管阴极的短脉冲爆炸场发射与等离子体膨胀

采用碳纳米管制备了一种强流电子束发射阴极,并对碳纳米管阴极在双脉冲条件下的强流发射性能进行了研究.在双脉冲条件下获得了245 A/cm²的强发射电流密度,阴极的开启时间约为40 ns.采用高速分幅相机和CCD相机对强流电子束在空间和时间的分布进行了研究.研究表明连续脉冲实验时,离子体及其膨胀对发射电子束的强度和分布影响很大,双脉冲时脉冲间隔时间内等离子体的膨胀速率约为8.17 cm/μs.等离子体形成时没有优先位置,电子束发射的局部增强位置是随机的.结果表明碳纳米管阴极可以作为强流阴 关键词： 碳纳米管 爆炸场发射 等离子体膨胀 强流电子束     

Combined bulk and laser heat treatment for optimizing the structure and phase composition of high-speed steels

The influence of laser radiation on the formation of an austenitic grain and on the alloying ability of a solid solution of high-speed cutting steels is investigated using as an example the steel R18 subjected beforehand to various bulk heat treatments. Notice is taken of the role of laser tempering during the initial stage of heating to the temperature at which an austenitic transformation with release of secondary carbides takes place, accompanied by laser action of plastic de formation with formation of a corresponding structurally stressed state. The content, in the solid solution, of the main alloying element tungsten in the R18 high-speed steel is determined from Mössbauer measurements and from x-ray phase analysis data on the degree of the dissolution of the carbide phase in laser-action zone. Determination of the carbon content in the solid solution of the zones of laser action, yields data on the degree of the carbide-phase solution and on the displacement of the diffraction -lines; these data can be attributed to anomalous diffusion of the carbon as the laser heating is accelerated.Quantum Physics Division, Kuibyshev Branch. Translation of Preprint No. 19, Lebedev Institute of Physics, Academy of Sciences of the USSR, Moscow, 1991.     

Ni-WC composite coatings by carburizing electrodeposited amorphous and nanocrystalline Ni-W alloys

In situ formation of tungsten carbide in the matrix of FCC nickel has been achieved by carburizing of the electrodeposited Ni-W alloy coatings. The size of the carbide particles ranges between 100 and 500 nm. The carbide phase is also present in the form of very small precipitates inside the nickel grains. The size of such precipitates is between 10 and 40 nm. The carburizing environment was created by introducing a flowing mixture of vaporized 95.5% alcohol (0.25 ml/min, liquid) and argon (0.5 L/min, gas) into the carburizing furnace. Supersaturated nature of electrodeposited amorphous and nanocrystalline alloys, in addition to high diffusivity, have been attributed for the formation of carbide phase in the deposits at a temperature range of 700-850 °C. The carbide-metal interface is clean and the composite coatings are compact. Hardness values up to about 1100 KHN are achieved. Hardness increases with tungsten content and carburizing temperature.     

Deformation twinning in boron carbide particles within nanostructured Al 5083/B4C metal matrix composites

The presence of deformation twins is documented in boron carbide reinforcement particles within a nanostructured Al 5083/B₄C metal matrix composite. High resolution transmission electron microscopy analysis suggests that these are (0001) twins. This work discusses the mechanisms responsible for their formation based on crystallographic analysis and mechanical loading conditions. Specifically, we propose that there are two potential models that can be used to describe twin formation in boron carbide particles. The structural models involve slip in the 1/3[1100] (0110) or 1/3[0110] (0110) planes of C–C–C chains and the appropriate reconfiguration of B–C bonds. Analysis of the loading conditions experienced by the boron carbide particles indicates that local high stress intensity and the presence of a high shear force around the boron carbide particles are two factors that contribute to twin formation.     

Substructural phase transitions during intense plastic deformation of low-carbon ferrite-perlite steel

We have studied the evolution of the defect structure and phase composition of low-carbon ferrite-perlite steel subjected to intense plastic deformation using diffraction electron microscopy. It has been shown that a high degree of deformation is accompanied by disruption of the perlite columns. We have found and described two perlite decay mechanisms: decay of the carbide plates by a path of their granulation due to dislocation slip and dissolution of cementite arising from the outflow of carbon atoms from the carbide phase into ferrite crystal lattice defects. We have described the phenomenon of morphological reconstruction of the cementite-phase particles (a transition from layers to spheres) under plastic deformation conditions. Tomsk State Architectural and Construction University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 63–71, March, 1998.     

Breakdown formation in a transient hollow cathode discharge-astatistical study

Discharge formation at low pressure is found to be greatly influenced in the presence of a suitable hollow cathode region. The formation of a moving virtual anode which extends the anode potential to within the hollow cathode region is thought to be responsible for the enhanced ionization growth which subsequently leads to gas breakdown. In this paper, the spatial evolution of the local potential in the discharge region of a pulsed hollow cathode discharge has been measured in a range of pressures with two different cathode apertures. An extensive data set has been collected and analyzed using a statistical technique. From the characteristic of the statistical distribution of the data, unique features associated with the role of hollow cathode at the different stages of discharge formation have been identified. It was found that the influence of the hollow cathode region is strongest in the start of ionization growth and in the final change over to high current breakdown     

Silicon carbide as electrode material of a pseudospark switch

Through the last years, the pseudospark switch, a low-pressure gas discharge switch with hollow cathode geometry, became established as a promising element of pulsed power technology and a serious alternative to other high-power switches. The use of a novel electrode material silicon carbide yields performance improvements in two main areas. Quenching phenomena, a long-standing problem for several applications, are suppressed completely and the switch lifetime can be distinctly increased, approaching that of thyratrons for operation with high repetition rate. As a crow-bar switch, the lifetime is nearby unlimited due to cold electrode usage. Spatial and temporal resolved spectroscopy revealed new insight into the extraordinary discharge behavior of silicon carbide electrodes. The radial plasma expansion from the central bore hole to the outer electrode regions, forming vesicular shells of different ionization stages of Si and C, are described in detail. The remaining problem, a significant loss of deuterium gas during discharge, has been long-term tested and is assumed to be the outcome of absorption in the silicon carbide electrodes. An envisaged promising remedy is presented     

ПОВЕДЕНИЕ КАТОДНЫХ П ЯТЕН В СИЛЬНОТОЧНОМ ЭЛЕКТРИЧЕСКОМ РАЗРЯ ДЕ

A magnetooptical rapid-action shutter was used to study the formation of cathode spots on the electrodes of a short-time high-intensity electric discharge. It was found that there exist two types of cathode spots. Spots of the first type are transported with a high velocity to the cathode surface not yet reached by the discharge. Spots of the second type, on the other hand, move in the neighbourhood of the centre of evaporation. On metals which evaporate with difficulty (Ni, Cu, Al) only spots of the first type may exist. On metals which easily evaporate (Cd, Sn, Zn) spots of both types exist simultaneously. The formation of cathode spots is explained by means of the cold emission of electrons from the cathode.     

Cathodic needle growth from Mo(CO)6 and Cr(CO)6 vapors at lower electric fields

A cathodic needle growth which is possibly associated with the temperature-field (T-F) electron emission is described. Experimental data disclosed that densely populated needle crystals exhibiting a dendritic configuration growth at the tip area of a pointed cathode when it is operated in an Mo(CO)₆ or Cr(CO)₆ atmosphere at a field strength just insufficient to draw field electrons. The needle growth occurs always at elevated temperatures of ∼600–∼700 K, indicating that it is triggered by T-F electrons emitted from the cathode tip. Needle crystals produced are not single crystals but composed of linearly packed micro-crystals, and those crystals obtainable from Mo(CO)₆ are a face-centered cubic phase of Mo₂C. Needles arising from Cr(CO)₆ have not been identified, but they are believed to be an unknown phase of chromium carbide.