Hopping conductivity in polycrystalline diamond films

The results of the experimental study of the conductivity of a polycrystalline diamond film by dielectric spectroscopy are presented. A diamond film about 200 μm thick was grown by microwave plasma-assisted chemical vapor deposition for 180 h. Two pronounced local peaks in the frequency dependence of the conductivity were observed. The relevant frequencies were temperature-dependent. These data permit us to hypothesize that the hopping mechanism of conductivity occurs in the polycrystalline diamond film. Two types of electrically active centers with different activation energies and relaxation times are involved in the conductivity.     

Structural and phase characteristics of the diamond/matrix interfacial zone in high-resistant diamond composites

The structural-phase state of the contact zone and the factors that influence on the strength of diamond retention in the diamond carbide composites were determined. Composites were obtained by the new hybrid technology that eliminates the reheating of the metalized coating. The elaborated technology combines the thermal diffusion metallization of a diamond and the sintering by the scheme of self-dosed impregnation in a one-stage technological cycle. By the methods of electron microscopy, X-ray diffraction analysis, and Raman spectroscopy the structural and phase characteristics of the interphase boundary were investigated. The improvement of chemical and mechanical adhesion between the diamond and carbide matrix was obtained. It was shown that the specific productivity of the samples with a metalized diamond component is 39% higher than those without metallization.     

High pressure sintering behavior of polycrystalline diamond with tungsten

Polycrystalline diamond was investigated under high pressure and high temperature of 5.0 GPa and 1100–1500 °C in the presence of tungsten. In situ resistance measurements indicated that reactions between diamond and tungsten happened at about 960 °C. Phase analysis demonstrated that WC increased and meta-stability of W₂C decreased clearly at the higher temperature. It is clear from the characterization of the sintered body that the electrical resistance decreased and the density of specimens increased as the sintering temperature rose. The specimen sintered at 1500 °C has a homogeneous microstructure and good conductivity.     

Effects of initial crystal size of diamond powder on surface residual stress and morphology in polycrystalline diamond (PCD) layer

Polycrystalline diamond compacts (PDC) were synthesized using diamond powder of average crystal size 3-20 μm by the Ni 70 Mn 25 Co 5 alloy infiltration technique at high temperature and high pressure (HPHT).The surface residual stress of polycrystalline diamond (PCD) layer was measured using micro-Raman spectroscopy with hydrostatic stress model and X-ray diffraction (XRD).Measurements of the stress levels of PCDs show that the residual compressive stresses range from 0.12 to 0.22 GPa,which increase with th...     

纳米引晶法选择性生长金刚石薄膜

通过传统的光刻工艺和纳米引晶技术,在抛光的单晶Si衬底上形成带有 超细金刚石纳米粉的引晶图案,并利用该图案与抛光Si处金刚石成核密度的巨大差异,实现 金刚石薄膜的高选择比生长。该方法具有工艺简单、沉积效率高、选择比高、对底无任何损 伤等优点。同时,这种方法很容易在不同衬底上实现金刚石薄膜的大面积选择性生长。     

Plastic deformation behaviour of layer-grained silver polycrystalline from atomistic simulation

Two types of nanocrystalline polycrystalline silver models in bulk, film and nanowire forms were constructed with layer-grained or equiaxed grain morphologies and average grain sizes of ~7.8 and ~14.7 nm. Uniaxial tensile deformation was performed to investigate the effect of grain morphology and free surface on the plastic deformation behaviour under the strain rate of 5 × 10⁸ and 10⁷ s^?1 at 0.1 K. Grain Boundary (GB) orientation and dimensions in layer-grained morphology promoted the formation of sessile dislocation structures. Some dislocations interacted with each other and some dislocations got obstructed by stacking faults. However, the resulting configurations did not last long enough to cause strain hardening. Strain softening was observed in all models except for the layer-grained models in bulk form, where steady plastic flow was observed after yield. The location and orientation of free surfaces with respect to GBs imposed geometric constraints on the deformation mechanisms (GB sliding and formation of sessile dislocations) which produced asymmetric stress states that influenced the elastic as well as plastic response of the material. The yield stress and flow stress were much smaller at lower strain rate simulations. The proportion of perfect dislocations increased as the strain rate decreased from 5 × 10⁸ to 10⁷ s^?1 due to the decrease of applied stress. Dislocations were mainly emitted from grain boundaries or triple junctions at both high and low strain rate deformations. These results provided insights into the understanding of layer-grained nanocrystalline materials and the synthesis of materials with both high strength and ductility.     

Data-driven exploration for pressure-induced superconductors using diamond anvil cell with boron-doped diamond electrodes and undoped diamond insulating layer

ABSTRACT

Data-driven exploration for pressure-induced superconductors was performed based on the high-throughput first-principles screening of electronic band structures. In the screening conditions, we focused on the characteristics including a narrow band gap, flat band feature, and possibility of metallization under high pressure. The 27 promising compounds were screened out from the database of Atomwork for the candidates of new pressure-induced superconductors. Among the candidates, we actually synthesized three compounds in a single crystal, and all candidates exhibited the pressure-induced superconductivity. For the in-situ electrical transport measurements, we developed a novel configuration of diamond anvil cell with boron-doped diamond electrodes and an undoped diamond insulating layer. The discovered new pressure-induced superconductors via the data-driven approach and the developed diamond anvil cell were summarized in this paper.     

Microstructure evolution during severe plastic deformation

Radiotracer diffusion studies of severely deformed, ultra-fine grained materials have revealed the presence of ultra-fast transport paths, which include “non-equilibrium” grain boundaries and free volume. Under some experimental conditions, percolating porosity is produced even in pure copper. Micro-cracks may form in metals, if the local maximum shear stress exceeds the shear yield stress. However, their growth and propagation is postponed till late in the deformation process owing to the ductility of metals, the hydrostatic component of the stress system and/or dynamic recovery/recrystallization. In other words, crack growth and propagation is present only when the scope for further deformation is highly restricted. Using this approach, the load required for equal channel angular pressing, the change in the slope of the Hall–Petch plot with decreasing grain size and the theoretical limit for the smallest grain size attainable in a metal in a severe plastic deformation process are predicted and validated by experimental results. Experimentally successful prevention of percolated crack formation by the superposition of a hydrostatic pressure is also accounted for using this model.     

Effect of deformation of diamond anvil and sample in diamond anvil cell on the thermal conductivity measurement

Studies show that the sample thickness is an important parameter in investigating the thermal transport properties of materials under high-temperature and high-pressure (HTHP) in the diamond anvil cell (DAC) device. However, it is an enormous challenge to measure the sample thickness accurately in the DAC under severe working conditions. In conventional methods, the influence of diamond anvil deformation on the measuring accuracy is ignored. For a high-temperature anvil, the mechanical state of the diamond anvil becomes complex and is different from that under the static condition. At high temperature, the deformation of anvil and sample would be aggravated. In the present study, the finite volume method is applied to simulate the heat transfer mechanism of stable heating DAC through coupling three radiative-conductive heat transfer mechanisms in a high-pressure environment. When the temperature field of the main components is known in DAC, the thermal stress field can be analyzed numerically by the finite element method. The obtained results show that the deformation of anvil will lead to the obvious radial gradient distribution of the sample thickness. If the top and bottom surfaces of the sample are approximated to be flat, it will be fatal to the study of the heat transport properties of the material. Therefore, we study the temperature distribution and thermal conductivity of the sample in the DAC by thermal-solid coupling method under high pressure and stable heating condition.     

高压熔渗生长法制备金刚石聚晶中碳的转化机制研究

在6 GPa和1500 ℃的压力和温度范围内, 利用高压熔渗生长法制备了纯金刚石聚晶, 深入研究了高温高压下金刚石聚晶生长过程中碳的转化机制. 利用光学显微镜、X-射线衍射、场发射扫描电子显微镜检测, 发现在熔渗过程中金刚石层出现了石墨化现象, 在烧结过程中金刚石颗粒表面形貌发生了变化. 根据实验现象分析, 在制备过程中存在三种碳的转化机制: 1)金属熔渗阶段金刚石颗粒表面石墨化产生石墨; 2)产生的石墨在烧结阶段很快转变为填充空隙的金刚石碳; 3)金刚石直接溶解在金属溶液中, 以金刚石形式在颗粒间析出, 填充空隙. 本文研究碳的转化机制为在高温高压金属溶剂法合成金刚石的条件下(6 GPa和1500 ℃的压力和温度范围内)工业批量化制备无添加剂、无空隙的纯金刚石聚晶提供了重要的理论指导.     

Study of stress localisation in polycrystalline grains using self-consistent modelling and neutron diffraction

The time-of-flight neutron diffraction technique and the elastoplastic self-consistent model were used to study the behaviour of single and multi-phase materials. Critical resolved shear stresses and hardening parameters in austenitic and austenitic–ferritic steels were found by analysing the evolution of the lattice strains measured during tensile tests. Special attention was paid to the changes of the grain stresses occurring due to transition from elastic to plastic deformation. Using a new method of data analysis, the variation of the stress localisation tensor as a function of macrostress was measured. The experimental results were successfully compared with model predictions for both phases of the duplex steel and also for the austenitic sample.     

非晶塑性变形机理研究新进展

非晶合金的塑性变形机理一直是材料科学和凝聚态物理研究的热点问题之一.文章简单介绍了近来中国科学院物理研究所在非晶合金塑性机理研究方面的最新进展,介绍了玻璃转变和塑性变形机制之间的关联性及最新的实验证据,以及从非平衡态统计力学角度对非晶塑性变形机制的理解,指出非晶合金的塑性和剪切带的动力学状态密切相关,发现韧性非晶合金在变形过程中可以演化到自组织临界状态.这对认识非晶合金的形成本质,探索具有实际应用价值的非晶合金具有重要意义.     

Modelling of grain boundary impurity segregation during high temperature plastic deformation

A modified theoretical model is proposed to predict the grain boundary segregation of impurity atoms during high temperature plastic deformation. The model is based on the supersaturated vacancy-impurity complex created by plastic deformation and involves quasi-thermodynamics and kinetics. Model predictions are made for phosphorus grain boundary segregation during plastic deformation in ferrite steel. The results reveal that phosphorus segregates at grain boundaries during plastic deformation. At a given te...     

To the synthesis of polycrystalline diamond

Abstract

The sintering system “Diamond-Graphite-Cobalt” is investigated by means of a High Pressure - High Temperature - Belt - Apparatus. Described are the sintering conditions used and some of the properties of the compacts obtained. Good sintering structures and hard PCD (up to 50 GPa) were obtained at 7,5 GPa and 1500°C.     

基于大型激光装置的材料塑性变形行为微介观研究进展

微介观尺度下材料结构的实时演化行为是决定其动态力学性能的关键因素。大型激光装置作为集加载和诊断能力为一体的综合实验平台,为高温、高压、高应变率等极端条件下材料动态力学性能的微介观尺度研究提供了重要支撑。随着激光功率密度和脉冲整形能力的不断提升,实验所能探索的压力（10¹～10³ GPa）及应变率（10⁶～10¹⁰ s⁻¹）范围不断突破;而利用激光打靶产生的高亮X射线脉冲作为探测源,建立动态衍射和成像技术,可以实现高空间和时间分辨率下材料塑性变形机制的实时原位研究。简要介绍了基于大型激光装置的原位微介观实验技术及其在材料塑性变形行为研究中的应用,系统梳理了近二十年来具有代表性的研究成果,阐明了相关研究对推动材料动态响应多尺度物理建模的重要价值。     

Evolution of triple junction defect structure at plastic deformation of metallic polycrystals

A method for the determination of the components of the Frank vector of disclinations in triple junctions (TJ) of real polycrystalline aggregates has been proposed. Using this method we found that real polycrystals contain junction disclinations (JD), the nature of which is related to their thermo-mechanical history. One of the elements of the evolution of the network of initial junction disclinations is the formation of disclination dipole configurations in the adjacent TJ. The influence of the types of boundaries forming the junctions on JD Frank vector power is discussed.     

高温塑性变形引起的P非平衡晶界偏聚

阐述了高温塑性变形引起的非平衡晶界偏聚的准热力学和动力学,并使用该模型预测了低合金结构钢中高温塑性变形导致的P在奥氏体晶界的非平衡偏聚.研究发现：当变形为20%,应变速率为1×10^-3 s^-1时,在800 ℃左右会出现一个P的晶界偏聚浓度峰值;在1000 ℃变形为20％时,晶界偏聚浓度随着应变速率的增加而增加.预测结果与现有的实验结果基本一致. 关键词： 非平衡偏聚 晶界 塑性变形 磷     

Structure of diamond nanoparticles grown by chemical vapor deposition

We studied the structure of diamond nanoparticles grown by chemical vapor deposition. SEM images show that the material contains cubic, hexagonal, and possibly icosahedral structures ranging in size from 10 to 200 nm. Raman spectroscopy shows bands, which are characteristic of crystalline diamond, E_2g mode of hexagonal diamond, a-C, and graphite.     

Simulation of particulate-filled composite deformation diagrams on the basis of a constitutive model of large plastic deformation for a polymer matrix

Deformation diagrams of particulate-filled polymers have been calculated on the basis of specific constitutive equations [1] for large plastic deformation of the polymer. Composite structure is represented by the Hashin polydisperse model [2]. Original finite-element (FE) code with triangular elements has been elaborated and used for the numerical solution of boundary value problems. Local achievement of a critical value by the elastic main strain was used as a fracture criterion. Engineering (force-elongation) diagrams were found to exhibit maxima for arbitrary filler fraction if the interfacial bond was perfect and for low loading at zero adhesion. Stress-strain diagrams with a yield maximum and draw minimum provide macroscopic neck-type localization. Further, the loading in the case of facilitated deboading results in the diminution of the difference between maximum and minimum drawing forces and then in the disappearance of the latter, which in turn provides the transition from localized to macrouni-form deformation. Young's modulus and the yield stress increase with filling in the case of absolute adhesion and decrease in the opposite case. Ultimate elongation sharply drops with an increase in filler fraction, and embrittlement occurs at a small fraction of inorganic particles if a perfect interfacial bond is present. Contrary, a decrease in ultimate elongation is much more gradual, and composites conserve ductile properties of the matrix up to a high portion of inclusions. The laws found qualitatively coincide with what is observed for real materials.