On the possible origins of natural diamonds

A brief survey of the current theories of formation of diamonds in nature leads to the hypothesis which suggests the possibility of natural diamonds having formed metastably—a hypothesis which has not been enjoying much experimental support in its favour. The authors present briefly some aspects of the investigations which have resulted in demonstrating a new process whereby the dissolution and metastable autoepitaxial (re)crystallization of diamond occurs at atmospheric pressure. A comparison of certain reported growth characteristics of natural (terrestrial) and synthetic (high pressure, solvent-catalyst) diamonds with those of the metastable diamond overgrowths reveal many striking similarities. These results therefore serve as evidence to support the metastable origin hypothesis.     

Fe40Ni40P12B8非晶合金的冲击晶化实验研究

 本文研究了Fe₄₀Ni₄₀P₁₂B₈非晶合金冲击波加载下的晶化行为，冲击波由二级轻气炮发射的告诉弹丸撞击靶产生。实验结果表明：Fe₄₀Ni₄₀P₁₂B₈非晶合金在冲击波加载下，晶化可在加载时间（微秒量级）内发生；晶化的阈值压力在30~50 GPa之间，相应的冲击温度约为510~800 K，晶化析出相与冲击压力有关，低压下析出相是面心立方γ-(Fe, Ni)固溶体和Fe₃(P_0.37B_0.63)化合物，高压下（大于60 GPa）析出相除了面心立方γ-(Fe, Ni)固溶体和Fe₃(P_0.37B_0.63)化合物之外，还包括(Fe, Ni)₃P化合物。     

Lindemann criterion and the anomalous melting curve of sodium

Recent reports of the melting curve of sodium at high pressure have shown that it has a very steep descent after a maximum of around 1000 K at 31 GPa. This maximum does not occur due to a solid-solid phase transition. According to the Lindemann criterion, this behaviour should be apparent in the evolution of the Debye temperature with pressure. In this work, we have performed an “ab initio” analysis of the behaviour of both the Debye temperature and the elastic constants up to 102 GPa, and find a clear trend at high pressure that should cause a noticeable effect on the melting curve.     

Predicting the Performance of Granulation Binders Through Micro‐Mechanistic Observations

The selection of an appropriate polymeric binder to be used to agglomerate drug with excipients is a critical issue for the development of high shear wet granulation processes for pharmaceutical tablet systems. The aim of the study reported here is to determine the potential for successful granulation through measurement of the interactions of two polymer binder solutions, hydroxypropyl methylcellulose (HPMC) and polyvinyl‐pyrrolidone (PVP), with individual paracetamol drug crystals. A novel micro‐force balance (MFB) has been used to measure different parameters of the crystal‐to‐binder interaction, including the forces exerted by axially strained liquid bridges formed between either two paracetamol crystals or between a reservoir of binder solution and a single paracetamol crystal, the paracetamol‐to‐binder wettability, the post rupture volume distribution and the residue deposited on each crystal. Video images of the separation sequences were obtained simultaneously for analysis of bridge geometry, contact angles, volume distribution and binder residues. It was found that the formation of liquid bridges and their ability to bond particles together depends on the amount of binder left on a crystal after contacting a reservoir of binder (for example, large binder drop). Crystals in contact with HPMC were able to retain more liquid from a binder reservoir than those contacted with PVP solutions. This behaviour is seen to be more important to the final granule strength than the liquid binding force holding particles together in the wet agglomerate.     

再结晶石墨对金刚石成核的影响

 本文结合实验现象，分析了在合成初期，碳源在溶剂-触媒金属中的分散，溶解及形成再结晶石墨的过程。并根据再结晶石墨与金刚石成核量的实验结果，初步定性地研究了膜生长法中，再结晶石墨对金刚石成核的影响。     

Novel model for the optical function of GaN

In this work we propose an analytical expression for the complex dielectric function which includes both discrete and continuum exciton effects. We have introduced Lorentzian broadening into Elliott’s formula. The introduction of broadening leads to equations for the dielectric function containing only elementary functions. We have applied the proposed model to the dielectric function of wurtzite GaN in the spectral region 1–10 eV. Excellent agreement with the experimental data has been obtained. We show that the Lorentzian-broadened dielectric function decays more slowly than the experimental data for hexagonal GaN at the low-energy side. This indicates that the broadening of the absorption edge in GaN is not purely Lorentzian. The agreement with the experimental data can be improved using adjustable broadening modification. Received: 29 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Plasma Surface Modification of Glass Fibre-Reinforced Nylon-6,6 Thermoplastic Composites for Improved Adhesive Bonding

The surface modification and adhesive bonding of a unidirectional GFRP Nylon-6,6 thermoplastic composite has been investigated. Wettability studies of plasma-treated specimens showed a significant reduction in the advancing and receding contact angles in water, relative to untreated material. The most effective treatment used oxygen plasma. The increases in wettability observed were determined to be the result of (a) an increase in the concentration of oxygen- and nitrogen-containing functional groups on the surface of the polymer and, (b) removal of fluoropolymer contamination, the source of which was identified as the PTFE mould release agent. This was established by SSIMS analysis. The surface modification resulted in significantly improved adhesion between the composite and an applied toughened epoxy adhesive; a considerable increase in the Mode II critical strain energy release rate, G _IIc, was observed following plasma treatment. Specimens treated in an oxygen plasma showed the greatest improvement in G _IIc, failing cohesively at a value of 1.6 kJ·m^–2 after only 15 seconds exposure. Without plasma treatment the specimens failed in an adhesive mode at very low values of G _IIc. Adhesion was further optimised by moulding the GFRP Nylon-6,6 against steel plates instead of PTFE.     

Acoustic nonlinearity parameter B/A determined by means of thermodynamic method under elevated pressures for alkanediols

The so-called Beyer nonlinearity parameter B/A is calculated for 1,2- and 1,3-propanediol, 1,2-, 1,3-, and 1,4-butanediol, as well as 2-methyl-2,4-pentanediol by means of a thermodynamic method. The calculations are made for temperatures from (293.15 to 318.15) K and pressures up to 100 MPa. The decrease in B/A values with the increasing pressure is observed. In the case of 1,3-butanediol, the results are compared with corresponding literature data. The consistency is very satisfactory. A simple relationship between the internal pressure and B/A nonlinearity parameter has also been studied.     

A new route for the synthesis of nitrides: The solvothermal preparation of GaN

Abstract

A new solvothermal route for the synthesis of nitrides is proposed using liquid NH3 as solvent in supercritical conditions, Such a preparation method was applied to the synthesis of GaN using gallium metal as starting material.

GaN is a wide band-gap semi-conductor (3.4eV). It is a very attractive nitride due to its various applications in micro- and opto-electronics [1,2]. Consequently, many research groups are interested in synthesising GaN. Two methods have been principally developed:

(i) synthesis of thin films by epitaxy [3,4]

(ii) synthesis of bulk GaN by high pressure method [5,6].

The new proposed process leads to fine microcrystallites of GaN with the wurtzite-type structure. The chemical purity can be optimised versus the synthesis mechanism. The size and shape of the crystallites would be influenced by the nature of the nitriding additive and the thermodynamical conditions (pressure and temperature) used for the synthesis.     

Bulk diamond formation from graphite in the presence of C-O-H fluid under high pressure

Abstract

C-O-H fluids have been successfully applied as catalysts for bulk diamond formation under high pressure. New insight into C-O-H fluids extends the understanding of the origin of natural diamond, which is presently of interest in materials and geological sciences. This review presents current literature data concerning the synthesis and characterization of bulk diamond formation assisted by C-O-H fluids at high pressure and high temperature. Based on a general survey of this subject, the pressure-temperature regime for diamonds formed in these fluids was established and the mechanism of conversion from graphite to diamond is discussed. Finally, a few questions are put forward that may be useful for the continued development of this research area.     

高温高压下闪锌矿相GaN结构和热力学特性的分子动力学研究

利用分子动力学方法和Buckingham经验势模型对重要半导体材料GaN立方闪锌矿相的晶格常数、相变压力(从闪锌矿到岩盐结构)、热膨胀、等温体模量、定压热容等结构和热力学特性在300—3000K的温度范围和0—65GPa的压力范围内进行了研究.研究表明，闪锌矿相GaN常态下的结构和热力学参数的模拟结果与实验数据及其他理论结果相符.同时在所选作用势模型可靠性检验的基础上，对等温体模量、定压热容诸非谐性参量在高温高压下的热力学行为进行了预测.所得结果在材料科学等领域的研究中具有一定的应用背景和参考价值. 关键词： GaN Buckingham势 分子动力学模拟 高温高压     

On intensities in high pressure neutron powder diffraction using single and polycrystalline diamond anvils: small versus large sample volumes

We report a quantitative comparison of measured intensities of neutron powder diffraction data collected in a single-crystal diamond anvil cell and in large-volume sintered diamond anvils. As expected from the difference in sample volumes, the latter provides 1–2 orders of magnitude higher intensities, depending on the anvil material. The remaining differences are due to effects of absorption and angular aperture.     

W过渡层结合界面对金刚石薄膜在WC-6%Co上的附着力的影响

 在微波等离子体化学气相沉积装置中,采用负偏压形核等方法,研究两种不同的W过渡层/基体结合界面对金刚石薄膜与WC-6%Co附着力的影响。采用氢等离子体脱碳、磁控溅射镀W、高偏压碳化等方法,在YG6衬底表面形成化学反应型界面,W膜在碳化时和基体WC连为一体,极大地增加了W膜与基体的附着力,明显优于直接镀钨、碳化形成的物理吸附界面。在高负偏压下碳化,能提高表面粗糙度,增加膜与基体机械钳合,而负偏压形核增加核密度,从而增加膜与基体的接触面积,结果极大地提高了金刚石薄膜的附着力。     

FeMn粉末触媒对合成金刚石的影响

 利用FeMn粉末触媒在国产六面顶压机上进行了合成金刚石单晶的实验,研究了高温高压条件下（5.7 GPa、1 550 ℃）,石墨-FeMn粉末触媒体系中金刚石单晶的生长特性。通过光学成像显微镜观测表明：合成出的金刚石单晶呈浅黄色,晶形完整,且都是八面体,晶体里含有白色物质,粒度集中在0.3~0.5 mm;通过扫描电镜观测了晶体的表面形貌,表面有熔坑;通过穆斯堡尔谱,发现粉末触媒里主要是FeMn合金和独立状态的Fe,金刚石内部主要是Fe和Fe₃C;利用X射线荧光光谱,检测出样品里有Fe和Mn元素。     

Measurement of unstable burning velocities of iso-octane–air mixtures at high pressure and the derivation of laminar burning velocities

A new technique is reported for measuring burning velocities at high pressures in the final stages of two inwardly propagating flame kernels in an explosion bomb. The flames were initiated at diametrically opposite spark electrodes, close to the wall, in quiescent mixtures. Measurements of pressure and flame kernel propagation speeds by high-speed photography showed the burning velocities to be elevated above the corresponding laminar burning velocities as a result of the developing flame instabilities. The enhancement increased with increase in pressure and decreased with increase in Markstein number. When the Markstein number was negative, instabilities could be appreciable, as could the enhancement. For the iso-octane–air mixtures investigated, where the mixtures had well-characterised Markstein numbers or critical Peclet numbers at the relevant pressures and temperatures, it was possible to explain the enhancement quantitatively by the spherical explosion flame instability theory of Bechtold and Matalon, provided the critical Peclet number was that observed experimentally, and allowance was made for the changing pressure. With this theoretical procedure, it was possible to derive values of laminar burning velocity from the measured values of burning velocity over a wide range of equivalence ratios, pressures, and temperatures. The values became less reliable at the higher temperatures and pressures as the data on Markstein and critical Peclet numbers became less certain. It was found that with iso-octane as the fuel the laminar burning velocity decreased during isentropic compression.     

Peculiarities of phase transitions when synthesizing diamond in high pressure apparatus of various types

Abstract

The influence of P, T-parmeters and duration of heat when synthesizing diamond in high pressure apparatus both of recessed anvil-type and cylindrical type (belt-type) on properties of diamond powders was studied. The dependence of pressure in reaction cells on temperature of force elements of apparatus in initial state and on efficiency of high pressure production in a reaction cell before heating was shown.     

Further study of the difference in nitrogen aggregation among (111) and (100) growth sectors of synthetic diamonds

Abstract

Single nitrogen atoms in synthetic diamond crystals aggregate in pairs by heat treatment under high pressure after electron irradiations. We find that the rate constants of nitrogen aggregation in the (111) growth sectors grown by the various solvents are about ten times greater than those of the (100) growth sectors. Furthermore the rate constant of nitrogen aggregation of the same sector is almost the same value. (shown Table-1). We have considered a few reasons for this fact. It may be accurate that a form of nitrogen atoms contained in (111) growth sectors of an as-grown diamond differs from that in (100) grown sectors and that the former accelerates nitrogen aggregation. Consequently we propose one nitrogen migration and aggregation model.     

Impact of dopants in GaN on the formation of two-dimensional electron gas in AlGaN/GaN heterostructure field-effect transistors

The two-dimensional electron gas distribution in AlGaN/GaN high electron mobility transistors is determined from the solution of the coupled Schr?dinger’s and Poisson’s equations. Considering the piezoelectric effect, the two-dimensional electron gas concentration is calculated to be as high as 7.7×10¹⁹ cm^-3. In order to obtain an understanding of how the two-dimensional electron gas distribution is influenced by dopants in GaN, we observed the two-dimensional electron gas concentration and occupation of sub-bands versus dopant concentration in the GaN layer of an AlGaN/GaN heterostructure. Our results show that the two-dimensional electron gas concentration is slightly increased at higher doping levels in GaN, while the quantum confinement in the AlGaN/GaN heterostructure is weakened with the increase of donor concentration in the GaN layer. Received: 26 May 2001 / Accepted: 23 July 2001 / Published online: 23 January 2002     

Schottky diode based on porous GaN for hydrogen gas sensing application

This article reports the study of Pd Schottky contact on porous n-GaN for hydrogen gas sensing. Upon exposure to 2% H₂ in N₂, porous GaN sensor exhibited significant change of current. Morphological studies revealed that Pd contact deposited on porous GaN has ridge-trench-like morphology, a dense porous network was found in between the ridges. The dramatic change of current was attributed to the unique microstructure at Pd/porous GaN interface, which allowed higher accumulation of hydrogen; this resulted in a stronger effect of H-induced dipole layer and led to a significant change in the electrical characteristics of the porous sensor.