石墨通过爆炸相变为金刚石的新结构位置转变模型(下)

石墨粉层受到击波高速冲击后,在高温高压下,会以固相结构方式转变为金刚石,文章作者创立了一个新的转变模型,在文中简称为“凯”模型,包括:在ABCA型石墨的转化理论中,找到了具体哪18个碳原子向金刚石晶胞转变的原子对应点结构,使活化能的计算可接近最小值.在ABA型石墨转化理论中,除石墨网格平面间的压缩之外,必须考虑层间剪切错动的作用,剪切变形与层间压缩的概率都同样多.标志其特性错动的距离是0.521h,h一层间压缩距离,理论被实验所征实.     

石墨重结晶现象与金刚石生长机制(上)

石墨材料在高温高压条件下,大量的C原子和原子团溶解于催化剂合金中,当回到常温常压下,过饱和的C会从合金晶格中析出来,形成的石墨晶体存在于合金中,成为重结晶石墨。文章详细地分析了高温高压合成金刚石过程中石墨的重结晶现象,通过实验论证了压力和温度对这一过程的影响,研究了石墨-合金界面两者的相互渗透以及此一界面上金刚石的成核与长大。认为金刚石优先在石墨-金刚石界面成核并长大,并且跟石墨的重结晶过程有关。     

用含硼T641石墨炭源合成含硼氮金刚石晶体的结构、性质与应用效果

介绍了硼元素对合成金刚石是晶体的结构,性质与应用技术的影响,来提高人们对它的进一步认识,以促进对它的研究和开发,使金刚石品种,质量等得到迅速的发展。     

柔性石墨的结构对其压缩回弹性能的影响

通过实验确立了柔性石墨气固二相结构的微观特征，并据此从理论上推导了柔性石墨的压缩率和回弹率与密度的关系。     

聚晶金刚石石墨化温度的研究

通过对PCD复合片中聚晶金刚石层加热前后的XRD图谱，以及在不同气氛环境下的DTA-TG分析，对聚晶金刚石的石墨化温度以及在加热过程中PCD的氧化和石墨化是否同时发生的问题进行了研究。结果表明：（1）聚晶金刚石在加热过程中，同时发生氧化还原反应，即氧化和石墨化同时进行；（2）随着加热温度的升高，聚晶金刚石石墨化程度增加；（3）聚晶金刚石的石墨化温度为960℃。     

金刚石钎焊(上)

金刚石磨料主要靠机械夹持力把持在金属(烧结或电镀)胎体中。由于这一弱点,在切割过程中,金刚石不可避免地会从胎体中脱落或掉出。此外,金刚石突出高度较低,故金刚石工具的切割速度受到限制。而且,金属胎体和工作对象(被切割岩石)相互磨擦,将会导致金刚石和其它材料的热损伤,而且工作的功率消耗将增加。金刚石采用钎焊的办法牢固地把持在金属胎体中,可形成强力的化学结合,金刚石磨料的突出高度将成倍提高,而不会从胎体中脱落或碎裂。因此,金刚石工具的切割速度将会成倍提高。当钎焊料熔融时,碳化物形成物将向金刚石方向迁移而在界面上形成碳化物。这种反应可能过分而使金刚石质量明显下降。此时,可能需要对金刚石进行镀覆以便缓和与控制这种反应。当金刚石钎焊在基体的表面时,熔化趋向于将金刚石聚集在一起,可促使钎焊层局部加厚。这种金刚石晶粒的成簇聚集将降低金刚石工具的切割效率。一种金刚石矩阵有序排列设计(grid)是必要的,它可保持钎焊层具有均匀的厚度。因此钎焊合金可控熔融,可使每一粒金刚石晶体周围形成较缓的坡度。这种整体均匀焊层的支承可使金刚石工具高速有效切割,而功率消耗较低。     

人造金刚石气相氧化除石墨的研究

通过实验研究,提出了一种经济合理、能有效避免传统酸煮工艺带来的严重环境污染的人造金刚石提纯新工艺——气相氧化法。分析了金属催化剂合金的相变及热解机理,建立了氧化助剂A促进石墨氧化的化学模型。     

石墨重结晶现象与金刚石生长机制(下)

石墨材料在高温高压条件下,大量的C原子和原子团溶解于催化剂合金中,当回到常温常压下,过饱的C会从合金晶格中析出来,形成的石墨晶体存在于合金中,成为重结晶石墨。文章详细地分析了高温高压合成金刚石过程中石墨的重结晶现象,通过实验论证了压力和温度对这一过程的影响,研究了石墨-合金界面两者的相互渗透以及此一界面上金刚石的成核与长大。认为金刚石优先在石墨-金刚石界面成核并长大,并且跟石墨的重结晶过程有关。     

采用非石墨材料合成金刚石的研究进展

综述了国内外在采用非石墨材料合成金刚石单晶、薄膜以及纳米金刚石方面的研究进展，并对21世纪金刚石的发展和应用前景做出了展望。     

人造金刚石用石墨性能的研究

用于合成金刚石的石墨具有三个功用——碳源、热源和受压介质，其性能直接关系着金刚石的质量。文章针对人造金刚石用石墨材料主要性能的研究进行了综述，包括石墨化度、气孔率（体积密度）、灰分（纯度）、电阻率以及晶体结构等等。提出在选择合成金刚石用石墨材料时，应综合考虑其满足不同功用的各项性能，同时还要结合具体的生产条件。认为满足合成设备大型化和粉末工艺的粉状石墨和辅助加热用的石墨材料将是人造金刚石用石墨材料发展的新亮点。     

Indentation hardness of nano-polycrystalline diamond prepared from graphite by direct conversion

Indentation hardness of nano-polycrystalline diamonds (consisting of fine particles of 10–30 nm size) prepared directly from graphite under high pressure and high temperature conditions were investigated. It was found that a measurable indentation with no cracking can only be formed using the Knoop indenter in a limited loading condition of 2–6 N, and a reliable and accurate measurement is obtained at a load around 4.9 N. The Knoop hardness measurement at the applied load of 4.9 N revealed that some of the nano-polycrystalline diamonds obtained at P≧15 GPa and T≧2300 °C have extremely high hardness (120–145 GPa), which is equivalent to that in the (001)〈100〉 of the synthetic high-purity (type IIa) diamond crystal (116–130 GPa).     

Synthesis of diamond films and nanotips through graphite etching

A hot filament CVD process based on hydrogen etching of graphite has been developed to synthesize diamond films and nanotips. The graphite sheet was placed close to the substrate and only hydrogen was supplied during deposition. No hydrocarbon feed gases are required for this process. High quality diamond films were synthesized with high growth rate on P-type (1 0 0)-oriented silicon wafers without discharge or bias. The diamond growth rate is approximately five times higher than that through conventional hot filament chemical vapor deposition using a gas mixture of methane and hydrogen (1 vol.% methane) under similar deposition conditions. The diamond films synthesized in this process exhibit smaller crystallites and contain smaller amount of non-diamond carbon phases. Synthesis of well-aligned diamond nanotips with various orientation angles was achieved on the CVD diamond-coated Si substrate when the substrate holder was negatively biased in a DC glow discharge. The nanotips grown at locations far enough from the sample edges are aligned vertically, while those around the sample edges are tilted and point away from the sample center. The alignment orientation of the nanotips appears to be determined by the direction of the local electric field lines on the sample surfaces.     

膨胀石墨基相变储能材料的研究

利用膨胀石墨孔隙结构的吸附性能,制备了石蜡/膨胀石墨、聚乙二醇/膨胀石墨、十四醇/膨胀石墨相变储能复合材料,用差示扫描热量法研究了材料的热性能.结果表明,复合材料的相变潜热随着石蜡、聚乙二醇、十四醇含量的增加而增加.复合材料的导热性能随着石蜡、聚乙二醇、十四醇含量的增加而小.膨胀石墨的多孔结构对石蜡、聚乙二醇、十四醇有很好的吸附性能,石蜡、聚乙二醇、十四醇在固一液相变时,未见有液态石蜡、聚乙二醇、十四醇的渗出.     

Shock-induced phase transition of oriented pyrolytic graphite to diamond at pressures up to 15 GPa

In studies of shock-induced phase transition of ordered pyrolytic graphite to a diamond-like phase, the lowest transition onset pressure was observed at 19.6 GPa. The phase transition in that case was considered to be martensitic. In the present study ordered pyrolytic graphite with voids between particles was loaded at pressures up to 15 GPa using a planar shock wave propagating along the basal plane of the graphitic crystal structure. As a result, both diamond-like carbon and diamond were observed in the postshock sample. The phase transition of graphite to diamond was assumed to occur by the release of distortional energy stored in the graphite particles, that is, diffusional-controlled reconstructive mechanism, on the basis of the data by high resolution electron microscopy together with electron energy loss spectroscopy.     

一种模拟气液相变过程的相变模型

提出了一种基于VOF（volume of fluid）方法的相变模型,用于计算气液相变过程的控制方程中的传热传质源项。在单位时间步长上相界面附近发生的传热以瞬态热扩散来考虑,并假设该传热导致了相变的发生。相变模型能够通过相界面单元的温度、流体热物性以及时间步长来计算传热传质源项。通过一维Stefan问题和二维水平膜沸腾问题对该相变模型进行验证,对比了相界面位置以及温度分布,结果与理论解吻合良好。进一步探讨了相变模型中的时间步长对计算精度的影响。结果表明时间步长越小,本相变模型模拟得到的结果与理论解的偏差越小。     

聚乙二醇/膨胀石墨相变储能复合材料

利用膨胀石墨孔隙结构的吸附性能,制备了聚乙二醇/膨胀石墨相变储能复合材料,用差示扫描热量法研究了材料的热性能.结果表明复合材料的相变温度不随聚乙二醇含量的改变而变化,材料的相变潜热随着聚乙二醇含量的增加而增加.复合材料的导热性能随着聚乙二醇含量的增加而减小.膨胀石墨的多孔结构对聚乙二醇有很好的吸附性能,聚乙二醇在固-液相变时,未见有液态聚乙二醇的渗出.     

Enhanced thermionic energy conversion and thermionic emission from doped diamond films through methane exposure

Thermionic electron emitters are a crucial component in applications ranging from high power telecommunication, electron guns, space thrusters and direct thermal to electrical energy converters. One key characteristic of diamond based electron sources is the negative electron affinity (NEA) properties of hydrogen terminated surfaces which can significantly reduce the emission barrier. Nitrogen and phosphorus doped diamond films have been prepared by plasma assisted chemical vapor deposition on metallic substrates for thermionic emitter application. Electron emission current versus temperature was measured and analyzed with respect to the Richardson-Dushman relation, with work function and Richardson constant deduced from the results. Initial emission measurements up to 500 °C in vacuum were followed by emitter characterization while the sample was exposed to methane. Vacuum measurements indicated a work function of 1.18 eV and 1.44 eV for phosphorus and nitrogen doped diamond films, respectively. Introduction of methane resulted in a significant increase of the emission current which was ascribed to contribution from ionization processes which increase charge transfer from the emitter surface. This phenomenon was utilized in a thermionic energy conversion structure by introduction of methane in the inter electrode gap where a two-fold increase in output power was observed upon introduction of the gaseous species.