Finite Element Analysis of Convection in Growth Cell for Diamond Growth Using Ni-Based Solvent

Thermal-electricaJ-fluid coupled finite element analyses are performed in the model of the growth cell in a high-pressure and high-temperature （HPHT） cubic apparatus in which the large diamond crystal can be grown by using Ni-based solvent with temperature gradient method （TGM）. The convection in the Ni-based solvent with different thicknesses at 1700-1800 K is simulated by finite element method （FEM）. The experiments of diamond crystal growth are also carried out by using Ni-based solvent at 5.7GPa and 1700-1800K in a China-type cubic high pressure apparatus （CHPA）. The simulation results show that the Rayleigh number in the solvent is enhanced obviously with the increasing solvent thickness. Good quality diamond single crystal cannot be grown if the Rayleigh number in the solvent is too high.     

高温高压下掺硼宝石级金刚石单晶生长特性的研究

本文在5.1—5.6 GPa,1230—1600℃的压力、温度条件下,以FeNiMnCo作为触媒,进行单质硼添加宝石级金刚石单晶的生长研究.借助于有限元法,对触媒内的温度场进行模拟.研究得到了FeNiMnCo-C-B体系下,金刚石单晶生长的P-T相图.该体系下合成金刚石单晶的最低压力、温度条件分别为5.1 GPa,1230℃左右.研究发现,在单晶同一{111}扇区内部,硼元素呈内多外少的分布规律.有限元模拟结果给出,该分布规律是由在晶体生长过程中,{111}扇区的增长速度逐渐减小所致.{111}晶向的晶体生长实验结果表明,硼元素优先从{111}次扇区进入晶体.研究发现,这是该扇区增长速度相对较快,硼元素扩散逃离可用时间短导致的.另外,同磨料级掺硼金刚石单晶生长相比,对于温度梯度法生长掺硼宝石级金刚石单晶,由于晶体的增厚速度较慢,即使硼添加量相对较高,也可以实现表面无凹坑缺陷的优质金刚石单晶的生长.     

Effects of catalyst height on diamond crystal morphology under high pressure and high temperature

The effect of the catalyst height on the morphology of diamond crystal is investigated by means of temperature gradient growth(TGG) under high pressure and high temperature(HPHT) conditions with using a Ni-based catalyst in this article. The experimental results show that the morphology of diamond changes from an octahedral shape to a cuboctahedral shape as the catalyst height rises. Moreover, the finite element method(FEM) is used to simulate the temperature field of the melted catalyst/solvent. The results show that the temperature at the location of the seed diamond continues to decrease with the increase of catalyst height, which is conducive to changing the morphology of diamond. This work provides a new way to change the diamond crystal morphology.     

Effects of carbon convection field on large diamond growth under high-pressure high-temperature conditions

Large diamond crystals were successfully synthesized by FeNi-C system using temperature gradient method under high-pressure high-temperature conditions. The assembly of the growth cell was improved and the growth process of diamond was investigated. Effects of the symmetry of carbon convection field around the growing diamond crystal were investigated systematically by adjusting the position of seed crystal in the melted catalyst/solvent. The results indicate that morphologies and metal inclusion distributions of the synthetic diamond crystals vary obviously in both symmetric and non-symmetric carbon convection fields with temperature. Moreover, finite element method was applied to analyze carbon convection mode of the melted catalyst/solvent around the diamond crystal. This work is helpful for understanding the growth mechanism of diamond.     

Effects of a carbon convection field on large diamond growth under high-pressure high-temperature conditions

Large diamond crystals were successfully synthesized by a FeNi-C system using the temperature gradient method under high-pressure high-temperature conditions. The assembly of the growth cell was improved and the growth process of diamond was investigated. Effects of the symmetry of the carbon convection field around the growing diamond crystal were investigated systematically by adjusting the position of the seed crystal in the melted catalyst/solvent. The results indicate that the morphologies and metal inclusion distributions of the synthetic diamond crystals vary obviously in both symmetric and non-symmetric carbon convection fields with temperature. Moreover, the finite element method was applied to analyze the carbon convection mode of the melted catalyst/solvent around the diamond crystal. This work is helpful for understanding the growth mechanism of diamond.     

基于有限元法分析宝石级金刚石的合成腔体温度场

以有限元法为理论分析手段模拟分析了温度梯度法合成宝石级金刚石大单晶的腔体温度场,实现了对宝石级金刚石的合成腔体内各位置温度同时测量.模拟结果表明:在宝石级金刚石合成过程中,其温度分布呈不均匀分布.腔体内高温区分布在样品(碳源+触媒)边缘,低温区分布在籽晶附近.样品腔内热量的传递方式和样品腔内的碳源输运方式相同,均由碳源的两侧向籽晶附近传输.籽晶附近轴向温度梯度大于径向温度梯度,导致单位时间内其轴向生长尺寸大于径向生长尺寸.宝石级金刚石腔体温度场分析的理论模型的成功构建,为新型宝石级金刚石腔体的研制提供了良好的设计基础,对促进优质宝石级金刚石的生长技术具有指导意义.     

Effects of additive NaN_3 on the HPHT synthesis of large single crystal diamond grown by TGM

In this paper,large single crystal diamond with perfect shape and high nitrogen concentration approximately 1671-1742 ppm was successfully synthesized by temperature gradient method (TGM) under high pressure and high temperature (HPHT).The HPHT synthesis conditions were about 5.5 GPa and 1500-1550 K.Sodium azide (NaN3) with different amount was added as the source of nitrogen into the synthesis system of high pure graphite and kovar alloy.The effects of additive NaN3 on crystal growth habit were investigated in detail.The crystal morphology,nitrogen concentration and existing form in synthetic diamond were characterized by means of scanning electron microscope (SEM) and infrared (IR) absorption spectra,respectively.The results show that with an increase of the content of NaN3 added in the synthesis system,the region of synthesis temperature for high-quality diamond becomes narrow,and crystal growth rate is restricted,whereas the nitrogen concentration in synthetic diamond increases.Nitrogen exists in diamond mainly in dispersed form (C-centers) and partially aggregated form (A-centers).The defects occur more frequently on crystal surface when excessive NaN3 is added in the synthesis system.     

Three-dimensional laser-induced thermal and stress analyses in diamond/ZnSe system

Thermal and stress responses of diamond/ZnSe system working as a IR window illuminated by a continue-wave (CW) laser have been investigated by means of finite element method (FEM). We find that the temperature and stress fields have a significant dependence on thickness of the diamond coating. If the increase of the diamond thickness, the laser-induced steady state temperature increases dramatically due to more laser power absorbed, which induces high stress in the interface between diamond and ZnSe. We further calculate the variation of the temperature and stress fields with changing the structural parameters in diamond/ZnSe system.     

Finite element modeling of laser-induced three-dimensional Transient Thermal Grating in two-layered systems

The three-dimensional (3D) temperature field of laser-induced transient thermal grating (TTG) for two-layered structures such as diamond/substrate is obtained by the finite element method (FEM). The numerical results indicate that the thickness of the diamond film has a significant influence on the temperature field. On the other hand, we further find that variation of the substrate significantly affects on the temperature distribution in the diamond film, which has been ascribed to the various thermal conductivities of the substrates.     

Laser heating in diamond anvil cells: developments in pulsed and continuous techniques

Developments in continuous and pulsed laser‐heating techniques, and finite‐element calculations for diamond anvil cell experiments are reported. The methods involve the use of time‐resolved (5 ns gated) incandescent light temperature measurements to determine the time dependence of heat fluxes, while near‐IR incandescent light temperature measurements allow temperature measurements to as low as 500 K. Further optimization of timing in pulsed laser heating together with sample engineering will provide additional improvements in data collection in very high P–T experiments.     

Inclusions in large diamond single crystals at different temperatures of synthesis

The inclusions in large diamond single crystals have effects on its ultimate performance, which restricts its industrial applications to a great extent. Therefore, it is necessary to study the inclusions systematically. In this paper, large diamond single crystals with different content values of inclusions are synthesized along the(100) surface by the temperature gradient method(TGM) under 5.6 GPa at different temperatures. With the synthetic temperature changing from 1200?C to 1270?C,the shapes of diamonds change from plate to low tower, to high tower, even to steeple. From the microscopic photographs of the diamond samples, it can be observed that with the shapes of the samples changing at different temperatures, the content values of inclusions in diamonds become zero, a little, much and most, correspondingly. Consequently, with the temperature growing from low to high, the content values of inclusions in crystals increase. The origin of inclusions is explained by the difference in growth rate between diamond crystal and its surface. The content values of inclusions in diamond samples are quantitatively calculated by testing the densities of diamond samples. And the composition and inclusion content are analyzed by energy dispersive spectroscopy(EDS) and x-ray diffraction(XRD). From contrasting scanning electron microscopy(SEM) photographs, it can be found that the more the inclusions in diamond, the more imperfect the diamond surface is.     

Homogenization of 3D finite photonic crystals with heterogeneous permittivity and permeability

We consider a heterogeneous magneto-dielectric photonic crystal and derive the so-called ‘homogenized Maxwell system’ via the multi-scale method and provide ad hoc proofs for the convergence of the electromagnetic field towards the homogeneous one using the notion of two-scale convergence. The homogenized medium is described by anisotropic matrices of permittivity and permeability, deduced from the resolution of two annex problems of electrostatic type on a periodic cell. Noteworthily, this asymptotic analysis also covers the case of photonic crystals with non-cuboidal periodic cells. We solve numerically the associated system of partial differential equations with a method of fictitious charges and a finite element method (FEM) in order to exhibit the matrices of effective permittivity and permeability for given magneto-dielectric periodic composites. We then compare our results in the 2D case against some Fourier expansion approach and provide duality relations in the case of magneto-dielectric checkerboards. We further compute some low-frequency eigenmodes of a photonic crystal fiber with metallic outer boundary and compare them with the eigenmodes of a corresponding effective anisotropic waveguide, thanks to the FEM. Finally, we derive the effective properties of a 3D photonic crystal both through classical homogenization (solving numerically two decoupled annex problems) and Bloch wave homogenization. In the case of spherical inclusions, the latter approach amounts to evaluating the slope of the first band around the origin on a Bloch diagram which we compute using finite edge elements.     

Shape-controlled synthesis of diamond crystal by epitaxial growth under high pressure and high temperature conditions

In this paper, the diamond epitaxial growth mechanism has been studied in detail by employing several types of diamond as a seed in a catalyst-graphite system under high pressure and high temperature (HPHT) conditions. We find that the diamond nucleation, growth rate, crystal orientation, and morphology are significantly influenced by the original seeds. The smooth surfaces of seeds are beneficial for the fabrication of high-quality diamond. Our results reveal that the diamond morphology is mainly determined by the original shape of seeds in the early growth stage, but it has an adjustment process during the growth and leads to well symmetry. Additionally, we have also established the growth model for the twinned diamond grown on several seeds, and proposed the possible growth processes by tracking the particular shapes of seeds before and after treatment under HPHT conditions. These results suggest that the shape-controlled synthesis of diamond with well morphology can be realized by employing certain suitable diamond seeds. This work is expected to play an important role in the preparation of trustworthy diamond-based electronic and photonic devices.     

B2O3添加宝石级金刚石单晶的生长特性

利用温度梯度法, 在5.3-5.7 GPa压力、1200-1600 ℃的温度条件下, 将B₂O₃粉添加到FeNiMnCo+C合成体系内, 进行B₂O₃添加宝石级金刚石单晶的合成. 研究得到了FeNiMnCo触媒生长B₂O₃添加宝石级金刚石单晶的相图分布规律. 结果表明B₂O₃添加会使晶体生长的“V”形区上移和低温六面体单晶生长区间变宽. 通过晶体生长实验, 研究合成了不同形貌的B₂O₃添加宝石级金刚石单晶. 研究同时证实, B₂O₃的过量添加会对宝石级金刚石单晶生长带来不利影响. 当B₂O₃的添加量高于约3 wt‰、生长时间超过20 h时, 很难实现优质B₂O₃添加宝石级金刚石单晶的生长. 但B₂O₃的适量添加(不超过1 wt‰), 有助于提高低温板状六面体宝石级金刚石单晶的成品率. 通过对晶体生长速度的研究发现, B₂O₃的添加使得优质晶体的生长速度明显降低, 随着晶体生长时间的延长, B₂O₃添加剂对晶体生长的抑制作用会越发明显. 扫描电镜测试结果表明, 合成体系内B₂O₃添加剂的引入, 导致晶体表面的平整度明显下降.     

灯丝温度对原位吸收光谱和金刚石薄膜生长的影响

 报道了原位红外吸收光谱在气相合成金刚石薄膜生长过程中的应用，研究了灯丝温度对原位红外吸收光谱和金刚石薄膜生长的影响。较高的灯丝温度使甲烷分解更充分，从而产生更多诸如C₂H₂等可能对金刚石薄膜生长有利的基团，导致金刚石薄膜质量和生长速率的提高。     

水热金刚石压腔结合拉曼光谱技术进行Na2SO4-水体系溶解结晶动力学研究

成矿作用过程中,温压条件改变导致矿物溶解重结晶,溶液中溶质浓度发生变化。从溶液中析出晶体的粒度同时存在着时间和空间的分布,是复杂的动力学过程。当前对矿物在流体中溶解重结晶动力学研究主要使用高压釜或活塞圆筒等封闭设备测定溶液溶质浓度的变化或固相矿物的形态变化,降温淬火反应会影响样品的真实组成。使用可进行原位观测的金刚石压腔结合拉曼光谱分析技术,研究无水芒硝-饱和Na₂SO₄溶液随体系温度压力变化所出现的晶体溶解重结晶过程。通过原位观测无水芒硝溶解、结晶变化来探究硫酸钠晶体在不同温压条件下的溶解结晶动力学反应机制。结果表明常温条件下无水芒硝拉曼位移分别位于449.9,620.5,632.9,647.4,993.3,1 101.8,1 132.2和1 153.1 cm-1。随着体系温度的缓慢升高,固相Na₂SO₄的晶形不断发生变化,温度至193 ℃时无水芒硝（Na₂SO₄）完全溶解,降温重结晶出现了新的1 196.5 cm-1拉曼特征峰,重结晶晶体为芒硝（Na₂SO₄·10H₂O）;金刚石原位观测结果显示迅速升温过程中无水芒硝发生部分溶解重结晶,重结晶区域拉曼特征峰显示依然为无水芒硝。拉曼光谱定量分析结果显示,溶液中SO2-₄,H₂O的拉曼谱峰面积比值参数更能反映SO2-₄浓度的变化, 体系达到溶解重结晶平衡状态时,SO2-₄/H₂O峰面积比值AR为(0.016 6±0.000 4),误差为2.4%。应用Johnson-Mehl-Avrami-Kolmogorov（JMAK）模型结合溶液中SO2-₄/H₂O峰面积比值变化对体系中固相无水Na₂SO₄的溶解重结晶过程进行动力学拟合,计算得出无水硫酸钠在109 ℃条件下的溶解结晶反应的反应级数为1.266 7,反应平衡常数为0.001 26。综上所述,水热金刚石压腔装置实验步骤少,过程简便,可避免由于淬火过程中退化交换作用等造成的误差,应用水热金刚石压腔原位观测的装置优势结合拉曼光谱定量分析技术可实现高温高压条件下矿物在水溶液中溶解结晶动力学过程的原位观察和测定,是一种高效的动力学研究手段。     

Effects of Al and Ti/Cu on Synthesis of Type-IIa Diamond Crystals in Ni70Mn25Co5-C System at HPHT

High-quality type-Ⅱa gem diamond crystals are successfully synthesized in a NiToMn25Co5-C system by temperature gradient method （TGM） at about 5.5 GPa and 1560 K. Al and Ti/Cu are used as nitrogen getters respectively. While nitrogen getter Al or Ti/Cu is added into the synthesis system, some inclusions and caves tend to be introduced into the crystals. When Al is added into the solvent alloy, we would hardly gain high-quality type-Ⅱa diamond crystals with nitrogen concentration Nc 〈 1 ppm because of the reversible reaction of Al and N at high pressure and high temperature （HPHT）. Piowever, when Ti/Cu is added into the solvent alloy, high-quality type-Ⅱa diamond crystals with Nc 〈 1 ppm can be grown by decreasing the growth rate of diamonds.     

用元胞自动机方法模拟镁合金薄带双辊铸轧过程凝固组织

用元胞自动机方法模拟铸轧凝固组织,采用有限元方法模拟铸轧过程中熔池内部的宏观传输现象,将二者耦合模拟了镁合金铸轧薄带凝固过程中晶粒的形核与长大过程.阐明了薄带铸轧工艺过程中的主要工艺参数（浇注温度、铸轧速度等）对镁合金薄带凝固组织中晶粒大小、取向等的影响规律.这为通过工艺优化来控制铸轧薄带的凝固组织提供了理论依据. 关键词： 双辊铸轧 镁合金 凝固组织 元胞自动机     

Simulation of continuous terahertz wave transient state thermal effects on static water

We report a numerical simulation of continuous terahertz beam induced transient thermal effects on static water. The terahertz wave used in this paper has a Gaussian beam profile. Based on the transient heat conduction equation, the finite element method (FEM) is utilized to calculate the temperature distribution. The simulation results show the dynamic process of temperature change in water during terahertz irradiation. After about 300 s, the temperature reaches a steady state with a water layer thickness of 5 mm and a beam radius of 0.25 mm. The highest temperature increase is 7 K/mW approximately. This work motivates further study on the interaction between terahertz wave and bio-tissue, which has a high water content.     

Prediction of photonic crystal fiber characteristics by Neuro–Fuzzy system

The most common methods applied in the analysis of photonic crystal fibers (PCFs) are finite difference time/frequency domain (FDTD/FDFD) method and finite element method (FEM). These methods are very general and reliable (well tested). They describe arbitrary structure but are numerically intensive and require detailed treatment of boundaries and complex definition of calculation mesh. So these conventional models that simulate the photonic response of PCFs are computationally expensive and time consuming. Therefore, a practical design process with trial and error cannot be done in a reasonable amount of time. In this article, an artificial intelligence method such as Neuro–Fuzzy system is used to establish a model that can predict the properties of PCFs. Simulation results show that this model is remarkably effective in predicting the properties of PCF such as dispersion, dispersion slope and loss over the C communication band.