首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 0 毫秒
1.
掺钛蓝宝石晶体(Ti∶Al2O3,简称钛宝石)是实现飞秒超短脉冲激光和拍瓦级超强超短激光器的核心材料。采用热交换法(HEM)生长大尺寸优质钛宝石激光晶体时,内辐射传热对晶体生长过程中的热量输运、温度及热应力分布具有显著影响,最终影响晶体质量。因此,本文采用有限体积法求解热交换法钛宝石晶体生长过程中晶体和熔体的内辐射传热,采用基于位移的热弹性应力模型求解晶体热应力,详细研究了内辐射传热对晶体温度及热应力分布的影响规律。数值模拟结果表明,内辐射传热显著强化晶体、熔体内的热量输运,导致晶体底部等温线密集分布,温度梯度和热应力显著升高。此外,随着晶体吸收系数(掺杂浓度)增大,晶体底部热应力呈现先升高后降低的非单调性变化规律。随着熔体吸收系数增大,晶体底部温度梯度和热应力略微降低。而随着晶体散射系数增大,晶体底部温度梯度和热应力逐渐降低,当晶体散射系数大于晶体吸收系数时,降低趋势变得显著。  相似文献   

2.
针对热交换法蓝宝石晶体各生长阶段的温场、流场和热应力进行数值模拟研究,并讨论了上部保温层结构、热交换器内管高度对晶体生长的影响.结果表明:长晶初期,固液界面呈椭球形;等径阶段,固液界面平坦,晶体与坩埚壁不接触;长晶后期,中心轴向晶体生长速率增加,晶体中心首先冒出熔体液面.随晶体高度增加,熔体对流由初期的两个涡胞变为等径阶段的一个涡胞,最大对流速度量级为10-3 m/s.晶体中最大热应力分布在晶体底部,热应力分布呈W型.增加炉体上部保温层,长晶后期固液界面变得平坦;降低热交换器内管高度,有利于降低晶体底部热应力.  相似文献   

3.
    
For a feasibility study of energy‐dispersive residual stress analysis under laboratory conditions, an X‐ray diffractometer that has been operated so far in the angle dispersive diffraction mode was equipped with a commercial tungsten tube and an energy‐dispersive solid‐state germanium detector. Starting from systematic investigations to find the optimum configuration regarding geometrical resolution, measuring time and stability of the applied detector system, different materials were characterized with respect to the near‐surface residual stress state. The results achieved with the modified laboratory equipment within reasonable measuring times are in good agreement with synchrotron measurements performed on the same samples. With the example of a shot‐peened Al2O3 ceramic with a highly non‐uniform near‐surface residual stress distribution it is furthermore shown that the different size and shape of the diffracting gauge volume used for the laboratory and synchrotron measurements might have a significant influence on the experimentally obtained Laplace‐space residual stress depth profiles σ||(τ).  相似文献   

4.
    
EDDIDAT is a MATLAB‐based graphical user interface for the convenient and versatile analysis of energy‐dispersive diffraction data obtained at laboratory and synchrotron sources. The main focus of EDDIDAT up to now has been on the analysis of residual stresses, but it can also be used to prepare measurement data for subsequent phase analysis or analysis of preferred orientation. The program provides access to the depth‐resolved analysis of residual stresses at different levels of approximation. Furthermore, the graphic representation of the results also serves for the consideration of microstructural and texture‐related properties. The included material database allows for the quick analysis of the most common materials and is easily extendable. The plots and results produced with EDDIDAT can be exported to graphics and text files. EDDIDAT is designed to analyze diffraction data from various energy‐dispersive X‐ray sources. Hence it is possible to add new sources and implement the device‐specific properties into EDDIDAT. The program is freely available to academic users.  相似文献   

5.
    
The different approaches for separating residual stress and composition gradients introduced in the first part of this series [ Klaus & Genzel (2017). J. Appl. Cryst. 50 , 252–264 ] are demonstrated with the experimental example of a graded Ti(C,N) coating layer deposited by a modified high‐temperature chemical vapour deposition process on a cemented carbide substrate. The coating layer features a depth gradient in the lattice parameter in the strain‐free direction of the biaxial stress state, ψ*,hkl , and tensile residual stresses σ|| which are nearly uniform over the coating thickness but drop down significantly towards the free surface. On the assumption that the out‐of‐plane stress component σ33 can be neglected, the gradient is related to the variation in the chemical composition with depth. Therefore, the example considered here corresponds to case (d ) of possible combinations of residual stress and composition gradients discussed in the first part of this series. The comparison of the results achieved by means of the different methods reveals the importance of choosing appropriate experimental conditions that fit best to the sample to be investigated. For the case of thin‐film analysis, it is shown that the X‐ray information depth is the crucial parameter which should match the film thickness.  相似文献   

6.
    
Different X‐ray measurement and data evaluation concepts are presented, which allow for residual stress analysis in thin films with pronounced gradients in chemical composition. These gradients lead to a variation in the strain‐free lattice parameter a 0 with respect to the film thickness and superimpose the lattice strain induced by the film's inherent stresses. Non‐consideration of a 0(z ) gradients is shown to lead to considerable errors (`ghost stresses') in the residual stress depth profiles. With the simulated example of a TiCx N1−x film with a pronounced carbon gradient, the first part of this series introduces four approaches, which permit the separation of residual stress and composition depth distributions at different levels of approximation. They are based on lattice spacing depth profile measurements performed in either the sin2ψ mode or the scattering vector mode, or in combinations of these two scanning modes. Depending on the approach used for separating the residual stress and composition gradients, angle‐ or energy‐dispersive diffraction has to be applied, employing monochromatic X‐ray sources available in the laboratory or either white high‐energy synchrotron radiation or the Bremsstrahlung of conventional X‐ray tubes, respectively. The methods introduced here assume a biaxial residual stress state within the film. For a triaxial residual stress state with σ33 ≠ 0, a separation of stress and composition gradients is not straightforward, because an a 0(z ) gradient cannot be distinguished from the hydrostatic part of the stress tensor.  相似文献   

7.
    
In this paper two evaluation methods for X‐ray stress analysis by means of energy‐dispersive diffraction are reassessed. Both are based on the sin2ψ measuring technique. Advantage is taken of the fact that the dψhkl–sin2ψ data obtained for the individual diffraction lines Ehkl not only contain information about the depth and orientation dependence of the residual stresses, but also reflect the single‐crystal elastic anisotropy of the material. With simulated examples, it is demonstrated that even steep residual stress gradients could be determined from sin2ψ measurements that are performed up to maximum tilt angles of about 45°, since the dψhkl–sin2ψ distributions remain almost linear within this ψ range. This leads to a significant reduction of the measuring effort and also makes more complex component geometries accessible for X‐ray stress analysis. Applying the modified multi‐wavelength plot method for data analysis, it turns out that a plot of the stress data obtained for each reflection hkl by linear regression versus the maximum information depth τψ=0hkl results in a discrete depth distribution which coincides with the actual Laplace space stress depth profile σ(τ). The sensitivity of the residual stress depth profiles σ(τψ=0hkl) to the diffraction elastic constants ½S2hkl used in the sin2ψ analysis can be exploited to refine the grain‐interaction model itself. With respect to the universal plot method the stress factors Fij which reflect the material's anisotropy on both the microscopic scale (single‐crystal elastic anisotropy) and the macroscopic scale (anisotropy of the residual stress state) are used as driving forces to refine the strain‐free lattice parameter a0 during the evaluation procedure.  相似文献   

8.
反向流动垂直喷淋式MOCVD反应器设计与数值模拟   总被引:8,自引:3,他引:5  
徐谦  左然  张红 《人工晶体学报》2005,34(6):1059-1064
本文分析了现有的MOCVD反应器存在的不足,提出了一种新型的反向流动垂直喷淋式反应器:反应气体从基片上方的许多平行小喷管喷入反应区,反应后的尾气又从基片上方出口排出,从而减少了反应物浓度沿衬底径向的不均匀性.通过对反应器进行三维数值模拟,改变喷管的中心距、喷管端与衬底的距离、流量、气体压强等参数,确定了反应室内衬底上方温度场与浓度场为最佳时的参数组合.  相似文献   

9.
    
A method for the evaluation of strongly inhomogeneous residual stress fields in the near‐surface region of polycrystalline materials is introduced, which exploits the full information content contained in energy‐dispersive (ED) diffraction patterns. The macro‐stress‐induced diffraction line shifts ΔEψhkl observed in ED sin2ψ measurements are described by modeling the residual stress state σij(z) in real space, based on Rietveld's data analysis concept. Therefore, the proposed approach differs substantially from currently used methods for residual stress gradient analysis such as the `universal plot' method, which enable access to the Laplace stress profiles σij(τ). With the example of shot‐peened samples made of either 100Cr6 steel or Al2O3, it is demonstrated that the simultaneous refinement of all diffraction patterns obtained in a sin2ψ measurement with hundreds of diffraction lines provides very stable solutions for the residual stress depth profiles. Furthermore, it is shown that the proposed evaluation concept even allows for consideration of the residual stress component σ33(z) in the thickness direction, which is difficult to detect by conventional sin2ψ analysis.  相似文献   

10.
    
In this work the momentum and heat transfer on a Bridgman system for the growth of GaSb has been studied. The main objective was to obtain some information about the role of the different processes like conduction, radiation and convective effects both in the melted material and the surrounding environment. These simulations are based on a 2D axi‐symmetrical model using a finite element method based code. The simulations have been carried out both in steady and transient states. It has been demonstrated that the consideration of a moving environment is important in the distribution of temperatures. The effects of the variations of thermal conductivities and emisivities on the thermal and velocity fields have been investigated. The results show that the key parameters are the thermal conductivities of the different materials present in the system, which produce significant changes in the convective flows inside the melt.  相似文献   

11.
本文采用专业晶体生长模拟软件CrysVUn对泡生法生长大尺寸蓝宝石单晶进行了计算机模拟.分析了气体压力对泡生法生长蓝宝石单晶的温场、气体速度场的影响.结果发现在气压P≥105Pa时,特别是在引晶初期,容易出现籽晶根部熔化,这在实验中得到了很好的验证,并提出了三种解决方案.  相似文献   

12.
    
Thermal properties play a vital role particularly for the materials which are used in high power laser irradiation. Photoacoustic technique is very much feasible to findout the thermal transport properties of solid materials. In the present work, nonlinear optical (NLO) single crystal of L‐tartaric acid (LTA) was grown by Sankaranarayanan‐Ramasamy (SR) method for dimensions of 140 mm in length and 11.5 mm in diameter. The grown LTA crystal has been subjected to UV‐Vis‐NIR spectral study to analyze the optical transmittance and absorbance characteristics. The thermal characterization studies were performed using photoacoustic spectrometer (PAS). Thermal characterization involves measurement of thermal parameters such as thermal diffusivity, thermal effusivity, thermal conductivity and specific heat capacity. The experimental results of photoacoustic spectrometer show that the thermal diffusivity of LTA is higher than the reported values of a few other well known NLO materials.  相似文献   

13.
    
This paper presents a set of numerical simulations on the effect of rotation on thermal convection in a shallow model Czochralski configuration with a heated crucible bottom. Results show that when the bottom is heated by low heat flux, the temperature fluctuation can be significantly weakened and the melt can be kept from solidifying. Once the heat flux exceeds a threshold value, the thermocapillary convection will transit to the Benard–Marangoni convection. The temperature fluctuation is amplified by increasing crystal rotation rate or heat flux. The crystal rotation can restrain the azimuthal traveling of the waves. However, the propagating direction of the waves is related to the temperature gradient and the crucible rotation, and is independent of the crystal rotation. The wavenumber is reduced by the crystal rotation individually. Furthermore, the crucible rotation can reduce the effect of the crystal rotation on the wavenumber.  相似文献   

14.
    
In this work, we present a theoretical analysis of the behaviour of the temperature profile for the Modified Markov Method. We have assumed different materials and different thicknesses for the insulating element. Comparison of preliminary results with experimental evidences, demonstrate us the reliability of our model. Discussion among our calculated results are provided and future outlooks for the experimental work are fixed. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

15.
    
On the basis of the theoretical concept for the use of small gauge volumes to study near‐surface residual stress fields with high spatial resolution [Meixner, Klaus & Genzel (2013). J. Appl. Cryst. 46 , 610–618], the experimental implementation of the approach is demonstrated. It is shown that specifically designed slit systems are required to avoid effects such as diffuse scattering at the slit blades and total external reflection, both giving rise to a reduced resolution. Starting from the characterization of the small gauge volume, practical guidance on how to control the alignment of the sample relative to the gauge volume for different geometrical conditions of energy‐dispersive diffraction is given. The narrow‐slit configuration as well as the formalism for data evaluation introduced in the first part of this series is applied to the analysis of a very steep in‐plane residual stress gradient in a shot‐peened Al2O3 ceramic sample. The results are compared with those obtained by means of a conventional wide‐slit setup using the classical universal plot method for residual stress analysis on the one hand, and with the simulations performed in the first part on the other hand.  相似文献   

16.
    
Exploiting the advantages of energy‐dispersive synchrotron diffraction, a method for the determination of strongly inhomogeneous residual stress depth gradients is developed, which is an enhancement of the stress scanning technique. For this purpose, simulations on the basis of a very steep residual stress depth profile are performed, and it is shown that conventional real space evaluation approaches fail, because they do not take into account the variation of the residual stresses within the gauge volume. Therefore, a concept facilitating the deconvolution of the diffraction signal by considering the effect of the gauge volume geometry as well as the influence of the material absorption on the average information depth is proposed. It is demonstrated that data evaluation requires a three‐dimensional least‐squares fit procedure in this case. Furthermore, possible aberrations and their impact on the analysis of the residual stresses by applying the `modified stress scanning' method are treated theoretically.  相似文献   

17.
    
The influence of the gauge volume size and shape on the analysis of steep near‐surface residual stress gradients by means of energy‐dispersive synchrotron diffraction is studied theoretically. Cases are considered where the irradiated sample volume is confined by narrow‐slit systems, in both the primary and the diffracted beam, to dimensions comparable to the `natural' 1/e information depth τ1/e of the X‐rays. It is shown that the ratio between τ1/e, defined by the material's absorption, and the immersion depth hGV of the gauge volume into the sample is the crucial parameter that shapes the dψhkl or ɛψhklversus sin2ψ distributions obtained in the Ψ mode of X‐ray stress analysis. Since the actual information depth 〈zGV to which the measured X‐ray signal has to be assigned is a superposition of geometrical and exponential weighting functions, ambiguities in the conventional plot of the Laplace stresses versuszGV may occur for measurements performed using narrow‐slit configurations. To avoid conflicts in data analysis in these cases, a modified formalism is proposed for the evaluation of the real space residual stress profiles σ||(z), which is based on a two‐dimensional least‐squares fit procedure.  相似文献   

18.
    
The modified stress scanning method [ Meixner, Fuss, Klaus & Genzel (2015). J. Appl. Cryst.48, 14511461 ] is experimentally implemented for the analysis of near‐surface residual stress depth distributions that are strongly inhomogeneous. The suggested procedure is validated by analyzing the very steep in‐plane residual stress depth profile of a shot‐peened Al2O3 ceramic specimen and comparing the results with those that were obtained by well established X‐ray diffraction‐based gradient methods. In addition, the evaluation formalism is adapted to the depth‐dependent determination of the residual stresses inside of multilayer thin‐film systems. The applicability for this purpose is demonstrated by investigating the residual stress depth distribution within the individual sublayers of a multilayered coating that consists of alternating Al2O3 and TiCN thin films. In this connection, the specific diffraction geometry that was used for the implementation of the stress scanning method at the energy‐dispersive materials science beamline EDDI@BESSYII is presented, and experimental issues as well as limitations of the method are discussed.  相似文献   

19.
    
The top‐seeded solution growth method is a promising technique for growing high‐quality silicon carbide single crystal. Some inherent issues in this growth process, such as morphological instability, polycrystalline growth, and low growth rate, should be clarified. A high temperature difference between the seed and the crucible wall in this system is needed to enhance growth. However, such a high temperature gradient makes the radial growth rate profile non‐uniform due to the effect of Marangoni convection below the seed crystal, which leads to poor crystal quality. In the present work, the effects of crucible size and crucible rotation are numerically investigated to minimize the effect of Marangoni convection. The possibilities of the occurrence of growth‐rate non‐uniformity and undesired impurity incorporation are examined. A smaller crucible (in radius) leads to a more uniform growth rate profile. However, it gives rise to a higher possibility of impurity incorporation. It is also predicted that crucible rotation is ineffective in suppressing the Marangoni flow near the seed edge. This leads to a flow stagnation in the center of the melt, and consequently, it does not enhance the carbon transport below the seed. It also does not reduce the possibility of undesired impurity incorporation.  相似文献   

20.
光伏发电以绿色、可再生、能源质量高和不受资源分布地域的限制等优点被广泛使用,单晶硅又以低衰减率和高转换效率等优点渐渐超过了多晶硅光伏电池在市场中的份额,但成本问题和产能问题一直束缚着单晶硅太阳能产业的发展.本文提出了一种在晶体生长过程中随硅液面下降而下降的直拉单晶炉热屏结构,来解决在拉晶过程中坩埚上升所造成的拉晶速度和...  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号