Experimental study of stress relaxation during annealing

In order to calculate the residual stresses in a flat sheet of glass during annealing on the basis of its thermal history, the viscosity and coefficient of thermal expansion must be determined throughout the transformation range. The structural modifications of the material are taken into account by using fictive temperature concept. Three constitutive assumptions for the rate of the fictive temperature are discussed. A very good agreement is obtained between measured and calculated residual stresses.     

负热膨胀铁电晶体研究进展

近些年来,负热膨胀材料已成为材料科学的研究热点之一.本文总结了负热膨胀材料发展历程及负热膨胀机理,重点介绍钙钛矿结构和钨青铜结构铁电晶体的生长和热膨胀特性,探讨了晶体结构、铁电相变及负热膨胀机理的关系,对负热膨胀铁电晶体研究和应用提出了建议.     

Polymorphic Modifications of Chitosan

This work describes the analysis of the crystal structures of chitosan, its main polymorphic modifications, and its characteristic mutual chain packing and hydrogen bond systems in the crystalline regions of conformers. The analysis takes into consideration the crystal structures of chitosan complexes (salts) with organic and inorganic acids and their structural transformations. Notably, the transformation of the hydrated form of chitosan into anhydrous is found to be irreversible and occurs either at high temperatures or through a less stable form of hydrated salts. The interaction with polyanions during the formation of multilayer films can be considered as a way to form the anhydrous crystalline form of chitosan.     

Control of crystal growth in bridgman furnace

Control of crystal quality during crystal growth requires accurate implementation of thermal boundary conditions. We identify this problem as the furnace temperature control problem. The thermal boundary conditions, in turn, dictate the interface shape between the solid and the liquid region of the material. Determination of the boundary conditions for a given desired interface shape is considered as the material temperature control problem in this paper. We outline the current efforts for the solution of the furnace temperature control and the material temperature control problems. We restrict our review to Bridgman growth control techniques.     

Curie symmetry principle and growth of single crystals

Application of the symmetry principle, proposed by Pierre Curie, to the growth of single crystals is considered. The basic concepts of this principle and the rules for alignment of the crystallographic symmetry of a seed crystal with the external symmetry of the thermal field formed by the crucible with a melt are shown. The modified Stepanov method, developed at the General Physics Institute, Russian Academy of Sciences, on the basis of the Czochralski and Stepanov methods, is described, and its advantages are demonstrated. Examples of application of this technique to the growth of complex oxide crystals are given.     

蓝宝石晶体热性能的各向异性对SAPMAC法晶体生长的影响

采用有限元法对冷心放肩微量提拉法蓝宝石晶体生长过程中晶体内的温度、应力分布进行了模拟计算,结合实验结果讨论了蓝宝石晶体热性能的各向异性对晶体生长的影响.研究结果表明,对于冷心放肩微量提拉蓝宝石晶体生长系统,较大的轴向热导率有利于提高晶体的生长速率和界面稳定性,而稍大的径向热导率则有利于保持微凸的生长界面.晶体内的热应力受径向热膨胀系数的影响显著,随着径向热膨胀系数的增大而增大,最大热应力总是出现在籽晶与新生晶体的界面区域.在实验中选α轴为结晶取向,成功生长出了直径达230mm、高质量蓝宝石晶体.     

Investigation of the effect of coefficient of thermal expansion on prediction of refractive index of thermally formed glass lenses using FEM simulation

During the cooling phase of thermal forming processes, the properties of glass undergo minor changes. These changes are a result of structural relaxation near the glass transition region. One such property, refractive index, is the most important material property that governs the optical performance of a glass lens. Therefore, it must be properly considered prior to manufacturing. A finite element model is established in this research to simulate structural relaxation behavior of glass during cooling. This is done in order to accurately predict the refractive index change which occurs during and after the forming process. This study shows that the success of the simulation model depends largely on the accuracy of material information available to the lens manufacturers. Specifically, the comparison between simulated and experimental data demonstrates that the published information for the generic coefficient of thermal expansion is inadequate for precisely predicting volume change. To resolve the issue, a reverse calculation method was presented. In addition, simulated finite element results showed that the value of the liquid coefficient of thermal expansion has a major effect on the simulated results, as compared to fraction parameter.     

Epitaxial growth of ferroelectric oxide films

This review paper begins with a brief overview of the most common ferroelectric materials, the perovskites and the Aurivillius families. The epitaxial growth of ferroelectric epitaxial films can be a viable approach to improve the ferroelectric properties, in particular for the layered perovskites. Defects related to a polycrystalline structure, which lead to a degradation of ferroelectric properties like remanent polarisation, piezoelectric coefficient, charge retention, and may cause time-dependent fatigue problems, can be prevented. However, it is also to be considered that effects connected with thin films like substrate clamping, strain or finite thickness may limit the film properties. Substitution of elements allows the adjustment of the film characteristics to the device function. Additionally, the orientation of the films can be controlled by the appropriate choice of the substrate, which is important due to the anisotropy of the ferroelectrics.The deposition methods commonly used for ferroelectric oxide layers are reviewed, particularly with regard to epitaxial growth. The conditions under which stoichiometric, crystalline growth can be obtained are described. The paper primarily focuses on the MO-CVD technique.Furthermore whether epitaxial growth of ferroelectric films occurs or not depends on several conditions like lattice mismatch between film and substrate, surface orientation and crystal symmetry of the substrate, thermal expansion of film and substrate. The influence of these parameters on epitaxial growth is discussed. Local epitaxial growth of ferroelectric layers on metallic electrodes is also mentioned due to its importance in device fabrication. The site-engineering concept is shortly reviewed as the substitution of elements constitutes a simple way to modify film properties in thin film technology.     

Peculiarities of the melt growth of metal single crystals with specified crystal-geometric and structural parameters

This paper describes the authors' modifications of Kapitza and Bridgman apparatus and growth methods of metal single crystals with specified crystal geometric and structural parameters. The effects associated with defect formation are discussed. The features of the effects in their relation to the crystal lattice types and crystallographic orientation are indicated. It is concluded that one of the main factors responsible for the defect formation during single crystal growth in the mould is strain. This is due to thermomechanical stresses arising during the crystal growth and cooling to room temperature, from the difference in thermal expansion coefficients of the crystal and mould materials.     

近零膨胀Zr0.5Hf0.5V1.4P0.6O7材料的制备及其性能研究

采用传统的固相法合成了近零膨胀氧化物功能陶瓷材料Zr_0.5Hf_0.5V_1.4P_0.6O₇,用X射线衍射(XRD)、Raman光谱和热膨胀法对Zr_0.5Hf_0.5V_1.4P_0.6O₇的热膨胀系数、各向同性和相变进行了测试,通过Hf⁴⁺/P⁵⁺共掺杂使得材料具有较低的热膨胀系数,研究发现合成的Zr_0.5Hf_0.5V_1.4P_0.6O₇具有Pa3立方相结构,从334 K附近到673 K较宽的温度范围内的线性热膨胀系数为-1.56×10^-6 K^-1,表现出稳定的近零热膨胀特性。由于固溶体内部微结构的影响造成膨胀仪实验结果与变温X射线衍射结果存在一定的差距。Zr_0.5Hf_0.5V_1.4P_0.6O₇具有的近零膨胀特性为通过负热膨胀材料合成膨胀系数可控的材料提供了基础。     

Effect of crystal content on the properties of willemite glass-ceramics

Helium permeability, thermal expansion properties, and density have been measured for a series of glass-ceramics based on willemite as a lone crystalline phase. These glass-ceramics were prepared such that the compositions of both the glassy and crystalline phases were fixed, and only the relative concentrations of glass and crystal varied. Results of this study demonstrate that the helium permeability and glass transformation temperature of glass-ceramics are controlled by the glassy phase composition, whereas the density, thermal expansion coefficient, and dilatometric softening temperature are a function of both phases. All of these conclusions can be explained readily by simple continuity and mixing arguments.     

Combination of colored patterns and cubes for crystallographic point group visualization

A concept based on the application of a cube as a general polyhedron is presented for the visualization of point group symmetry. The cube is used to represent both the highest and lowest crystal symmetries, with differences observable as patterns characteristic to each point group. Patterns are generated using direction‐specific color keys, which enable the recognition of point group‐specific distribution of vectors in an external reference frame. For the visualization of the incoherent hexagonal crystal classes, two twinned cubes are applied in order to generate symmetry operators that would otherwise be missing. The resulting hexagonal dipyramid is described in the frame of a cube, reducing the number of used symmetry operators from 72 to 60. The complete set of 32 polyhedra are suitable, for example, as a visual aid for understanding the crystal symmetry and/or sub‐ and supergroup relationships.     

A Facile Method to Pull Crystals

A facile crystal puller based on thermal expansion property of metal was set up. In order to enlarge the displacement produced by the thermal expansion rod, a lever with 100 cm long was utilized. As a demonstration, a NaCl crystal of 45 mm in length and about 12 mm in diameter was successfully grown by using this apparatus.     

A novel method of temperature control for a crystal growth puller

A facile method to control the contracting rate of the thermal expansible bars for pulling crystal is first suggested. The thermal expansible bars, set in a modified Dewar flask whose vacuum degree is controlled, are heated to designed temperature and then switch off the power to let it cool down at a desired rate, which depends largely on the changeable vacuum degree. This new approach is expected to completely eliminate the effects, which possibly reduces the smooth extent of thermal expansion, caused by the minor temperature fluctuations during crystal growth process and to realize the utmost smooth and slow pulling rate. It is expected to install this apparatus in optical floating zone furnace, instead of traditional motor, to grow peritectic crystal, such as crystal Bi‐2223, since for the peritectic reaction, in principle, the extremely slow growing rate is considerably essential.     

Bridgman法晶体生长的热物性各向同性坩埚的选择

本文对不同坩埚热物性组合时计算得到的结果进行了比较.对各向同性坩埚而言,应该优先选择具有与晶体和熔体的导热系数相当导热系数的材料,也可选择导热系数较大的材料.在强度允许的情况下,减小坩埚壁厚对晶体生长有利.     

Solid mechanics and material strength studies on the melt growth of bulk single crystals

Microdefects such as dislocations and macrocracking should be controlled during the crystal growth process to obtain high-quality bulk single crystals. Solid mechanics and material strength studies on the single crystals are of importance to solve the problems related to the generation and multiplication of dislocations and the cracking of single crystals. The present paper reviews such research activities that comprise the thermal stress analysis during crystal growth process, the dislocation density estimation during crystal growth process, and the cracking of single crystal due to thermal stress.     

Thermal properties of a Nd:LuVO4 crystal

A Nd‐doped lutetium orthovanadate Nd:LuVO₄ crystal has been grown using a modified Czochralski method. The thermal properties of this crystal have been studied by measuring the thermal expansion, specific heat and thermal diffusivity. The thermal expansion coefficients are α₁₁ = 1.7 × 10^‐6, α₂₂ = 1.5 × 10^‐6 and α₃₃ = 9.1 × 10^‐6/K in the temperature range of 298–573 K along the three respective crystallographic axes. The specific heat is almost linear and increases from 0.442 to 0.498 Jg^‐1K^‐1 in the measured temperature range. The thermal diffusivity is anisotropic and decreases with increasing temperature from 295 to 548 K. At room temperature the calculated thermal conductivities κ₁₁ and κ₃₃ are 7.96 and 9.77 Wm^‐1K^‐1, respectively. These thermal parameters of Nd:LuVO₄ crystal have indicated that it is an excellent candidate laser material. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Symmetrical polyhedra (simple crystal forms) as orbits of noncrystallographic point symmetry groups

Simple crystal forms are analyzed as the orbits of noncrystallographic point symmetry groups on a set of smooth or structured (“hatched”) planes of crystal space. Polyhedra with symmetrically equivalent faces, obtained using noncrystallographic point symmetry groups, are considered. All possible versions of simple forms for all noncrystallographic groups are listed in a unified table.     

Thermal expansion of amorphous metals

The thermal expansion of three commercial amorphous metals has been measured from room temperature to above the crystallization temperature. The results clearly demonstrated the glassy nature of these materials and show the reduction in volume which results from crystallization. The glass transformation temperature is more evident here than on the usual differential scanning calorimeter curves. The activation energies associated with the glass transformation and crystallization are derived from data taken as a function of heating rate. The measured temperatures for the glass transformation and crystallization from the thermal expansion measurements are the same as those obtained from DSC measurements at the same heating rate.