Internal stress,lattice deformation,and modulus of polymers

The relationships among internal stress, lattice deformation, and the moduli of polymers, such as poly(ethylene terephthalate) (PET), polypropylene (PP), polyethylene (PE), polyacryonitrile (PAN), and poly(p-phenylene terephthalamide) (PPTA), have been studied systematically. Compared to small molecule crystals, polymer crystals have many more defects and their parameters of the crystal unit cells and the density of the crystalline region are easy to vary to a certain extent. During the processing, external stresses were put on the polymer fibers and polymer molecules were drastically deformed, which induced polymer crystallization and preferred orientation. After processing, there are many macromolecules frozen in the nonequilibrium status, which is the cause of internal stress. Heat treatment could give energy to the macromolecules to release the internal stress and to approach thermodynamic equilibrium. For polymer crystals, the higher the annealing temperature, the shorter the lengths of the unit cell axes, the more integral and regular the crystal lattice, and the smaller the unit cell volume. Crystalline modulus changes not only with different kinds of polymer materials, but also with measurement temperature and processing history. Polymer processing, mainly drawing, causes orientation and internal stress which causes lattice deformation and raises tensile modulus of polymer materials.     

Evolution of the Microstructure of Dynamically Loaded Materials

The paper deals with the evolution of the microstructure in materials after explosive loading by the method of a hollow thick-walled cylinder. The materials considered differ in the type of crystal lattice and initial state (grain size and initial defect density). The role of crystal structure in the formation of the microstructure of single crystals and coarse-grain copper specimens formed under explosive deformation is investigated. The microstructures formed are compared with the corresponding strains. It is shown that during high-rate deformation, fragmentation of the structural elements occurs at all scale levels. The fragmentation mechanism and the associated properties depend on the initial structure and state of the material. The special features of the microstructure evolution in materials revealed in this work are taken into account in producing new materials by dynamic and quasidynamic methods.     

A method of characteristics for solving population balance equations (PBE) describing the adsorption of impurities during crystallization processes

Possible hindering effects of impurities on the crystal growth were shown to take place because of the adsorption of impurity species on the crystal surface. Transient features of this adsorption were observed, such that the growth of a given crystal does not depend on supersaturation only, but also on the time a given particle spent in contact with impurities present in the mother liquor. Meanwhile, few kinetic models describe the effect of impurities on the growth of crystals in solution, and published models are usually derived from data obtained, thanks to specific experiments based on the evaluation of the growth rate of single crystals. Such models are obviously questionable because, in the industrial practice, distributed properties of crystals are actually involved. Considering the “time of contamination” of particles as a new internal variable is thus made necessary. This is the reason why a specific PBE resolution algorithm is presented in this paper. The numerical scheme for the resolution of PBEs is based on the method of characteristics and shown to allow fast and accurate simulation of transient features of the crystal size distribution in the particular case when the growth or nucleation rates are assumed to exhibit unsteady-state dynamics. The algorithm is finally used to simulate the isothermal desupersaturation crystallization of citric acid in water.     

镁铟共掺杂铌酸锂晶体的生长及光学性能

在同成分铌酸锂(LiNbO3,LN)体中掺入3%(摩尔分数,下同)MgO,并分别掺入0.5%,1%,1.5%In2O3,用提拉法生长了一系列Mg:In:LN晶体.通过紫外坷见吸收光谱测试确定了晶体样品的组成和缺陷结构.通过透射光斑畸变法检测Mg:In:LN晶体抗光损伤能力.结果表明:Mg:In:LN晶体抗光损伤能力比纯LN晶体提高2个数量级.以波长为1 064nm的Nd:YAG激光为基频光源,对Mg:In:LN晶体的倍频性能进行了测试.结果表明:Mg:In:LN晶体的相位匹配温度在室温附近,Mg:In:LN晶体的倍频效率要高于In:LN晶体和Mg:LN晶体.     

Quality Evaluation of RDX Crystalline Particles by Confined Quasi‐Static Compression Method

A confined, quasi‐static uniaxial compression method, named as the compressive stiffness test (CST), is promoted to evaluate the quality of different RDX crystalline particles which are obtained from commercial and reprocessed lots. This method is based on the fact that the fragmentation or fracture behavior in the compression process could be correlated to the external/internal defects of targeted crystalline particles. The quality difference between the commonly used RDX and reprocessed RDX can be easily discriminated by compaction curves and the defined initial secant modulus can be used as a quantitative parameter to grade RDX particles. The results show that the RDX recrystallized from mixed solvents and spheroidized by treatment with solvent is of much higher quality than the commonly used RDX, and the particle size as well as external characteristics exert limited effects on the compaction curves, but the key factor is internal defects, which affect the coherence strength of RDX crystalline particles. Qualitative optical image analysis and quantitative particle apparent density measurements by liquid density gradient tubes also give consistent results with those from the CST.     

Investigation of HMX‐Based Nanocomposites

Advanced munition systems require explosives which are more insensitive, powerful, and reactive. For this reason, nano‐crystalline explosives present an attractive alternative to conventional energetics. In this study, formulations consisting of 95 % octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) and 5 % polyvinyl alcohol (PVOH) were prepared with mean crystal sizes ranging from 300 nm to 2 μm. The process to create these materials used a combination of mechanical particle size reduction and spray drying, which has the advantages of direct control of crystal size and morphology as well as the elimination of ripening of crystals (which occurs during slurry coating of nanomaterials). The basic physical characteristics of these formulations were determined using a variety of techniques, including scanning electron microscopy and X‐ray diffraction. Compressive stress‐strain tests on pressed pellets revealed that the mechanical properties of the compositions improved with decreasing crystal size, consistent with Hall‐Petch mechanics. The 300 nm HMX/PVOH composition demonstrated a 99 % and 129 % greater strength and stiffness, respectively, than the composition with 2 μm HMX. The formulations were subjected to the Small Scale Gap Test, revealing a significant reduction in shock sensitivity with decreasing crystal size. The formulation containing 300 nm HMX registered a shock initiation pressure 1.6 GPa above that of the formulation with 2 μm HMX, a 44 % improvement in sensitivity. These results serve to highlight the relevance of structure‐property relationships in explosive compositions, and particularly elucidate the substantial benefits of reducing the high explosive crystal size to nano‐scale dimensions.     

黑梯炸药力学性能与感度的分子动力学研究

为了研究黑梯炸药配方对其力学性能与感度的影响,用Materials Studio软件建立了黑梯炸药的晶胞模型。采用分子动力学方法,计算了不同配方的黑梯炸药的力学性能、引发键键长分布、键连双原子作用能与内聚能密度,并对其变化情况并进行了比较。结果表明,在黑梯炸药中,随着RDX的质量分数从30%增加到80%,黑梯炸药的力学性能参数在一定范围内波动,其中拉伸模量变化范围为1.772 3~2.825 1GPa,剪切模量变化范围为0.636 6~1.042 8GPa,体积模量变化范围为2.734 1~3.747 9GPa,柯西压变化范围为1.203 2~2.181 6GPa,泊松比变化范围为0.354 6~0.397 0,而最大键长从0.155 4nm增至0.162 6nm,键连双原子作用能从167.6kJ/mol减至152.3kJ/mol,内聚能密度从0.899kJ/cm~3减至0.678kJ/cm~3,表明炸药的感度增大。     

Growth strategy for controlling dislocation densities and crystal morphologies of single crystal diamond by using pyramidal-shape substrates

The growth of millimetre-thick diamond single crystals by plasma assisted CVD is complicated by the formation of unepitaxial defects, particularly at the edges of the crystal. These defects tend to encroach on the top surface hence limiting the maximum thickness to typically a few hundreds of micrometres. Dislocations are another type of defects that are also particularly formed at the edges of the crystal. They thread through the diamond film, strongly affecting its characteristics. The growth on pyramidal-shape substrates having different angles and orientations was carried out in an attempt to solve those issues. It was found that the pyramidal-shape tends to disappear after a certain thickness is grown. The inclined faces of the pyramid not only helped in preserving the crystal morphology over a large thickness but also deviated dislocations towards the edges of the crystal, hence limiting their occurrence at the surface. Using this strategy, millimetre-thick diamond single crystals presenting a reduced dislocation density were successfully grown.     

THE NARROWING OF CRYSTAL SIZE DISTRIBUTIONS IN A SONICATOR-MSMPR CRYSTALLIZER SYSTEM

Obtaining small crystals of a narrow crystal size distribution is an important problem in many pharmaceutical, biotechnological, and ceramic processing applications. This paper describes the use of a two-stage system to reproducibly produce pilot-scale quantities of a ceramic precursor powder.

The population balance is used to determine the crystal size distributions obtained from a staged crystallization system. The staged crystallization system consists of two units, a mininucleator (sonicator) and an MSMPR crystallizer. Experimental and theoretical studies of this system showed that the use of the mininucleator leads to narrowing of the crystal size distribution obtained from the MSMPR crystallizer. The resulting expression for the population density distribution allows direct fitting of experimental log population density data from the second stage to determine the growth and nucleation kinetics for both the mininucleator and the MSMPR crystallizer     

THE NARROWING OF CRYSTAL SIZE DISTRIBUTIONS IN A SONICATOR-MSMPR CRYSTALLIZER SYSTEM

Obtaining small crystals of a narrow crystal size distribution is an important problem in many pharmaceutical, biotechnological, and ceramic processing applications. This paper describes the use of a two-stage system to reproducibly produce pilot-scale quantities of a ceramic precursor powder.

The population balance is used to determine the crystal size distributions obtained from a staged crystallization system. The staged crystallization system consists of two units, a mininucleator (sonicator) and an MSMPR crystallizer. Experimental and theoretical studies of this system showed that the use of the mininucleator leads to narrowing of the crystal size distribution obtained from the MSMPR crystallizer. The resulting expression for the population density distribution allows direct fitting of experimental log population density data from the second stage to determine the growth and nucleation kinetics for both the mininucleator and the MSMPR crystallizer     

Processing–morphology–property relationships in epoxy resins

An investigation was conducted into processing–morphology–property relationships of a series of epoxy resin formulations. Diglycidyl ether of bisphenol A (DGEBA) epoxy resin was cured with diethylene triamine (DETA) and 2,5-dimethyl 2,5-hexane diamine (DMHDA). The two systems were compared by electron microscopic investigation and thermomechanical and fracture property measurements. Transmission electron microscopy has revealed a difference in the morphology of fracture surfaces. On the other hand, thermomechanical and fracture properties of DETA- and DMHDA-cured formulations were found to be very similar. Three different processing (curing) conditions were used for DMHDA-cured formulations, without an apparent effect on their properties. The previously reported improvement in impact energy of DMHDA-cured formulations is unfounded.     

湿法研磨制备改性HMX及其机械感度研究

通过湿法研磨制备出不同粒度的改性HMX晶体,采用折光匹配法对其形貌进行表征,并测试了改性HMX的机械感度。讨论了研磨速率对HMX粒径的影响,以及改性前后HMX的机械感度,分析了湿法研磨对HMX机械感度的影响机理。结果表明,研磨速率为0.524m/s时得到的HMX粒度最小,为43.1μm,改性后的HMX摩擦感度降低60%,撞击感度降低68%,且研磨晶体粒度越小,晶体特性落高数值越大。晶体粒度及内部缺陷对机械感度的影响机制主要是研磨后晶体粒度减小且晶体内部缺陷减少,受到外力作用时,晶体内部热点产生和传播的概率降低。     

Defects in banded spherulites of polymers

In order to investigate the detailed structure of a banded spherulite observed by polarized light microscopy, we develop a new image processing technique that can visualize defects (band defects) in the concentric bands and determine the growing directions of crystals everywhere in a spherulite. This technique is applied to a banded spherulite of poly(vinylidene fluoride) and reveals that the spherulite has many defects (colliding defects), on which crystals collide with neighboring ones. It is found that the band defects are included in the colliding defects. The number of colliding defects increases linearly with the radius to give a constant density. Between the defects, the orientations of crystals are well correlated to form a coherent area. On the basis of these findings, a mechanism of the formation of the coherent band pattern is discussed.     

The Gibbs Energy Minimization Approach in the estimation of detonation,explosion and work production parameters of confined charges

A method is suggested for the estimation of the physicochemical parameters associated with the entire phenomenon of detonation of explosive mixtures, exclusively containing the atomic species C,H,N,O, and expected to yield gaseous products. The equation of state applied for the products is a modified Abel equation. The calculation algorithm developed, constitutes the frame of construction of a computer program. The input required are the initial composition and apparent density of the explosive mixture and the final work production pressure. Sample applications for pure pentaerythrite tetranitrate detonation, TNT-ammonium nitrate-water slurries, and dry mixtures explosions are provided.     

6H-SiC单晶的生长与缺陷

采用升华法,在一定的温度、气体压力和流量的条件下,生长了尺寸ф50．8mm的6H—SiC单晶。利用光学显微术观察了原生晶体的表面形貌,发现了微管在晶体表面的露头点具有明显的多个螺位错成核特征。采用透射模式对抛光晶片进行观察,发现了SiC晶体内的典型缺陷,如：负晶、微管、碳颗粒等,并对它们的形成机理进行了讨论。     

粉煤灰制备轻质高强混凝土的试验研究

以烧结粉煤灰陶粒作为粗骨料,复掺超细粉煤灰与一级粉煤灰部分替代水泥作为胶凝材料,制备轻质高强混凝土.主要研究了两种粉煤灰的掺配比例与总掺量对轻骨料混凝土力学性能、干表观密度及微观形貌的影响.试验结果表明:掺超细粉煤灰能够细化水泥水化产物的晶体尺寸,打乱氢氧化钙的生长取向,减少混凝土内部结构缺陷,使胶凝材料浆体更均匀;当超细粉煤灰与一级粉煤灰的比例为1:1,粉煤灰的总掺量为40%时,可以配制出28 d抗压强度为58.6 MPa,干表观密度为1900 kg/m3的LC50轻质高强轻骨料混凝土.     

A Theoretical Analysis of the Formation of Nonstoichiometric Point Defects in SiC Single Crystals Grown under Equilibrium Conditions at Different Partial Pressures of Silicon Vapor

Formation of Schottky and Frenkel vacancy-type point defects in silicon carbide single crystals is analyzed. Relevant equations for numerical analysis are derived and a method for their solution is proposed. The partial pressure of silicon vapor over the growing crystal is shown to play a role in the formation of defects in SiC single crystals.     

熔铸炸药研究现状与发展趋势

熔铸炸药是目前战斗部最主要的装药方式之一,但是现有以TNT为载体的熔铸炸药配方在能量、安全性、装药质量和力学性能等方面存在明显缺陷。本文详细综述了熔铸炸药连续相、高能量密度材料、综合降感技术、流变性研究和装药工艺5个方面国内外的研究现状,特别是归纳了以NTO、DNTF等为代表的新型含能材料的应用情况,提出了熔铸炸药在新型载体物质、高能钝感单质炸药、共晶炸药、功能助剂、高固相含量熔铸体系装药工艺等方面未来的主要发展方向。     

锗酸铋晶体缺陷和小面生长

对坩埚下降法生长锗酸铋闪烁晶体的缺陷进行了研究，除使用通常研究透明晶体的光学方法，由于ＢＧＯ晶体在受到光辐照损伤对短波长光具有高的吸收系数，因此还采用近紫外光吸收形貌法研究晶体缺陷，以及缺陷与晶体小面之间的关系，并根据ＢＧＯ晶体的结晶习性和小面形成机理提出了减少和消除晶小面生长及缺陷的方法。     

Substrate crystal recovery for homoepitaxial diamond synthesis

Homoepitaxial chemical vapor deposition (CVD) of diamond requires high quality substrate crystals. This paper describes the process of diamond substrate crystal recovery so that the original substrate can be reused for multiple synthesis processes. A three-stage treatment is applied after homoepitaxial CVD growth. First the original substrate is separated by laser cutting, then the cut surface is mechanically polished, and finally polycrystalline material at the edges of the recovered seed plate is laser trimmed. This recovery process yields reusable diamond substrates that do not differ appreciably from their original state in terms of stresses and impurity concentrations. While the recovery process was demonstrated using HPHT seed substrates the process can also be applied to the as-grown CVD diamond plates. Infrared absorption spectral analysis, surface profilometry, birefringence imaging and Raman spectroscopy are performed after each processing step to monitor crystal quality. The nitrogen concentration in the substrate crystal remains constant throughout CVD and recovery processes. When using HPHT type Ib substrates the detected nitrogen concentration is 110–180 ppm. The nitrogen is mainly incorporated in form of C center defects and no transformation to other forms of defect centers occurs during the CVD process. Birefringence imaging showed a low level of internal stress within the HPHT crystals. No change is observed during CVD growth and recovery processes. It is shown that the polycrystalline rim removal is essential for repeatable CVD deposition on the same seed substrate. Substrate crystal recovery allows growth of up to 20 crystals from one original seed.