Neutron Diffraction Measurements and Micromechanical Modelling of Temperature‐Dependent Variations in TATB Lattice Parameters

Triaminotrinitrobenzene (TATB) is a highly anisotropic molecular crystal used in several plastic‐bonded explosive (PBX) formulations. TATB‐based explosives exhibit irreversible volume expansion (“ratchet growth”) when thermally cycled. A theoretical understanding of the relationship between anisotropy of the crystal, crystal orientation distribution (texture) of polycrystalline aggregates, and the intergranular interactions leading to this irreversible growth is necessary to accurately develop physics‐based predictive models for TATB‐based PBXs under various thermal environments. In this work, TATB lattice parameters were measured using neutron diffraction during thermal cycling of loose powder and a pressed pellet. The measured lattice parameters help clarify conflicting reports in the literature as these new results are more consistent with one set of previous results than another. The lattice parameters of pressed TATB were also measured as a function of temperature, showing some differences from the powder. This data is used along with anisotropic single‐crystal stiffness moduli reported in the literature to model the nominal stresses associated with intergranular constraints during thermal expansion. The texture of both specimens were characterized and the pressed pellet exhibits preferential orientation of (001) poles along the pressing direction, whereas no preferred orientation was found for the loose powder. Finally, thermal strains for single‐crystal TATB computed from lattice parameter data for the powder is input to a self‐consistent micromechanical model, which predicts the lattice parameters of the constrained TATB crystals within the pellet. The agreement of these model results with the diffraction data obtained from the pellet is discussed along with future directions of research.     

The Effect of Shear Strain on Texture in Pressed Plastic Bonded Explosives

The insensitive explosive PBX 9502 contains 95 wt‐% of TATB crystals and a plastic bonding agent (Kel‐F). The TATB crystals have plate‐like morphology, similar to that of graphite or boron nitride. We have used X‐ray diffraction to measure the preferred orientation (texture) of the TATB crystals in parts fabricated by pressing PBX 9502 powder. Independently, we have used finite‐element calculations to derive the direction and magnitude of the shear imposed during the consolidation of this composite material. Based on our results, we propose that the texture develops because the applied shear causes the TATB crystals to rotate such that their (002) basal planes are parallel to shear planes. The texture predicted by this model agrees qualitatively with that measured at various locations within the PBX 9502 compact. Further validation of this model is obtained by the measurement of the thermal expansion coefficient of PBX 9502, which is highly anisotropic.     

溶剂对FOX-7晶体相变和热性能的影响

采用重结晶法在二甲基甲酰胺(DMF)/H2O、二甲基亚砜(DMS)O/H2O和N-甲基吡咯烷酮(NMP)/H2O等不同溶剂体系中得到FOX-7晶体,用扫描电子显微镜(SEM)测试晶体的形貌、变温X-射线粉末衍射(XRD)分析晶型和相变、差示扫描量热仪(DSC)测试其热性能。结果表明,不同溶剂重结晶得到的FOX-7晶体形貌有较大差别,在DMSO/H2O溶剂中得到的晶体质量要优于其他两种;3种溶剂中得到FOX-7晶体的晶型和相变过程相同,即常温下FOX-7的晶型为α晶型,在120℃时,FOX-7完成α→β相变,至185℃时,完成β→γ相变;重结晶的FOX-7晶体5s爆发点温度提高了4~9℃,说明热稳定性增强。     

Density Distributions in TATB Prepared by Various Methods

The density distribution of two legacy types of 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB) particles were compared with TATB synthesized by new routes and recrystallized in several different solvents using a density gradient technique. Legacy wet (WA) and dry aminated (DA) TATB crystalline aggregates gave average densities of 1.9157 and 1.9163 g cm⁻³, respectively. Since the theoretical maximum density (TMD) for a perfect crystal is 1.937 g cm⁻³, legacy TATB crystals averaged 99% of TMD or about 1% voids. TATB synthesized from phloroglucinol (P) had comparable particle size to legacy TATBs, but significantly lower density, 1.8340 g cm⁻³. TATB synthesized from 3,5 dibromoanisole (BA) was very difficult to measure because it contained extremely fine particles, but had an average density of 1.8043 g cm⁻³ over a very broad range. Density distributions of TATB recrystallized from dimethylsulfoxide (DMSO), sulfolane, and an 80/20 mixture of DMSO with the ionic liquid 1‐ethyl‐3‐methyl‐imidazolium acetate (EMImOAc), with some exceptions, gave average densities comparable or better than the legacy TATBs.     

添加剂作用下阿司匹林结晶模拟和实验研究

晶体形貌作为晶体产品的重要质量指标,不仅会影响产品流动性、稳定性、溶出速率和生物可利用度等产品的质量指标,还会对过滤、干燥、压片等下游操作造成影响。通过分子模拟的方法指导阿司匹林冷却-反溶剂结晶过程的添加剂筛选,以添加剂作为晶体形貌的改性剂,降低阿司匹林晶体的长径比优化晶体形貌。通过单因素实验考查了添加剂浓度、晶种加入量、降温速率、搅拌速率和加水速率对阿司匹林晶体产品形貌、流动性和粒度分布等的影响,确定了较优的工艺条件。实验结果表明加入聚乙烯吡咯烷酮（PVP）作为添加剂可以降低阿司匹林晶体长径比,获得形貌为短棱柱状的晶体产品,能够显著改变晶体形貌优化产品的流动性。     

SEEDING EFFECT ON CRYSTAL GROWTH IN HYDROTHERMAL SYNTHESIS OF LAYERED OCTOSILICATE

In the hydrothermal synthesis process of layered octosilicate ilerite, seed crystals were added to the starting solution (water glass) for control of the crystallization rate. Octosilicate platelets of 4.4 μm and their ground products with a size less than 1 μm were used as the seed. The crystallization rate increased with an increase in the additional amount of platelet seed. The crystalline products were obtained even if the seed had a relatively low crystallinity. The size of products depended strongly on both the additional amount and size of seed. When the seed-loading mass ratio of seed to the starting solution was 0.01, the crystal growth was a maximum and the size of products was about twice of that of seed. However, in the use of the ground seed, a finer product was obtained at the optimum seed-loading mass ratio due to increase in the number of seed crystals. To control crystal growth for reducing synthesis time and for obtaining crystals of a specific size, both number and size of seed crystals must be optimized because the silanol groups on the edge of seed crystals added become new reaction sites of crystallization.     

CL‐20 Evaporative Crystallization under Reduced Pressure

The effect of parameters of the CL‐20 crystallization process carried out by solvent removal by evaporation in vacuo on shape, polymorph type, crystal size, and on their shock sensitivity was studied. The CL‐20 crystallization process by this technique was shown to allow a precise control of the crystallization process parameters and of the process run. The o‐xylene/ ethyl acetate system proved to be highly effective. Selecting suitable values of the parameters such as: pressure, process time, temperature, stirring rate, CL‐20 crystals were obtained in the ε form (even with no need for inoculation of the crystallization system with polymorph ε seeds) and of the shape close to a spherical one. The crystal growth modifiers added allowed to additionally control the shape and size of the CL‐20 crystals formed and to produce crystals of reduced impact and friction sensitivity.     

The Influence of Shear Stress on Crystallization in an Ultrasound Levitator

Industrial precipitation processes often use chemical agents to influence crystal morphology and size distribution. This experimental study deals with the investigation of physical parameters including an alternative method to affect crystal growth, thus, avoiding the presence of additives as intrinsic impurities. The influence of shear stress acting on growing crystals within a droplet is investigated in an ultrasound levitator. An ultrasound levitator enables the suspension of a single droplet against gravity and the study of containerless precipitation with specific mechanical forces acting on crystals. The levitator is used as a three‐phase reactor with precipitation from the gas and liquid, and as a reactor for precipitation from two different solutions. Calcium carbonate is used as a model system. The variation of temperature and the amount of applied shear stress leads to different amounts of calcium carbonate morphologies. An increase in the shear stress results in more rounded or spherical crystals. The intensity of the shear stress also influences the particle size distributions of the precipitated crystals, i.e., with increasing shear stress, particle size distributions are shifted to smaller sizes.     

Fabrication of high‐quality silica photonic crystals on polyester fabrics by gravitational sedimentation self‐assembly

A series of monodispersed silica microspheres with favourable sphericity and different diameters in the range 180–380 nm were prepared by adjusting the concentrations of tetraethylorthosilicate, ammonia, and deionised water by the Stöber method. In different self‐assembly conditions, the quality of silica photonic crystals on a polyester fabric substrate was investigated by field‐emission scanning electron microscopy for the morphology of the crystal structures and by spectrometry for their stop band intensities. High‐quality silica photonic crystals with a face‐centred cubic (fcc) array on polyester fabrics were produced at a low evaporation rate by adopting a high‐relative‐humidity controlled condition with a medium mass fraction of colloidal silica, and exhibited brilliant and variable structural colours, depending on the photonic band gap regulated by the spherical size of the silica microspheres and the viewing angles.     

Effect of Crystal Quality and Particle Size of HMX on the Creep Resistance for TATB/HMX Composites

Two kinds of reduced sensitivity high explosive 1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocane (RS‐HMX) with different particle sizes were selected to enhance the energy output and the mechanical properties of insensitive high explosive 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB). Mechanical sensitivities, dynamic mechanical analysis, and non‐linear time dependent creep behaviors of TATB/HMX composites were investigated and discussed in relation to the structural characteristics. Compared with TATB/conventional HMX (C‐HMX) sample, both the impact and friction sensitivities of TATB/RS‐HMX were reduced. It revealed that TATB/fine grains RS‐HMX composites had the highest storage modulus and minimum steady‐state creep strain rate due to the increased coherence strength and the inhibited slide of the single layer of TATB crystal. The creep resistance also showed clear dependence on the particle size of RS‐HMX. The overall results indicated that RS‐HMX had good potential in high energetic, safe, and load‐bearing material applications.     

Synthesis and crystallization behavior of nylon 12 14. II: Chain‐folded lamellar crystals and crystalline transformation behavior of nylon 12 14

Chain‐folded lamellar crystals of nylon 12 14 have been grown from a dilute 1,4‐butanediol solution with the “self‐seeding” technique. The morphology and structure of nylon 12 14 lamellar crystals were studied by both transmission electron microscopy (TEM) and wide‐angle X‐ray diffraction (WAXD). Two kinds of electron diffraction patterns were detected when different areas were selected for diffraction, which indicates that the α crystal phase and the β crystal phase coexist for nylon 12 14 under the present crystallization conditions. The WAXD diffractograms of the crystal mats confirm the results obtained from electron diffraction (ED). In addition, the changes of the crystal structure as a function of temperature for melt‐crystallized and dilute solution‐crystallized nylon 12 14 were monitored by variable‐temperature WAXD and variable‐temperature infrared spectroscopy (IR). It was found that the melt‐crystallized sample undergoes a Brill transformation at 80°C–90°C, but no Brill temperature can be observed for the dilute solution‐crystallized nylon 12 14.     

Influence of NaCl on Granulometric Characteristics and Polymorphism in Batch‐Cooling Crystallization of Glycine

The effect of NaCl added in different quantities on thermodynamic properties, granulometric characteristics, and structure of glycine in a crystallization process was investigated. Solubilities of α‐ and γ‐polymorphs in the presence of varying amounts of NaCl were analyzed. In order to examine the impact of the additive on granulometric properties of glycine, crystal morphology was examined by observing crystals under a scanning electron microscope. Crystal size distribution was determined by sieve analysis. By X‐ray diffraction analysis, the critical concentration of NaCl at which the structure of glycine changed, could be defined. The purity of obtained polymorphs was confirmed by Fourier transform infrared spectroscopy.     

Structural Defect Evolution of TATB‐Based Compounds Induced by Processing Operations and Thermal Treatments

2,4,6‐Triamino‐1,3,5‐trinitrobenzene (TATB) compounds are commonly used in high performance explosives because of their thermal stability and high detonation velocities compared to other materials. The insensitivity and mechanical properties are related to the stability of their crystalline structure. Crystallographic structure and structural defects evolution of TATB and TATB‐based compounds were studied by X‐ray diffraction for powders, molding powders, and pressed compounds, using Rietveld refinement. The effects of synthesis conditions, thermal treatments, coating and pressing operations on the structure of TATB compounds were evaluated. The results show that the pressing operation results in anisotropic crystallite size, leading to an increase of the structural defects density. It could be due to the anisotropic mechanical response of the TATB crystal under pressure, possibly plasticity. Finally, it is shown that increasing thermal treatment temperature on TATB powders decreases the structural defects density.     

Effect of Homolog Doping on Surface Morphology and Mass‐Loss Rates from PETN Crystals: Studies using Atomic Force Microscope and Thermogravimetric Analysis

Pentaerythritol tetranitrate (PETN) is an important energetic material, whose performance as a secondary explosive depends strongly on the density as well as flow porosity of powdered material, which in turn is governed by the size and surface properties of the PETN crystallite particles. Historically there has been evidence that the surface properties of PETN particles can be strongly influenced by the presence of homolog impurities of PETN, in particular, dipentaerythritol hexanitrate (diPEHN) and tripentaerythritol octanitrate (triPEON), although not many systematic studies characterizing such influence exist. In this work we employ thermogravimetric analysis (TGA) to measure mass‐loss rates at elevated temperatures and show that doping with a small amount of diPEHN and triPEON can reduce the mass‐loss rate from PETN single‐crystal surfaces by as much as 35 % as compared to undoped crystals. Arrhenius plots of mass‐loss rates as a function of temperature suggest that the reduction in evaporation is not due to the change in activation barrier of the molecular evaporation process, but perhaps due to the impedance to the receding motion of the steps by the immobile impurities on the surface. Removal of surface impurities through gentle washing with ethanol leads to enhanced mass‐loss rate relative to pure PETN suggesting a roughened surface morphology. Some surface roughening in doped crystals is supported by Atomic force microscopy (AFM) images of growth layers that show evidences of growth layer stacking and rough edges. We also find that a larger amount of impurity added to the original solution does not necessarily lead to a more highly doped crystal, which could perhaps be interpreted as PETN crystals being able to accommodate only up to a certain weight percent of homolog impurities.     

Internally generated seeding policy in anti-solvent crystallization of ceftriaxone sodium

A promising seeding strategy of internally generated seeds in producing ceftriaxone sodium with a uni-modal crystal size distribution (CSD) was experimentally investigated in batch anti-solvent crystallization. Effects of different seeding policies on product CSDs, both external seeding and internally generated seeding, were experimentally determined. A uni-modal product CSD was obtained without obvious nucleation at high external-seed loadings, while a bi-modal CSD was obtained with plenty of secondary nuclei at low external-seed loadings. The internally generated seeds, prepared by adding pure acetone into the non-seeded saturated solution of ceftriaxone sodium, were fine crystals with a uni-modal, narrow CSD, irrespective of the amount of the added diluent. These fine crystals were grown continuously as seeds by slowly adding a relatively low concentration diluent. The product CSD was uni-modal with relatively large mass-weighted mean size, and the product quality was as good as the one obtained by external seeding. Therefore, such an internally generated seeding policy is expected to be a favorable seeding technology to hopefully substitute for the external seeding policy in the pharmaceutical industry, where external seeding is not welcome because the external seeding policy would destroy the axenic conditions in crystallizer and lead to unstable quality of different batches.     

Partial Seeding Policy for Controlling the Crystal Quality in Batch Cooling Crystallization

Partial seeding, in which the nuclei originating from seed crystals grow to yield the product crystals, was simulated and optimized for controlling the batch cooling crystallization process of L-arginine. The product quality was evaluated by the coefficient of variation (CV) of the crystal size distribution. First, the optimum seed amount for partial seeding was estimated by simulation. Then, the simulated values of the optimum seed amount and the resulting local minimum CV were correlated with the seed crystal size and the cooling rate. The correlation can be utilized for estimating the seed amount in the case that the seed crystals are added in a slurry. Finally, these simulated values were compared to the measured ones. Consequently, the optimum seed amount was suggested to be reasonably predicted.     

Stereo imaging of crystal growth

Significance A methodology that directly images the full three‐dimensional (3‐D) shape of crystals within a crystallizer is reported. It is based on the mathematical principle that if the two‐dimensional (2‐D) images of an object are obtained from two or more different angles, the full 3‐D crystal shape could be reconstructed. A prototype instrument is built and proof of concept study performed to demonstrate the potentials in using the system for 3‐D measurement of crystal shape and shape distribution. It is our belief that 3‐D measurement of crystal shape represents a significant step forward from existing work of 2‐D measurement of crystal morphology and is potentially of great significance to research toward closed‐loop control of crystal morphology. © 2015 American Institute of Chemical Engineers AIChE J, 62: 18–25, 2016     

Precipitation of pharmaceuticals using a supercritical anti‐solvent (SAS) dechnique: A preliminary study

The aims of this research were to investigate the applicability of the supercritical anti‐solvent (SAS) process on the precipitation of pharmaceuticals (andrographolide and acetaminophen). In particular, the goal of this research was to study the influence of pressure at 10 and 24 MPa on particle characteristics (morphology, crystalline structure, polymorphic form, size, size distribution, and precipitation yield), and to compare the precipitation efficiency of SAS process and evaporation process. Scanning electron microscope (SEM), X‐ray diffraction (XRD), and high performance liquid chromatography (HPLC) showed a significant change in particle size, size distribution, morphology, and precipitation yield, respectively. From an analysis of the results it was found that the crystal size of andrographolide and acetaminophen decreased with increasing pressure. The morphology of andrographolide particles changed from slice‐like to column‐like when the pressure was increased. On the other hand, the acetaminophen particles obtained were found to be monoclinic form (I) under both operating pressures. The SAS process produced small uniform shaped crystals, with a narrow size distribution, high precipitation yield and selective precipitation were also observed.     

尼龙-6球晶形貌的研究

利用偏光显微镜，研究了不同培养条件下制备的PA-6熔融结晶球晶形貌。结果表明：随着培养时间的增加，PA-6球晶的尺寸明显增大且可观察到完整的大小不一的多面体球晶；PA-6的晶核产生以均相成核为主；降温速度对PA-6球晶的生长程度影响显著；在晶体样片的不同区域，PA-6球晶的形貌呈现出明显差异；在负荷作用下培养的晶体，在显微镜下具有较高的衬度。     

Molecular Dynamics Simulations of Polymer‐Bonded Explosives (PBXs): Modeling,Mechanical Properties and their Dependence on Temperatures and Concentrations of Binders

Two models, i.e. “covering” and “cutting” models, for the polymer‐bonded explosives (PBXs) were proposed for different researching aspects. Used for choosing polymeric binders, the “covering” models are mainly applied to find the relations of temperatures and concentrations respectively with elastic properties of the PBXs. The “cutting” model is especially used to describe the highly anisotropic behavior of 1,3,5‐triamino‐2,4,6‐trinitrobenzene crystals (TATB). These models were realized by using molecular dynamics methods. It is found that the ductility of crystalline TATB can be effectively improved by blending fluorine‐containing polymers in small amounts. The moduli for the PBXs decrease with increase in temperature and concentration of binders. Different crystalline surfaces interacting with the same polymer binder have different modulus‐decreasing effects due to the highly anisotropic behavior of TATB. The modulus‐decreasing effect for different crystalline surfaces ranking order is (010)≈(100)>(001).