RDX和HMX晶体压制方程的对比研究

分别选取经过不同重结晶工艺处理的RDX和HMX晶体和一种工业级原料颗粒样品进行准静态压制实验，由实验应力／位移曲线获得压制曲线，采用Kawakita和Heckel方程对压制曲线进行拟合。结果表明，拟合所得的常数具有模量倒数量纲，能区分重结晶和原料样品，用作含能晶体品质评价的定量参数。比较两个压制方程的模拟情况，对RDX颗粒两个方程均拟合得很好，而对HMX颗粒存在一定的误差，尤其是Heckel方程误差较大。选取压制过程的形变破碎阶段的数据所得结果其区分度有明显提高，同时两个方程的拟合情况均得到明显改善。对于含能材料颗粒，Kawakita方程更合适。     

RDX晶体颗粒压制中的声发射现象

在RDX晶体颗粒压制实验中,采用声发射信号检测系统获取压制过程产生的声发射信号.特征参数分析显示,颗粒压制时的声发射信号极为丰富,且信号幅度、波击计数(率)和上升时间随时间的变化曲线均能分成明显的3段,分别与颗粒压制过程的3个阶段,即流动重排、变形破碎和压实阶段相对应.声发射监测结果反映了压制过程中颗粒材料力学行为的变化,为研究颗粒压制过程的行为提供了可行的依据.     

Quantitative Characterization of Internal Defects in RDX Crystals

Size, shape, internal defects are very important properties of explosives crystals. These parameters play a role on both the explosive formulation processing and the detonic behavior of the explosive formulations. The use of explosive crystals free of solvent inclusions leads to decrease the shock sensitivity of cast explosive formulations. Many efforts for processing such high quality explosive crystals have been done and are still in progress. Qualitative observations of internal crystal defects can be performed by optical microscopy with matching refractive index. The purpose of this paper is to provide two accurate quantitative tools for internal crystal defects measurements. The first method is based on accurate measurements of the crystal apparent density. The second method records the mass of the species entrapped in the crystal internal cavities. Experiments are performed on two RDX batches. The strong correlation recorded between the results of the two complementary methods validates the measurements. Apparent density measurements provide an accurate global characterization of the internal defects population of a crystal batch sorting the crystals in function of their apparent density. The second method is a tool to identify the species entrapped in the crystals.     

Concomitant crystallization for in situ encapsulation of organic materials

Concomitant crystallization leads to process intensification through the synergistic combination of the partial processes of particle formation and encapsulation within a single process step. Both cooling and electrospray crystallization in multi-component solutions were used to create (sub-)micron sized particles of different crystalline materials. Concentrations were varied to control core and shell material. Depending on the relative initial concentrations used, concomitant electrospray crystallization of isonicotinamide and caffeine leads to encapsulated particles. Only limited encapsulation was achieved during concomitant cooling crystallization. Concomitant cooling crystallization of cyclotrimethylenetrinitramine (RDX)–2,4,6-trinitrotoluene (TNT) resulted in separate RDX and TNT particles. Using electrospray crystallization, spherical nano-particles were produced, for which the component distribution within the particles could not be determined. Whereas crystallization from bulk solvent starts with a nucleus that grows gradually outward, whereby heterogeneous growth of a coating material on this core particle is not guaranteed, it appears that crystallization from evaporating solvent droplets starts at the surface of the droplets, and moves gradually inward. The resulting RDX–TNT powders have been tested for impact and friction sensitivity. The impact sensitivity has decreased compared to the raw materials, and the friction sensitivity did not change.     

The variation of pore sizes in,and the surface area of,powder compacts of magnesium oxide

The surface area of and pore size distribution in magnesium oxide and magnesium oxide compacted in the pressure range of 350 – 500 MNm^?2 has been determined by low temperature nitrogen adsorption.Pore closure and particle fracture, resulting from uniaxial compression, can be observed by using compacted—uncompacted comparison curves (C—U curves). This method eliminates the use of arbitrary t-curves or capillary condensation pore models to show the variation in particle surface due to direct compaction and the formation of voids between solid particles.     

Physicochemical Parameters of Nitramines Influencing Shock Sensitivity

TNO Prins Maurits Laboratory has actively followed and contributed to the research on the development of insensitive munitions (IM). One of the initial research topics at TNO focused on the improvement of the shape of RDX crystals and its relation to the shock sensitivity. The variation of crystal shape has been studied by crystallization from different solvents and/or by post‐treatment of the crystals. The role of the mean particle size on shock sensitivity was also included in these analyses. The decrease in shock sensitivity is even more pronounced when controlling the internal quality of crystals. In the meantime research has shifted to other energetic materials as well – in particular HMX and CL‐20 – in this way revealing step by step the important physicochemical parameters which play a role in determining the shock sensitivity of formulations containing these types of nitramines. Various characterization techniques, to determine the internal and external quality of crystals will be discussed, and their relation to shock sensitivity in PBXs will be shown. Two different grades of I‐RDX have been subjected to different characterization tests. The objective is to gain more understanding about which of the physicochemical parameters enables one to discriminate between a reduced sensitivity RDX and normal RDX.     

Dependence of Compaction Efficiency in Dry Pressing on the Particle Size Distribution

The compact densification with pressing pressure (compaction efficiency) was determined to be sensitive to the particle size distribution. For the three types of alumina powders used in this research, the compaction efficiency increased with increasing particle size. It has been demonstrated that if the compact density versus log (pressure) has a linear relationship for any two types of powders, so do the blends of the two powders. A model is proposed which can predict the compaction efficiency of a binary particle system based on the Furnas particle packing model and consider the packing efficiency as a function of forming pressure. The composition of the binary mixture at which the highest density is obtained under high pressures is also the composition having the largest compaction efficiency. When coarse particles were added to this composition, the compaction efficiency slowly decreased, and when fine particles were added, the compaction efficiency rapidly decreased. For a continuous particle size distribution, the highest compaction efficiency is related to the average value of -log (porefraction).     

Internal Stresses in Model Ceramic Systems

Model ceramic systems of crystals and glass fabricated by vacuum hot compaction were used to study methods of detection of internal stress on the crystalline phase. Effects of the volume fraction of the glass, compaction temperature and time, and crystal particle size in modifying induced internal stress are presented. These effects, together with X-ray diffraction and wetting studies, indicate that internal stress development is closely connected to wetting of the crystal by the glass and the formation of a second crystalline phase. Limited results regarding influence of internal stress on strength of ceramic bodies indicate that increasing internal stress on the crystal phase decreases the modulus of rupture when the particle size of the crystal component remains constant. The results of this work indicate that the full potential of ceramic systems may not be realized with the presence of internal stress.     

Simulation of cavity formation hysteresis in the presence of granular compaction

The cavity formation hysteresis phenomenon in packed and spouted beds of particles has been simulated using the Eulerian–Eulerian two-fluid model, incorporating a particle compression mechanism via changing the particle packing limit in Fluent software. The cavity height for the internal jet determined from the simulated results agreed well with the published experimental data. The simulated results also showed that the compaction was most significant in a small region in close contact with the internal jet, causing the extra pressure drop across the whole column. The simulated static pressure drops captured the trend of hysteresis but underestimated the experimental data for the flow ascending process and overestimated the flow descending data. Improved understanding of the particle compaction mechanism in packed and spouted beds is required in order to improve the accuracy of numerical simulations.     

Investigation of Crystal Morphology and Shock Sensitivity of Cyclotrimethylenetrinitramine Suspension by Rheology

Crystal morphology and shock sensitivity of a series of cyclotrimethylenetrinitramine (RDX) particles suspended from ethylene glycol were investigated. Flow rheology was employed to measure the rheological properties of the suspensions at constant temperature; it was observed that the stress‐shear rate and viscosity behavior of the suspensions were controlled by the particle morphology. The viscosity of the RDX suspensions changed with the roundness/smoothness of RDX crystals at all applied shear rates. The suspensions containing crystals with smoother morphology showed reduced viscosity. When the viscosity data was compared to the shock sensitivity results from the RS‐RDX Round Robin study, a good correlation was obtained. This study has validated the use of flow rheology to indicate the morphology and shock sensitivity of crystalline particles.     

Mechanism of Solid Propellant Combustion Submitted to a High Plasma Flux

A simplified mechanism of the plasma effect on the combustion on different types of propellants is presented. The model provides a semi‐quantitative prediction of the burning rate enhancement that should be expected for a given propellant composition and structure. Depending on the propellant structure and composition, one expects a burning rate enhancement either to disappear or to survive a certain time after the plasma injection has been turned off. In order to estimate the internal front propagation rate, we have built a simplified model of the RDX ignition inside the matrix. A cubic lattice topology has been assumed and the ignition front extends from one layer to its nearest neighbor. The propagation rate was found to be dependent on the lattice constant, the particle size, the thermo‐chemical properties of the RDX particles and the matrix background.     

The Flow Characteristics of High Explosives

A comparative study of flow properties of the explosive compound RDX is presented. The results were used to identify desirable free-flowing characteristics in the powders. Various static and free-flowing tests were conducted on the explosive powders. The results suggest that flow properties of the RDX powders tested were not influenced by particle size and surface roughness. However density, moisture content and cohesive strength between the particles had some effect on the flow properties. An increase in moisture content and cohesive strength of the particles reduced the flow rate. Whereas an increase in bluk density of the powders increased the flow rate.     

The importance of the amount/thickness of die wall lubricant for UO2 pellets pressing

External lubrication is often used to complete compaction process of powder materials. The main goal of this method is generally to reduce the amount of admixed internal lubricant (Zinc stearate powder) within the raw material. The application of external lubricants enhances the density uniformity and the mechanical strength of the resulting compacts. This study investigates the effects of the external lubricant amount for UO₂ powder compaction and the properties of the corresponding green pellets (corresponding to the compacts before sintering) without any admixed lubricant in the raw powder in order to evaluate the feasibility of this route in the case of nuclear powder. Results show that there is a quantity or number of layers from which the external lubricant applied on the die wall becomes detrimental to the friction index and the ejection force measured during the pressing cycle. The quality (surface defects, mechanical strength) of the green pellets can also be affected by the amount of lubricant. Thus the quantity and the thickness of the die wall lubricant must be optimized in order to assure an efficient mixed lubrication mode corresponding to the better lubrication mode in our study case.     

Study of fragmentation in cBN powders under ultra-high pressure

Studying the fragmentation law and refinement of cubic boron nitride powder under ultra-high pressure is crucial to producing a high-strength, high-density polycrystalline cubic boron nitride. In this paper, brown and black cBN crystalline powders with different micron sizes were selected as initial raw materials for an ultra-high-pressure simulation experiment. Single and mixed particles were extruded under 80MPa low pressure and 5.5GPa ultra-high pressure at ambient temperature for 1 min. The crushing behavior and particle size distribution of cBN powders with different particle sizes and ratios were investigated using a laser particle size analyzer and scanning electron microscopy. Results revealed no particle breakage or deformation at low pressure, and the compaction density was low. However, under ultra-high pressure, the cBN particles showed cracks, plastic deformation, and fragmentation, resulting in crushed fine particles filling in the voids of coarse particles, which led to a higher pressing density. Small-sized or mixed cBN particles with high density ratios were not easily crushed; the coarser the particle size, the more severe the ultra-high-pressure extrusion and crushing. The pressing density also declined, and brown cBN crystal particles with higher impact toughness demonstrated a lower particle breakage rate. The ultra-high-pressure crushing law should be considered and appropriate binders should be selected to improve the sintering performance of PcBN materials; ultra-high-pressure crushing of cBN powder contributes to cBN-cBN and cBN-M-cBN bonds under high temperatures and ultra-high pressure.     

Review: An investigation of single particle breakage tests for coal handling system of the gladstone port

Aspects of the literature on single particle breakage test have been reviewed in this article. The test procedures that are commonly used by the researchers in examining and measuring the breakage characteristics of the ore and coal particles are also discussed. It appears that most of the common size distribution function fitting techniques were not suitable for accurate representation of the size distributions obtained from a pendulum breaking process. The single impact test, double impact test (drop weight test, pendulum test) and slow compression test can be used to study the behaviour of the single particle breakage events. The single impact test, slow compression test and drop weight test cannot measure the energy utilization pattern in single particle breakage events, but this can be determined from the pendulum test.The energy utilized for breakage was predominantly dependent upon the size and shape of the specimen, level of input energy and the breakage properties of the specimen. This review highlights that the size distribution curves were linear in the fine particle region and have varying curvature in the coarser region, the gradient of the linear fine particle region of the size distribution curves increases with an increase in the specific comminution energy. The comminution energy increases with input energy at lower levels of input energy but at the higher levels of input energy the comminution energy did not show the same proportional increase. At a given level of input energy, the size distribution resulting from the breakage of the particles by the pendulum apparatus can be represented by a one-parameter family of curves.     

Aging of Standard and Insensitive RDX Crystals Investigated by Means of X‐Ray Diffraction

Coarse particles of the high explosive RDX in different qualities (S‐RDX, I‐RDX, VI‐RDX) were aged artificially in air and argon, equivalent up to 25 years at 25 °C. The samples were investigated by means of X‐ray diffraction and rocking curves, revealing the behavior of microstrain during the artificial aging. The investigations revealed that the improved crystal quality of RDX survives artificial aging in contrast to a standard quality, where aging increases microstrain significantly. Besides aging details and mechanisms on a crystal level are described and discussed, such as eutectic mixtures with HMX impurities, crystal growth, defect healing, surface diffusion and smoothing, and reconstruction of crystal faces, edges and corners in rounded particles.     

Nanoparticle generation by intensified solution crystallization using cold plasma

In this study, atmospheric pressure cold plasma (surface dielectric barrier discharge) was used as an alternative energy form to intensify solution crystallization and produce nano-sized organic crystals. Nano-sized particles can have beneficial product properties such as improved internal quality and dissolution rate, compared to conventionally sized crystalline products. In cold plasma intensified crystallization a nebulizer system sprays the solution aerosol into the plasma with the help of a carrier gas. The plasma simultaneously heats and charges the droplets causing fast solvent evaporation and Coulomb-fission to occur, after which nucleation and crystal growth commence within the small, confined volume offered by the small droplets. In this manner, nano-sized crystals of the energetic material RDX and the pharmaceutical niflumic acid were produced.     

Investigating the Mechanical Properties of RDX Crystals Using Nano‐Indentation

The shock sensitivity of RDX is of major interest for the development of insensitive munitions. Previous research has implied that internal defects that form within RDX crystals have a strong sensitising effect, increasing the probability of shock initiation. During the NATO Reduced sensitivity RDX Round Robin (R⁴) program the number of internal defects within RDX crystals of differing qualities were assessed by optical microscopy and a scoring system. The results suggested that RDX crystals with many internal defects were more sensitive. Studies investigating the mechanical properties of bulk RDX have shown that lots consisting of poorer crystals are weaker. This study investigates the mechanical properties of individual crystals from different RDX lots using nano‐indentation. It is shown that crystals with many internal defects have reduced modulus of elasticity, stiffness and prone to greater deformation under applied load. The results also show a correlation between these parameters and previously reported shock sensitivity data.     

Shock Sensitivity of Pressed RDX‐Based Plastic Bonded Explosives under Short‐Duration and High‐Pressure Impact Tests

The shock sensitivities of plastic bonded explosives were studied with a thin flyer impact test by using two types of pressed RDX. The thin flyer, driven by an electrically exploding plasma, exerts a short‐duration, high‐pressure pulse to the samples to trigger a shock‐to‐detonation process. It was found that the duration and magnitude of the incident shock strongly influence the dominant mode of hot‐spot formation, promoting a fast pore collapsing mechanism while suppressing other slower shear or friction mechanisms, as proposed by Chakravarty et al. [1]. The pressed PBX based on reduced sensitivity RDX had higher shock threshold pressure, compared to the pressed PBX based on commercial RDX. The difference was observed even with a certain portion of external extragranular defects. It is postulated that the internal crystal defects are more efficient than the external porosity in terms of the rapid reaction of hot spots.     

Effects of acid dyes on crystallization and mechanical properties of melt‐reprocessed nylon 66

Nylon fibers dyed with different types of acid dyes were melt reprocessed using a compression‐molding machine. The crystalline structure and mechanical properties of the melt‐reprocessed nylon were experimentally evaluated. It was found that metal complex acid dyes showed much more distinct effects on the structure and mechanical properties of melt‐reprocessed nylon than nonmetallized acid dyes. They decreased the crystallization rate of the molten nylon and reduced its crystallinity. They also reduced the imperfect form I structure in the crystalline region. Compression‐molded nylon samples showed inferior mechanical properties in the presence of metal complex acid dyes. The coordinate bonds between the Cr³⁺ ions and amide groups are possibly formed in melt‐reprocessed nylon, which could be the reason for the changes in the structure and properties of melt‐reprocessed nylon. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2386–2396, 1999