Electric spark and esd sensitivity of reactive solids (primary or secondary explosive,propellant, pyrotechnics) part one: Experimental results and reflection factors for sensitivity test optimization

An experimental study shows that spark energy transfer mechanisms are different for the explosive alone (HMX) and for the explosive containing 1% of graphite. This study is completed by literature data and underlines the need for parametric studies by systematically varying the electrodes gap as well as the spark duration to determine the true minimum value of the ignition energy. Grain size and shape are very important factors. The influence of the electrode shape is also discussed, as well as the environmental conditions. These data allow us to determine the conditions for testing the electric spark sensitivity of solids. The tests are then performed at a small scale. Moreover, for ionic solids or substances made of an electrically insulating matrix containing conductive inclusions, particular phenomena appear when they are submitted to electric fields for a certain time. These phenomena justify the development of specific tests to determine the capacitive discharge sensitivity.     

电火花加工中放电不稳定现象产生的原因及改善措施

简述了电火花加工放电过程的机理，介绍了加工中放电状态的判断方法，重点分析了放电不稳定现象产生的原因，并根据相对应的原因提出了改善的措施，对电火花加工具有一定指导作用。     

Formation of Nitrite- and Nitrate-Ions in Aqueous Solutions Treated with Pulsed Electric Discharges

The accumulation of nitrogen-containing ions formed in aqueous solutions dispersed in air and treated with pulsed corona, dielectric barrier and spark electric discharges, was studied dependent on electric conductivity and pH of treated solutions. The impact of conductivity to the spark and corona discharge is determined by the increased ohmic losses in the reactor. In contrast, the character of dielectric barrier discharge is significantly changed with growing conductivity resulted in increased nitrite-to-nitrate ratio. In alkaline solutions the production of nitrites is increased for the spark and the barrier discharge; the corona discharge produce only nitrates. The amount of nitrates produced in pulsed corona discharge at energy doses characteristic for potable water treatment is about 100 times lower than their maximum permissible concentration.     

Introducing Novel Tetrazole Derivatives as High Performance Energetic Compounds for Confined Explosion and as Oxidizer in Solid Propellants

This paper introduces five novel high‐nitrogen content (N>50%) tetrazole derivatives with desirable physicothermal properties, high detonation and combustion performance as well as suitable sensitivities with respect to external stimuli electric spark and heat. Suitable density functional theory (DFT) and empirical methods were used to predict their crystal density, melting point, condensed phase heat of formation, enthalpy of fusion, Gibbs free energy of formation, velocity of detonation, detonation pressure, Gurney velocity, heat of detonation, power (strength), brisance, impact sensitivity, electric spark sensitivity, heat sensitivity and specific impulse. Two compounds 5,5′‐[(1Z,5Z)‐3,4‐dinitrohexaaza‐1,5‐diene‐1,6‐diyl]bis(1‐nitro‐1H‐tetrazole) and 3,3′,7,7′‐tetranitro‐3,3a,3′,3′a‐tetrahydro‐7H,7′H‐6,6′‐bitetrazolo[1,5‐e]pentazine as compared to the other new derivatives can be introduced as high performance explosives for confined explosion and oxidizers in solid propellants.     

The Correlation between Electric Spark Sensitivity of Polynitroaromatic Compounds and Their Molecular Electronic Properties

In this paper, a relationship between electric spark sensitivity and molecular electronic properties is studied for polynitroaromatic compounds. The lowest unoccupied molecular orbital energy and the Mulliken charges of the nitro group, and the number of the aromatic rings as well as certain substituted groups attached to the aromatic ring can be used for the prediction of the electric spark sensitivity. Electric spark sensitivities calculated by such a correlation are reasonably close to the experimental data for both 17 polynitroaromatic explosives as training set and 11 polynitroaromatic explosives as test set.     

Rapid formation of α-sialon during spark plasma sintering: Its origin and implications

α-Sialons with varied chemical compositions and starting precursors were sintered in spark plasma sintering (SPS) furnace to verify the influence of electrical conduction, mechanical pressure and heat transfer efficiency. The internal temperature that the sample was exposed to and its evolution during the heating were experimentally estimated and discussed. XRD and SEM grain size analysis were carried out to evaluate the phase and microstructure evolution. Mechanical pressure was eliminated to create a free sintering condition under SPS. By free sintering and surrounding the sample with graphite wool the heat transfer was sufficiently retarded and a usual slow reaction was demonstrated. It is verified that the efficiency of heat transfer is more influential in the enhancement of α-sialon formation and grain growth by SPS than the efficiency of electric conduction.     

Thermochemical,Sensitivity and Detonation Characteristics of New Thermally Stable High Performance Explosives

The M06‐2X/6‐311G(d,p) and B3LYP/6‐311G(d,p) density functional methods and electrostatic potential analysis were used for calculation of enthalpy of sublimation, crystal density and enthalpy of formation of some thermally stable explosives in the gas and solid phases. These data were used for prediction of their detonation properties including heat of detonation, detonation pressure, detonation velocity, detonation temperature, electric spark sensitivity, impact sensitivity and deflagration temperature using appropriate methods. The range of different properties for these compounds are: crystal density 1.51–2.01 g cm⁻³, enthalpy of sublimation 346.4–424.7 kJ mol⁻¹, the solid phase enthalpy of formation 500.4–860.6 kJ mol⁻¹, heat of detonation 13.64–17.57 kJ g⁻¹, detonation pressure 33.0–37.0 GPa, detonation velocity 8.5–9.5 km s⁻¹, detonation temperature 5488–6234 K, electric spark sensitivity 7.89–9.47 J, impact sensitivity 21–38 J, deflagration temperature 560–586 K and power [%TNT] 207–276. The results show that two novel energetic compounds N,N′‐(diazene‐1,2‐diylbis(2,3,5,6‐tetranitro‐4,1‐phenylene))bis(5‐nitro‐4H‐1,2,4‐triazol‐3‐amine) (DDTNPNT3A) and 1,1′‐(diazene‐1,2‐diylbis(2,3,5,6‐tetranitro‐4,1‐phenylene))bis(3‐nitro‐1H‐1,2,4‐triazol‐5‐amine) (DDTNPNT5A) can be introduced as thermally explosives with high detonation performance.     

Microstructure and flexural strength of hafnium diboride via flash and conventional spark plasma sintering

Microstructure evolution in bulk hafnium diboride ceramics prepared by spark plasma sintering in flash regime was compared with conventional spark plasma sintering. The conventional and flash spark plasma sintering resulted in ceramics with a high relative density exceeding 96% of their theoretical density. A remarkably fine grain size distribution was noticed for the specimen prepared in the flash regime. This atypical microstructure evolution provides a possible insight into the mechanism of flash sintering for conductive bulks. The room temperature flexural strength of the hafnium diboride processed by flash SPS was 650 MPa which is 140 MPa higher than the sample produced by conventional SPS.     

Initiation of detonation in liquid explosive mixtures by an incomplete electric spark discharge

An experimental study is conducted of the detonation initiation by means of an electric spark in solutions of a number of nitrocompounds in high-concentration nitric acid. When the duration of the voltage pulse applied to the spark discharger is limited, the process of detonation initiation by incomplete discharge (leader initiation) is registered. The basic laws of leader initiation are established. The results of experiments confirm the hypothesis which states that the initial heating-up is determined by the local electric field strength near the leader head. The indicated mechanism may be valid for the case of initiation by an electric spark of liquid explosive mixtures with electrolytic and dielectric properties when the critical diameter of the mixture is comparable with the leader cross section.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 1, pp. 129–135, January–February, 1996.     

Simulation of thermal and electric field evolution during spark plasma sintering

Finite element simulations have been conducted to determine the temperature distribution (both in radial and axial direction), heat and electric flux-field in the powder compact/die/punch assembly during the spark plasma sintering (SPS) process. A thermal–electrical coupled model with temperature dependent thermal and electrical properties is implemented. The simulation studies were conducted using both ABAQUS and MATLAB and a range of power input, varying thermal conductivity of powder compact were considered. Also, the effect of time variation on both thermal and electric field evolution was assessed. During SPS, the heat transmission pattern and the temperature difference between the specimen center and the die surface depend on thermal conductivity of the powder compact, time of sintering and power input. The simulation results also revealed that the temperature gradient across the powder compact/graphite die wall is higher for conditions of higher power input and/or powder compact with lower thermal conductivity.     

Properties of spark plasma sintered pseudocubic BiFeO3–BaTiO3 ceramics

Perovskite solid solution materials, namely, 0.67BiFeO₃-0.33BaTiO₃, were synthesized by spark plasma sintering method. The effects of the spark plasma sintering temperature on phase purity, microstructure, and electric properties of the as-prepared materials were investigated. The materials could be referred as pseudocubic phases based on the X-ray diffraction patterns. The bulk density first increased and then decreased. The 880 °C-sintered-ceramics had the maximal density and a compact microstructure with grain size of 0.77 ± 0.34 μm. The dielectric constant as a function of temperature exhibited a broad peak. At the optimal spark-plasma-sintering temperature, enhanced ferroelectric properties were observed with a value of P_r ～ 21 μC/cm². This investigation on the spark plasma sintering process confirms it as an efficient approach to prepare outstanding performance BiFeO₃–BaTiO₃ ceramics.     

High throughput nanoparticle generation utilizing high-frequency spark discharges via rapid spark plasma removal

Scaling up the production capacity of spark discharge generated nanoparticles is crucial for their industrial application. Among various options, high-frequency spark discharge generation is a promising route as the mass production rate of nanoparticles should simply scale linearly with the spark frequency. However, reports of spark discharge generators operating above 1 kHz are scarce in the literature, as spark discharges at higher frequencies have been observed to be irregular, leading to premature spark discharges that result in a significantly lower mass production rate. In this study, we present a wire-to-plate electrode configuration that suppresses premature spark discharges during high-frequency operations above 1 kHz, and investigate the factors that contribute to the occurrence of premature spark discharges by comparing the performance of the wire-to-plate electrode configuration to those of rod-to-rod and wire-to-rod configurations. We identified that spark duration should be minimized to achieve stable spark discharge events as it correlates to the time required for the spark plasma to be completely removed from the spark zone, and found that increasing local electric field intensity and carrier gas velocity are both beneficial to achieving stable high frequency spark discharges. Lastly, using the wire-to-plate electrodes, we show that stable operation without premature spark discharges can be achieved up to 17.9 kHz and the copper nanoparticle production rate scales as expected (up to 7.6 mg/h) when premature spark discharges are eliminated during high frequency operations.

Copyright © 2017 American Association for Aerosol Research     

电火花沉积技术的优缺点和应用实例

金属表面电火花沉积技术是近年发展起来的新技术、新工艺，具有较强的实用性和发展性。于电火花沉积工艺是瞬间的高温——冷却过程，金属表面不仅会因迅速淬火而形成马氏体。在狭窄的沉积过渡区还会得到超细奥氏体组织。在这项新的技术应用中，也存在着明显的优缺点，本文通过实例进行探讨和说明。     

Synthesis and microstructure evolution of WCoB based cermets during spark plasma sintering

WCoB based cermets were prepared by spark plasma sintering at sintering temperature among 600°C-1200 °C. The phase evolution was investigated by detecting density behavior, phase composition, microstructure and mechanical properties during sintering process. The sintering process can be divided into three stages: powder densification, solid phase reaction and liquid phase sintering. WCoB hard phase forms at 1000 °C during solid phase sintering, showing better mechanical properties than Co₂B, especially on Vicker's hardness. WCoB hard phase forms on the basis of Co₂B binary boride and its content increases in liquid phase sintering stage with high density. The Vicker's hardness and transverse rupture strength (TRS) reach the maximum value of 1262 Hv and 1212 MPa at 1200 °C and 1170 °C, respectively. The fracture toughness reaches the maximum value of 21.8 MPa m^1/2 at 1050 °C, and the inter-granular fracture is the main fracture mechanism.     

Spark plasma sintering and hot pressing of ZTA-NbC materials – A comparison of mechanical and electrical properties

Zirconia toughened alumina can be made electrically conductive and thus electric discharge machinable by addition of a percolating dispersion of niobium carbide. In order to boost the productivity of the sintering process spark plasma sintering was tested at identical temperature and pressure but shorter dwell than in hot pressing. SPS sintering parameters for ZTA-NbC are developed and spark plasma sintered ceramics are compared to the hot pressed benchmark.During SPS a percolating NbC backbone of niobium carbide grains is formed which enhances electrical conductivity but impedes densification. Identical strength at however higher sintering temperature is achieved by SPS but the fracture resistance and hardness were always superior in hot pressed samples. The monoclinic content of zirconia grains in as fired SPS samples is higher despite smaller average grain size and the transformation toughening effect is less pronounced. SPS promises economic benefits due to shorter dwell and cooling cycles.     

Optimization of Spark and ESD Propellant Sensitivity Tests. A review

All cured propellants can pass small-scale spark sensitivity tests, yet some, generally involving larger propellant samples, are electrostatic discharge (ESD) sensitive. These tests fail to rank propellants because the test methods themselves are not optimized and frequently use instrumentation which does not indicate the energy transferred to the sample. For spark ignition of premixed combustible gases or fuel sprays in air, minimum energies are measured and considered thermochemical properties for a given fuel/air ratio, initial temperature and pressure, and for a spray, a given droplet size distribution. Obtaining such data requires monitoring the total energy discharged across the gap during the spark and parametric studies where gap length and spark duration are varied systematically to establish the true minimum (essential to hazard/risk analysis). Electrode geometry and tip design, as well as electrode material, are other important considerations. Present spark and ESD sensitivity tests used for solid propellants are reviewed in the context of methods more likely to provide meaningful initiation thresholds.     

Improvement in microstructure and mechanical properties of Ti(C,N) cermet prepared by two-step spark plasma sintering

A fine grained Ti(C, N) cermet tool material was prepared by two-step spark plasma sintering. Microstructure evolution and densification mechanisms of Ti(C, N) during spark plasma sintering were studied. Effect of two-step sintering process and Ni content on microstructure and mechanical properties were also investigated. The critical activated densification temperature of Ti(C, N) is about 1300?℃, and the rapidest densification rate takes place at 1300?℃~1400?℃. Grains are in the size of 1?µm when the Ti(C, N) cermet was prepared by two-step spark plasma sintering. The optimal flexural strength, fracture toughness and Vickers hardness are 1094?±?42?MPa, 7.2?±?0.5?MPa?m^1/2 and 18.3?±?0.4?GPa, respectively. The Ti(C, N) cermets containing more content of Ni have higher toughness, which is due to the remarkable toughening effect of crack bridging by large grains.     

Preparation and Electric Properties of Dense Nanocrystalline Zinc Oxide Ceramics

This communication reports on the preparation and electric properties of dense nanocrystalline ZnO ceramics. By spark plasma sintering, nanocrystalline (∼100 nm) ZnO ceramics with a high density of 98.5% were obtained at a very low temperature of 550°C. Electric property measurement revealed a novel conduction nonlinearity in the sample sintered at 500°C. This phenomenon is due to the nanometerization of ZnO crystal and the grain boundary layer with an amorphous interfacial layer.     

Creep and recovery behaviors of a polythiophene-based electrorheological fluid

We investigate the creep response of poly(3-thiopheneacetic acid) (PTAA) particles doped with perchloric acid. With increase in applied stress, these suspensions exhibit an evolution from a linear viscoelastic response, with three components of instantaneous elastic strain, retarded elastic strain and viscous strain, to a nonlinear viscoelastic response, where the retarded elastic and viscous strains monotonically decrease and a plastic contribution to the instantaneous strain grows, followed by a viscoplastic solid behavior, with fully plastic instantaneous strain, and finally a transition from plastic solid to a plastic liquid at the yield stress. With increase in electric field strength at fixed particle concentration and applied stress, the viscoplastic response diminishes, and more elastic behavior ensues. For highly doped samples, at high-electric field strengths, a fully elastic solid response is observed in the linear viscoelastic regime. The equilibrium compliance, J_C and steady state recoverable compliance J_R, were investigated as a function of electric field strength, particle concentration and particle conductivity. The results are interpreted in terms of the field-induced formation of thick fibrillar aggregates spanning the gap between the electrodes, each consisting of bundles of particle strings. Strings, which are fully connected to both electrodes generate an elastic response to the applied stress, whereas strings which are attached at only one end or are unattached generate a viscoplastic response. The net effect of an increase of the electric field strength, particle concentration, or particle conductivity is an increase in elasticity, i.e. predominantly creation of fully connected particle strings.     

浅谈数控电火花加工的编程

介绍了数控电火花加工编程方法的要点，在编程中的一些使用技巧及注意事项。