Thermal–mechanical analysis for confined HMX-based polymer-bonded explosives

It is known that the reaction rate of the thermal decomposition of polymer-bonded explosives exposed to cook-off has a certain relation with temperature, confining pressure and some other factors, which were verified by many experiments. Temperature-dominated thermal-decomposition models were developed for various high explosives and applied to study their decomposition process, such as HMX- and TATB-based polymer-bonded explosives. These models have reasonable accuracy. For example, the multistep thermal-decomposition model of PBX 9501 (which consists of 95% HMX, 2.5% Estane and 2.5% BDNPA/F) proposed by Tarver considers decomposition of both HMX and polymer binders. The temperature-dominated thermal-decomposition model only applies to preignition thermal decomposition. After ignition occurs, the dominant mechanism of the reaction transforms to deflagration and subsequent explosion, where the effect of pressure can no longer be neglected. Furthermore, the time scale of deflagration and explosion (millisecond or microsecond) differs significantly from the time scale of slow thermal decomposition (hours or minutes). The numerical model of postignition phenomenon (deflagration and final explosion) is still under investigation and is far from maturity. The predicted violence scale resulting from thermal explosion does not agree with experiment very well. An alternative method is to conduct a thermal–mechanical analysis for preignition stage, which takes advantage of a developed temperature-dominated thermal-decomposition model, and to analyze the stress caused by quasi-static thermal expansion. Herein, a thermal–mechanical analysis is implemented for a one-dimensional time-to-explosion experiment (ODTX) and a scaled thermal explosion experiment (STEX) with HMX-based polymer-bonded explosives inside using the finite element method. Then, the finite element model is applied to investigate the thermal decomposition of PBX 9501 inside an explosive device exposed to cook-off. The regions that have maximum temperature, maximum hydrostatic pressure and maximum von Mises stress are identified based on simulation results, which can benefit future improvement of the explosive device.     

Thermal–mechanical modelling around the cavities of underground coal gasification

Underground coal gasification (UCG) is an efficient method for the conversion of the deep coal resources into energy. This paper is concerned with a feasibility study of the potential of deeply lying coal seams (>1200 m) for the application of UCG combined with subsequent storage of CO₂ for a site located in Bulgaria. A thermal–mechanical coupled model was developed using the ABAQUS software package to predict the heat transfer, the stress distributions around the UCG and the consequent surface subsidence. Material properties of rocks and coal were obtained from existing literature and geomechanical tests which were carried out on samples derived from the demonstration site in Bulgaria. Three days of gasification has been simulated by assigning a moving heat flux on a cell of 2 m × 2 m × 2 m at a velocity of 2 m/day. Results of temperature and stress distribution showed that the developed numerical model was able to simulate the heat propagation and the stress distribution around cavities under a thermal–mechanical coupled loading during the UCG process. Also, the surface subsidence was found to be 0.08 mm after three days of gasification for the case studied. It is anticipated that the results of this paper can be used for the prediction and optimization of the UCG process in deep coal seams.     

On the preparation of amorphous Mg–Ni alloys by mechanical alloying

This paper deals with the preparation of the amorphous Mg–Ni alloys. By mechanical alloying (MA) the amorphous Mg–Ni alloys with different compositions have been prepared of pure elemental magnesium and nickel powder. The as-milled powder was characterized by X-ray diffraction analysis and transition electron microscope (TEM) observation. The results showed that the 30 < Ni < 70 at.% composition could be amorphized with a milling time strongly dependent on the starting chemical compositions. The investigation on the early stage of MA showed that the different compositions amorphized by two different paths. On the magnesium rich side of 30–70 at.% Ni, the as-milled powders first formed the intermetallic compound Mg₂Ni, which subsequently destabilized into the amorphous phase. For the nickel rich side, the amorphous was obtained directly from the mechanical blend of magnesium and nickel powder by the suppression of the formation of thermodynamic equilibrium phase MgNi₂. © 1999 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.     

Electrochemical performance of hydrogen evolution reaction of Ni–Mo electrodes obtained by mechanical alloying

Electro active Ni–Mo electrodes have been prepared by mechanical alloying and pressure-less sintering $\text{(1173}\text{K}\text{)}$ Ni and Mo powders. The electrochemical performance of obtained electrodes has been evaluated in KOH 30% at $\text{343}\text{K}$ as a function of the milling time, applied pressure for green compaction as well as the effect conferred by the addition of a process control agent (PCA). Cathodic slope of the best specimen is $\text{279}\text{mV}\text{/}\text{dec}$. Faster reaction kinetics is observed for the specimens treated with PCA addition. The longer milling time and applied pressure on the specimens the better cathodic response. The activation overpotential, i.e. cathodic-Tafel slopes found at high overvoltages are in the range of 274–$\text{481}\text{mV}\text{/}\text{dec}$, whereas the exchange current density for the hydrogen evolution reaction ranged from 27.3 to $\text{1.4}\text{mA}\text{/}\text{cm}{}^2$.     

Hydrogen storage thermodynamics and kinetics of RE–Mg–Ni-based alloys prepared by mechanical milling

The nanocrystalline/amorphous NdMg₁₁Ni + x wt.% Ni (x = 100, 200) composite hydrogen storage alloys were synthesized by ball milling, and the effects of Ni content and milling time on the hydrogen storage thermodynamics and dynamics of the alloys were systematically investigated. The results reveal that the variation of the Ni content has a slight effect on the thermodynamic properties of the alloys, but it significantly improves their absorption and desorption kinetics performance. The variation of the milling time clearly affects the hydrogen storage properties of the alloys. Hydrogen absorption capacity and hydrogen absorption saturation ratio have maximum values with milling time varying. But hydrogen desorption ratio always increases with milling time prolonging. It is found that the hydrogen desorption activation energy of the alloys clearly decreases with increasing Ni content and milling time, which is responsible for the improved hydrogen desorption kinetics of the alloys.     

Synthesis and hydrogenation properties of Mg–La–Ni–H system by reactive mechanical alloying

We have synthesized Mg–30 mass%LaNi_2.28 composite material and investigated its hydrogenation behaviour. The reactive mechanical alloying process of the mixture of Mg and LaNi_2.28 was studied. It is found that a composite of _MgH2${\mathrm{MgH}}_2$, _La4H12.9${\mathrm{La}}_4{\mathrm{H}}_{12.9}$ and _Mg2NiH4${\mathrm{Mg}}_2{\mathrm{NiH}}_4$ formed after 80 h ball-milling under 3.0 MPa hydrogen. Scanning electron microscopic analysis indicated that these new phases are distributed homogeneously. This composite shows excellent hydriding properties even at moderate temperature. Under 3.0 MPa hydrogen pressure it absorbed more than 80% of its full capacity in the temperature range of 473–553 K within less than 1 min. The maximum hydrogen absorption capacity at 553 K is 5.4 mass%. The enhanced hydriding properties could be attributed to the fine and uniform particles and a synergeticly catalytic effect generated by mechanical milling.     

Effect of organoclay on the mechanical/thermal properties of microcellular injection molded PBT–clay nanocomposites

An organically modified montmorillonite (MMT) was compounded with polybutylene terephthalate (PBT) in a twin-screw extruder. The organoclay PBT nanocomposites were then injection molded by conventional and microcellular methods. Nitrogen was used as the blowing agent. The effect of organoclay content, organoclay size (8 and 35 μm), and speed of the screw (80 and 100 rpm) on the mechanical and thermal properties were investigated.     

Chemical–mechanical pretreatment of wood: Reducing downsizing energy and increasing enzymatic digestibility

A chemimechanical (CM) pretreatment method was devised, wherein wood chips are acid-treated to weaken the physical structure then disc-refined to produce a fibrous substrate. CM pretreatment was directly compared with a common dilute acid (DA) pretreatment method, wherein wood is mechanically downsized to a powder or fiber substrate and then acid-treated. It was hypothesized that the CM pretreatment sequence would reduce the energy required for size reduction and increase enzymatic digestibility of the pretreated substrate. By treating wood chips in a dilute sulfuric acid liquor before the mechanical downsizing step, the net specific energy (NSE) of disc-refining was reduced by up to 95%. At the optimal acid treatment and disc-refining conditions determined within this work, CM pretreatment could produce a highly digestible lignocellulose substrate (95% cellulose conversion) while requiring less than 100 kWh/tonne od NSE for mechanical downsizing. A comparison of CM and DA pretreated hardwood revealed that CM pretreatment produced a significantly more digestible substrate than DA pretreatment. Differences in the digestibility of CM and DA pretreated substrates were attributed to differences in physical structure. CM pretreatment produced a substrate that consisted primarily of single fibers and small fiber bundles, while DA pretreatment produced larger fiber bundles. Furthermore, the CM pretreated substrate had a more accessible pore structure, and an altered distribution of surface lignin.     

Evaluation of the mechanical performance of a composite multi-cell tank for cryogenic storage: Part II – Experimental assessment

This study focuses on the understanding of the thermal and structural behavior of an innovative Type IV multi-spherical composite-overwrapped pressure vessel through an experimental assessment that consists of hydrostatic testing at ambient conditions and pressure cycling with a cryogenic medium (LN₂). During hydro-burst testing at a high displacement rate, the strain and damage progression is monitored with Digital-Image-Correlation (DIC) and Acoustic Emission (AE) techniques respectively. The effect of filling with LN₂, pressure cycling and draining on the composite overwrap temperature gradient and strain evolution is additionally obtained with Fiber Bragg Gratings (FBGs) and thermocouples. Utilization of AE helped to reveal the different damage mechanisms occurring and enabled the evaluation of the pressure window of the multi-sphere. The experimental measurements in the cryogenic regime verified the suitability of the involved stiffness and coefficient of thermal expansion (CTE) fitting functions developed in [32] that enable to establish of a relationship between strain and temperature during cryogenic chill-down and pressure cycling. This study provides a framework about the suitability of conformal Type IV multi-spherical COPVs for cryogenic storage.     

Hydrogen storage properties of Mg–Ni–C system hydrogen storage materials prepared by hydriding combustion synthesis and mechanical milling

Mg–Ni–C composite hydrogen storage materials were prepared by first ball milling the powder mixtures of carbon aerogel and nano-Ni, and then mixed with magnesium powder followed by hydriding combustion synthesis (HCS). The HCS product was further treated by mechanical milling for 10 h. The effect of Ni/C ratio on the structures and hydrogen absorption/desorption properties of the materials were studied by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and pressure–composition–temperature (PCT) measurements. It is found that 90Mg–6Ni–4C system shows the best hydriding/dehydriding properties, which absorbs hydrogen at a saturated capacity of 5.23 wt.% within 68 s at 373 K and desorbs 3.74 wt.% hydrogen within 1800 s at 523 K. Moreover, the dehydriding onset temperature of the system is 430 K, which is 45 K lower than that of 90Mg–10Ni system or 95 K lower than that of 90Mg–10C system. The improved hydriding/dehydriding properties are related greatly to the Ni/C ratio and the structures of the composite systems.     

Electrochemical performance of crystalline Ni–Co–Mo–Fe electrodes obtained by mechanical alloying on the oxygen evolution reaction

Electro-active _Co30Ni70${\mathrm{Co}}_{30}{\mathrm{Ni}}_{70}$, _Co30Mo70${\mathrm{Co}}_{30}{\mathrm{Mo}}_{70}$, _Ni30Mo70${\mathrm{Ni}}_{30}{\mathrm{Mo}}_{70}$, _Co10Ni20Mo70${\mathrm{Co}}_{10}{\mathrm{Ni}}_{20}{\mathrm{Mo}}_{70}$, _Fe10Co30Ni60${\mathrm{Fe}}_{10}{\mathrm{Co}}_{30}{\mathrm{Ni}}_{60}$ and _Co10Fe30Ni60${\mathrm{Co}}_{10}{\mathrm{Fe}}_{30}{\mathrm{Ni}}_{60}$ wt% alloys have been prepared by mechanical alloying. The electrocatalytic behavior of obtained electrodes has been evaluated in 30 wt% KOH aqueous solutions as a function of different temperatures (298, 323 and 343 K) and overvoltages (η=200$\eta =200$, 300, 400 mV). The effect of Fe contamination during mechanical milling was also analyzed. The electrode performance was studied by cyclic voltammetry, ac–impedance and steady-state polarization techniques. Appreciable current values for oxygen evolution reactions (OER) were measured at Ni–Co–Mo–Fe electrodes produced by this technique. It was also found that the electrochemically formed Co and Fe oxides and/or hydroxides did not show an activity for OER as good as reported on hydrogen evolution reaction (HER). Tafel plots of preoxidized at 1 cycle and prolonged cycled (50 cycles) for _Co30Ni70${\mathrm{Co}}_{30}{\mathrm{Ni}}_{70}$, _Co30Mo70${\mathrm{Co}}_{30}{\mathrm{Mo}}_{70}$, _Ni30Mo70${\mathrm{Ni}}_{30}{\mathrm{Mo}}_{70}$, _Co10Ni20Mo70${\mathrm{Co}}_{10}{\mathrm{Ni}}_{20}{\mathrm{Mo}}_{70}$, _Fe10Co30Ni60${\mathrm{Fe}}_{10}{\mathrm{Co}}_{30}{\mathrm{Ni}}_{60}$ and _Co10Fe30Ni60${\mathrm{Co}}_{10}{\mathrm{Fe}}_{30}{\mathrm{Ni}}_{60}$ crystalline electrodes were very different, which might be related to changes in the surface enrichment of one or two of the alloy constituents. The Mo electrocatalytic effects seem to be more important on the OER at higher temperatures than those showed previously for HER. Molybdenum containing crystalline Co and Ni powders (_Co10Ni20Mo70)$({\mathrm{Co}}_{10}{\mathrm{Ni}}_{20}{\mathrm{Mo}}_{70})$ showed the best catalytic behavior for the OER.     

A mechanical–electrical finite element method model for predicting contact resistance between bipolar plate and gas diffusion layer in PEM fuel cells

Contact resistance between the bipolar plate (BPP) and the gas diffusion layer (GDL) plays a significant role on the power loss in a proton exchange membrane (PEM) fuel cell. There are two types of contact behavior at the interface of the BPP and GDL, which are the mechanical one and the electrical one. Furthermore, the electrical contact behavior is dependent on the mechanical one. Thus, prediction of the contact resistance is a coupled mechanical–electrical problem. The current FEM models for contact resistance estimation can only simulate the mechanical contact behavior and moreover they are based on the assumption that the contact surface is equipotential, which is not the case in a real BPP/GDL assembly due to the round corner and margin of the BPP.     

Effect of high temperature deformation on the microstructure,mechanical properties and hydrogen embrittlement of 2.25Cr–1Mo-0.25 V steel

In this paper, the effects of high temperature deformation on the microstructure, mechanical properties and hydrogen embrittlement (HE) of the 2.25Cr–1Mo-0.25 V steel was investigated by a scanning electron microscope (SEM), a transmission electron microscope (TEM) and tensile tests. The SEM and TEM images demonstrated that high temperature plastic deformation (HTPD) led to the coarsening of carbides and the dislocation density increase. The tensile tests displayed that the HTPD resulted in the cracking susceptibility increase, as indicated by the increased numbers and sizes of cracks at the fractured surface. This was attributed to the coarsening of carbides during high temperature deformation. In contrast, the HTPD highly decreased the loss of ductility compared to the un-deformed specimens, although the amount of ductility losses (elongation or reduction of area) did not change significantly as the HTPD increased. The correlations among carbides, hydrogen and cracks were discussed.     

Hydrogen storage properties of Mg–Ni–Cu prepared by hydriding combustion synthesis and mechanical milling (HCS + MM)

The effect of Cu-doping on the hydrogen storage properties of Mg₉₅Ni₅Cu_x (x = 0, 0.5, 1, 2) prepared by hydriding combustion synthesis and mechanical milling (HCS + MM) was studied. For dehydriding properties, the dehydriding temperature onset decreases from 450 K for Mg₉₅Ni₅ to 420 K for Mg₉₅Ni₅Cu₂. Additionally, the activation energy for dehydriding decreases from 116 kJ/mol for Mg₉₅Ni₅ to 98 kJ/mol for Mg₉₅Ni₅Cu₂, indicating that the dehydriding reaction is activated by the catalytic effect of Cu. Moreover, the hydrogen absorption capacity of Mg₉₅Ni₅Cu₂ at 373 K in 100 s increases from 0.95 to 4.16 wt.% by MM pretreatment before HCS. The factors for the improvement of the hydrogen storage properties are discussed in this paper.