Ab initio molecular dynamics studies on the ignition and combustion mechanisms,twice exothermic characteristics,and mass transport properties of Al/NiO nanothermite

Ab initio molecular dynamics was used to simulate the ignition and combustion reaction process of the Al/NiO nanothermite. The exothermic characteristics and the role of oxygen migration in the whole reaction were disclosed. The results suggest that the primary and secondary heat release are mainly caused by the interfacial and bulk thermite reaction, respectively. The interfacial oxygen migration and the breaking of Ni–O bonds cause the initial ignition. The destruction of the bulk NiO lattice is the precursor process of the secondary heat release and retards the rapid thermite reaction. The ¹⁸O isotope leads to a lower oxygen mass transport rate, having a great influence on the ignition delay time and reaction progress. The metal–oxygen flip mechanism ensures the continuous propagation of the thermite reaction. Our work may be helpful for understanding the reaction mechanisms of the metal/metal oxide thermites and optimizing the formulation design and performance of the nanothermites.

     

Reactive hot pressing of Al2O3-Ni composites

Reactive hot pressing has been used to form Al₂O₃-Ni composites from Al and NiO. The effect of attrition milling on the precursor powder and subsequent composite formation was examined. The surface area, phase assemblage, reaction temperature, and morphology of precursor powders were characterized as a function of milling time, which ranged from 0 (unmilled) to 480 min (8 hrs). During milling, particle surface area increased from less than 1 to more than 11 m²/g as the size of the Al and NiO particles decreased. At the same time, the temperature at which Al and NiO reacted to form Al₂O₃ and Ni decreased from more than 1000°C to around 600°C. Formation of Al₂O₃ or Ni during milling was not detected, regardless of time. Precursor milling time also affected the morphology and phase assemblage of composites produced by reactive hot pressing. Composites formed from unmilled powders contained a small amount of unreacted NiO and had a Ni ligament size greater than 10 m. The composite forming reaction went to completion when powders milled for one hour or more were hot pressed. Based on microstructural evidence and analogy to similar reactions, it appears that the composite forming reaction proceeds by Al diffusing into and reacting with NiO.     

Effects of metal oxide nanoparticles on combustion and gas-generating performance of NaN3/Al composite powders ignited using a microhotplate platform

We investigated the effects of different metal oxide (MO) nanoparticles (e.g., CuO, KIO₄, Fe₂O₃) on the combustion and gas-generating characteristics of sodium azide microparticle (NaN₃ MP; gas-generating agent) and aluminum nanoparticle (Al NP; heat source) composite powders. The NaN₃ MP/Al NP/MO NP composite powders were stably ignited using a microhotplate (MHP) heater. The addition of CuO and KIO₄ to the NaN₃ MP/Al NP composite powders resulted in relatively high burn rates and high pressurization rates upon MHP-assisted ignition. This suggests that the highly reactive CuO and KIO₄ NPs significantly increased the combustion of the Al NPs; as a result, sufficient heat energy was generated via the active aluminothermic reaction to thermally decompose the NaN₃ MPs. Finally, the gas generating properties of NaN₃ MP/Al NP composite powders mixed with various MO NPs were tested using homemade inflatable small airbags. The airbags were fully inflated within ~20 ms when CuO and KIO₄ NPs were added to the NaN₃ MP/Al NP composite powders. However, the addition of Fe₂O₃ NPs to the NaN₃ MP/Al NP composite powder resulted in a slow and only partial inflation of the airbag due to an incomplete aluminothermic reaction, which was due to a slow combustion reaction between the Al NPs and relatively weak oxidizer of the Fe₂O₃ NPs. This suggests that the rapid, stable, and complete thermal decomposition of NaN₃ MP/Al NP composites can be effectively achieved by employing highly reactive nanoscale oxidizers.     

AZ31B镁合金表面自蔓延反应火焰喷涂Al_2O_3基陶瓷层的组织结构及性能

为了提高火焰喷涂Al2O3基陶瓷涂层的致密度、结合强度和耐磨性,将铝热剂Al/CuO加入氧化铝钛粉(AT)陶瓷骨料中,采用自蔓延(SHS)反应火焰喷涂技术在AZ31B镁合金表面制备了Al2O3基复相陶瓷层,并对涂层的组织结构、致密度、抗热震性、结合强度和耐磨性进行了测试,分析了铝热剂对陶瓷层性能的影响。结果表明:与普通火焰喷涂制备的AT陶瓷层相比,SHS反应火焰喷涂制备的Al2O3基陶瓷层内部有Cu2+1O,Cu3TiO5,Al等新相生成,具有良好的致密度、结合强度和显微硬度,其耐磨性较普通火焰喷涂AT层提高较多。     

用于超级电容器的纳米氧化镍/石墨薄片/银复合电极材料的超声法合成及性能

通过简单超声法制备了球状NiO纳米颗粒、NiO/石墨薄片(NiO/GNS)和NiO/GNS/Ag纳米复合材料。在NiO/GNS和NiO/GNS/Ag复合材料中,GNS作为NiO和Ag纳米颗粒分散的模板,不仅有效避免了NiO和Ag纳米颗粒的团聚,还改善了复合材料的电化学性能。采用场发射扫描电子显微镜(SEM)、透射电子显微镜(TEM)和X射线衍射对样品的成分、形貌和结构进行了表征。NiO/GNS/Ag复合材料(GNS质量分数为0.5%,Ag质量分数为3%)电极表现出优异的电化学性能。在1A/g时,其初始比电容为501.66F/g,2000次循环后其比电容衰减为440.45F/g。这表明所制备的复合材料是一种有应用价值的超级电容器电极材料。     

基于Cu阻挡层的Al/CuO含能半导体桥的电爆性能研究

界面层的反应性是纳米含能复合薄膜(RMFs)制备中的重要因素,直接影响纳米RMFs的反应性能。为了研究纳米Al/CuO RMFs在半导体桥上集成后的电爆性能,采用磁控溅射工艺制备了Al/CuO含能半导体桥（Al/CuO-ESCB）和Al/Cu/CuO含能半导体桥(Al/Cu/CuO-ESCB),研究了Cu层作为阻挡层对Al/CuO-ESCB电爆过程的影响。结果表明:增加Cu阻挡层可以缩短ESCB的临界激发时间,增加ESCB的燃烧时间。     

基于海藻酸钠交联的NiO/C复合材料制备及其储锂性能

NiO作为过渡金属氧化物代表,具有能量密度较高、成本低的优点,在锂离子电池负极材料的应用中引起了广泛关注。通过海藻酸钠与金属离子的自主交联反应,以及碳化、氧化过程,制备了低成本的多孔纳米NiO/C复合材料。得到的复合材料中,NiO纳米颗粒分散均匀且被石墨化碳层包覆,并嵌入多孔相互连通的碳基体中,在提升复合材料整体导电性的同时抑制了活性材料在电化学反应中的体积膨胀。将其用作锂离子电池负极材料时,NiO/C复合材料在0.1,1 A/g的电流密度下分别具有608.2,307.2 mAh/g的比容量,并且在0.1 A/g电流密度下经过100圈循环后仍保持448 mAh/g的比容量,显示出优良的循环稳定性。优良的电化学性能充分显示出NiO/C复合材料在锂离子电池负极材料中的应用潜能。     

Synthesis of Bulk Nano-Al2O3 Dispersed Cu-Matrix Composite Using Ball Milled Precursor

Kinetics of solid-state reduction reaction during ball milling of CuO-Al and CuO-prealloyed Cu(Al) powder blends in dry and wet condition has been investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) techniques. Direct reduction of CuO by Al has resulted into Al₂O₃ dispersed Cu-matrix composite through a self-propagating reaction only during milling in dry condition. However, indirect reduction of CuO by prealloyed Cu(Al) resulted into formation of nano-Al₂O₃ dispersed Cu-matrix composite either by continued ball milling in dry condition or by subsequent thermal treatment of wet milled powder precursor. The influence of milling conditions, that is, milling speed, and milling media, on the occurrence of reduction of CuO by elemental Al or Al in prealloyed Cu(Al) during ball milling have been explained by considering their effects on the rise of powder temperature due to collisions between balls and powder particles, and the rate of reduction of ignition temperature of the reaction due to microstructural refinement. TEM investigation has revealed that the size of Al₂O₃ particles in the composite power blend formed by the indirect reaction route (CuO-prealloyed Cu(Al)) is much finer than the same in case of direct reaction route (CuO-Al). It is suggested that the kinetics of the reduction reaction in the indirect reaction route is relatively sluggish in nature and amenable to processing of large amount of nano-Al₂O₃ dispersed Cu-matrix composite powder for industrial purpose.     

Reactive Structural Materials: Preparation and Characterization

Reactive structural materials, which can serve both as structural elements as well as a source of chemical energy released upon initiation have emerged as an important class of metal‐based composites for use in various energetic systems. Such materials rely on a variety of exothermic reactions, from oxidation to formation of metal‐metalloid and intermetallic phases. The rates of these reactions are as important as the energy that may be released, in order for them to occur at the time scales compatible with the requirements of applications. Therefore, chemical composition, scale at which reactive components are mixed, and the structure and morphology of materials are important and can be controlled by the method of preparation and compaction of the composite materials. Methods of preparation of the composite structures are briefly reviewed as well as methods of characterization of their mechanical and energetic properties. In addition to common thermo‐analytical and static mechanical property measurements, dynamic tests of mechanical properties as well as ignition and combustion experiments are necessary to understand the fragmentation, initiation, and heat release expected for these materials when they are stimulated by an impact, shock, or rapid heating. Reaction mechanisms are studied presently for the thin layers and small samples of reactive materials initiated in carefully designed experiments. In other experiments, impact and explosive initiation are characterized for larger material compacts in the conditions imitating practical scenarios. Examples of results describing thermal, impact, and explosive initiation of some of the reactive materials are presented.

     

组元配比对球磨固态燃烧式反应和扩散型反应的影响

采用搅拌式高能球磨机研究了不同铝含量的Al/CuO球磨固态燃烧反应和Al-Cu及Al-Cu-Al2O3扩散型反应。结果表明：理想配比的Al/CuO的反应孕育期最短,偏离这一配比,孕育期延长,反应由整体燃烧式逐渐过渡到渐进燃烧式完成;球磨强度扩大以燃烧式进行的组元配比范围;当铝含量超过理想配比中的比例,随Al含量增加,反应由单一的还原反应向还原＋合成复合反应模式转化,反应产物为平衡组织,依次为Cu Al2O3、CuoAl4 Al2O3、CuAl2 Al2O3、Al(Cu) Al2O3;而球磨Al-Cu和Al-Cu-Al2O3体系的反应以扩散方式进行,产物是非平衡组织。     

A Combined Analytic,Numeric, and Experimental Investigation Performed on NiTi/NiTiCu Bi‐Layer Composites under Tensile Loading

Adjusting mechanical behavior and controlling deformation parameters are significant tasks in designing shape memory components. In this paper, an analytical model describes the deformation behavior of NiTi/NiTiCu bi‐layer composites under tensile loading. Different deformation stages are considered based on single mechanical behavior at each stage. Closed‐form equations are derived for stress–strain variations of bi‐layer composites under uniaxial loading–unloading. Bi‐layer composites made via the diffusion bonding method from single layers of NiTi alloy with a composition of Ti‐50.7 at.% Ni, as an austenitic layer, and Ti‐45 at% Ni‐5 at% Cu, as a martensitic layer, are produced by the vacuum arc remelting technique. The tensile behavior of single‐ and bi‐layers is investigated by using loading–unloading experiments to find the nominal stress–strain curves. Numerical simulations are also done by employing Lagoudas constitutive model to simulate stress–strain diagrams. The solutions of the analytical method presented are validated by using the numerical simulations as well as the experimental results. With regard to the results obtained from the analytical modeling, the numerical simulations, and the experiments, it is evident that the bi‐layer composites with different thickness ratios provide adjustable mechanical behavior that can be considered in different application designs, for example, actuators equipped with shape memory components.

     

In Situ Oxidation Synthesis of p‐Type Composite with Narrow‐Bandgap Small Organic Molecule Coating on Single‐Walled Carbon Nanotube: Flexible Film and Thermoelectric Performance

Although composites of organic polymers or n‐type small molecule/carbon nanotube (CNT) have achieved significant advances in thermoelectric (TE) applications, p‐type TE composites of small organic molecules as thick surface coating layers on the surfaces of inorganic nanoparticles still remain a great challenge. Taking advantage of in situ oxidation reaction of thieno[3,4‐b]pyrazine (TP) into TP di‐N‐oxide (TPNO) on single‐walled CNT (SWCNT) surface, a novel synthesis strategy is proposed to achieve flexible films of TE composites with narrow‐bandgap (1.19 eV) small molecule coating on SWCNT surface. The TE performance can be effectively enhanced and conveniently tuned by poly(sodium‐p‐styrenesulfonate) content, TPNO/SWCNT mass ratio, and posttreatment by various polar solvents. The maximum of the composite power factor at room temperature is 29.4 ± 1.0 µW m^?1 K^?2. The work presents a way to achieve flexible films of p‐type small organic molecule/inorganic composites with clear surface coating morphology for TE application.     

Highly Efficient Photoelectrocatalytic Reduction of CO2 to Methanol by a p–n Heterojunction CeO2/CuO/Cu Catalyst

Photoelectrocatalytic reduction of CO2 to fuels has great potential for reducing anthropogenic CO2 emissions and also lessening our dependence on fossil fuel energy.Herein,we report the successful development of a novel photoelectrocatalytic catalyst for the selective reduction of CO2 to methanol,comprising a copper catalyst modified with flower-like cerium oxide nanoparticles(CeO2 NPs)(a n-type semiconductor)and copper oxide nanoparticles(CuO NPs)(a p-type semiconductor).At an applied potential of−1.0 V(vs SCE)under visible light irradiation,the CeO2 NPs/CuO NPs/Cu catalyst yielded methanol at a rate of 3.44μmol cm^−2 h^−1,which was approximately five times higher than that of a CuO NPs/Cu catalyst(0.67μmol cm^−2 h^−1).The carrier concentration increased by^108 times when the flower-like CeO2 NPs were deposited on the CuO NPs/Cu catalyst,due to synergistic transfer of photoexcited electrons from the conduction band of CuO to that of CeO2,which enhanced both photocatalytic and photoelectrocatalytic CO2 reduction on the CeO2 NPs.The facile migration of photoexcited electrons and holes across the p–n heterojunction that formed between the CeO2 and CuO components was thus critical to excellent light-induced CO2 reduction properties of the CeO2 NPs/CuO NPs/Cu catalyst.Results encourage the wider application of composite semiconductor electrodes in carbon dioxide reduction.     

Thermodynamic analysis of thermite reactions for synthesizing Ni-based alloys

The equilibrium compositions of a thermite reaction system (Fe₂O₃+Cr₂O₃+CrO₃+NiO+O₂+N₂+TiO₂+C+S+CaO+Al), which was used to prepare Ni-based alloys, were predicted by means of the free energy minimization method. The effects of the reactant contents on the equilibrium compositions and the adiabatic temperature of the thermite reaction system were analyzed. The microstructure of the Ni-based alloys was investigated. The thermodynamic analysis indicated that the products of the thermite reaction system contained primarily Al₂O₃, Fe, Ni, Cr, Ni₃Al and NiAl, due to impurities C, N₂ and S in the raw materials, there could also exist TiC, Cr₇C₃, CrN, CaS and TiN in the equilibrium products. The amount of Ni₃Al and NiAl in the products increased with increase of the aluminum content in the starting materials. The microstructure investigation showed that the Ni-based alloys consisted of austenite, ferrite, Ni₃Al and NiAl. The existence of Ni₃Al and NiAl in the Ni-based alloys reconciled with the predictions of thermodynamic analysis, suggesting the free energy minimization method is useful in designing advanced materials with multiple components by thermite reaction technique.     

Enhancement of photocatalytic properties of TiO2 nanobelts through surface-coarsening and surface nanoheterostructure construction

In this work, heterostructures were obtained by uniformly assembling NiO nanoparticles on the surface-coarsened TiO₂ nanobelts through a precipitation process. The uniform assembling of NiO nanoparticles led to a large number of nano-p-n-junction heterostructures on the surface of the TiO₂ nanobelts, where NiO and TiO₂ are p- and n-type semiconductors, respectively. Compared with both pure NiO nanoparticles and TiO₂ nanobelts, NiO nanoparticles/TiO₂ surface-coarsened nanobelt heterostructured composite exhibited a greatly enhanced photocatalytic activity in the decomposition of a model dye compound methyl orange under ultraviolet light irradiation. It was argued that the nano-p-n-junctions effectively reduce the recombination of electrons and holes, thus resulting in enhanced photocatalytic property of the heterostructured composites. The better performance of the surface-coarsened nanobelts is due to the increased photo absorption and production of charge carriers, which renders the composites with further enhanced photocatalytic performance. The established approach allows the control of the nano-p-n junction heterostructure on the nanobelts, and hence, their photocatalytic effect.     

Al2O3/NiO包裹Ni纳米颗粒的结构和磁性

用电弧法蒸发Ni-Al合金(4%～5%Al,质量分数),制备了Al2O3/NiO包裹Ni及Ni-Al合金纳米颗粒.高分辨电镜显示该纳米颗粒具有壳核结构,核为纳米Ni及Ni-Al合金,壳为Al2O3/NiO复合氧化物.壳的厚度为2～4 nm,颗粒的尺寸为5～60 nm.壳核结构防止纳米Ni颗粒的进一步氧化和团聚.饱和磁化强度为29.6 Am2/kg,矫顽力为4.13 kA/m.由于铁磁和反铁磁性相界面处存在交换耦合作用,磁滞曲线出现小的偏置.     

CuO/Al反应自生复合材料的动力学研究

用XD法研究了CuO/Al反应自生粒子增强铝基复合材料的动力学机制。通过CuO/Al反应中的热效应测试和反应过程中对材料的显微组织实验观察及对CuO/Al的反应过程及微观机制的分析,建立了合成反应的动力学模型。     

Processing of In Situ Aluminium Matrix Composites by Micropyretic Reactive Sintering

In situ aluminium matrix composites were processed by the micropyretic reactions between the host aluminium matrix and powder containing nanodimensional Fe₂O₃ crystallites used as the precursor compound. Differential Scanning Calorimetric (DSC), studies have shown, use of nanosized Fe₂O₃ crystallites has decreased the initiation temperature of the in situ micropyretic reaction and the desirable reinforcements; iron-aluminide and alumina are formed during sintering. Microstructural features of the composite consist of well-dispersed reinforcements in the host Al matrix with size less than 1 µm. Wear tests performed on the composite samples have revealed coefficient of friction and wear volume can be reduced significantly by incorporating 20 vol% reinforcement.     

Formation of spinel (MgAl2O4), MgO and pure Cu particles in Al-2Mg alloy-CuO particle composites

CuO particles were introduced into liquid Al-2Mg alloy by the vortex method to prepare an Al alloy-MgAl₂O₄ in situ particle composite, by reaction between CuO particles and the Al-2Mg alloy melt. Pure Cu, MgAl₂O₄ and MgO particles were detected in the particles extracted from the composites. DTA study showed partial dissolution of Cu in the matrix. Microhardness and hardness of the composites are higher than those of the base alloy. Both microhardness and hardness are higher for the Al-2Mg-2CuO composite than those of the Al-2Mg-5CuO composite. The hardness of the Al-2Mg-2CuO composite is remarkably high. The increase in microhardness has been attributed to the solid solution hardening effect with Cu as well as to the difference in CTE between the Al matrix and the particles. On the other hand, the improvement in hardness resulted from both solid solution hardening as well as the presence of hard particles such as MgAl₂O₄ and MgO.Retired.     

影响内氧化生成Al2O3/Cu表面复合层厚度及组织的因素

采用真空条件,用CuO作氧化剂,在一定的温度下使Cu-Al合金内氧化,获得Al₂O₃/Cu表面复合材料。金相分析发现,在较低温度下内氧化,表面复合层中Al₂O₃颗粒晶界处多于晶内;在较高温度下内氧化,复合层中Al₂O₃颗粒呈弥散状分布;表面复合层厚度随Al含量的增加而减薄,显微硬度随Al含量的增加而升高。