Oxidation kinetics of metallic powders

Aluminum and magnesium are widely used in pyrotechnic formulations and other energetic materials; they are also common components of reactive alloys, e.g., Al-Mg and B-Mg, and others, which are potential fuels for explosives and propellants. Reaction mechanisms and oxidation kinetics of aluminum, magnesium and Al-Mg alloy powders in different oxidizing environments are investigated using thermo-analytical measurements. New methods of data processing are developed, relying on measured particle size distributions of the reactive spherical powders. It became possible to identify the reaction interface location for many heterogeneous metal oxidation processes; for several reactions, detailed kinetic descriptions are obtained. For aluminum powders, location of the reaction interface is established for oxidation in steam and liquid water. Stage-wise oxidation behavior is observed and interpreted. The oxidation of aluminum covered by a thin natural oxide layer in oxygen occurring at relatively low temperatures is quantitatively characterized using different types of thermo-gravimetric (TG) measurements with increased amount of powder for greater sensitivity. Activation energy and the pre-exponent are determined as a function of reaction progress using isoconversion processing and assuming a diffusion-limited reaction mechanism. The reaction kinetics is also established for aluminum nanopowders. It is shown that the oxidation mechanism established for micron sized aluminum remains valid for particles as small as 10 nm. Aluminum oxidation model is combined with a heat transfer model to describe ignition of aluminum particles exposed to a heated oxidizing environment. For magnesium powders, their oxidation by both oxygen and steam was studied by thermo-analytical measurements for micron-sized powders. The location of reaction interface is identified using experiments with spherical powders with different but overlapping particle size distributions. The reaction is found to occur at the interface of metal and the growing oxide layer for all oxidizing conditions. Thus, the reaction is rate limited by diffusion of oxidizer to the metal surface. Reaction rates for low and elevated temperatures are quantified using heat flow calorimetry and TG measurements, respectively. Simplified diffusion-limited reaction models are developed for oxidation of magnesium in both oxygen and steam. The models enable one to predict both pre-ignition reactions and the time of Mg powder aging when exposed to moisture or oxygen at different temperatures. Finally, the mechanisms of low-temperature, heterogeneous oxidation of differently prepared Al-Mg alloy powders in oxygen are studied using thermo-gravimetric measurements. Fully and partially oxidized samples are recovered and characterized using scanning electron microscopy and x-ray diffraction. Voids grow within oxidized alloy powders for both atomized and mechanically alloyed powders. Two oxidation stages are identified for both alloy powders. Both magnesium and aluminum are oxidized at first oxidation stage, producing MgO and amorphous alumina. Spinel MgAl2O4 is produced during the second stage. The reaction is found to occur at the internal surface of the oxide shell as determined by matching the oxidation dynamics for particles with the same size but belonging to powders with different particles size distributions. Apparent activation energies for both oxidation stages are obtained as a function of the thickness of the growing oxide layer. The switchover between oxidation stages occurs when the spinel structure starts forming

The aqueous supramolecular chemistry of crown ethers

This mini-review summarizes the seminal exploration of aqueous supramolecular chemistry of crown ether macrocycles. In history, most research of crown ethers were focusing on their supramolecular chemistry in organic phase or in gas phase. In sharp contrast, the recent research evidently reveal that crown ethers are very suitable for studying abroad range of the properties and applications of water interactions, from: high water-solubility, control of Hofmeister series, “structural water”, and supramolecular adhesives. Key studies revealing more details about the properties of water and aqueous solutions are highlighted

Reactivity of Al/CuO nanothermite composites with fluoropolymers

Aluminum/copper oxide (Al/CuO) nanothermite is a promising metal fuel due to its high combustion enthalpy and excellent gas-producing characteristics. Fluoropolymers are energetic binders that can potentially be used to make moldable reactive composites without compromising the ignitability of thermites for structural applications. Herein, we report the reactivity of Al/CuO nanothermite composites with two processable fluoropolymers, THV, and Viton. The reactivities of the prepared composites were investigated from pressure measurement. The corresponding reaction mechanism was also studied by post-reaction products analysis. The results showed that both THV and Viton composites, even those containing up to 30 wt.% of fluoropolymer, were ignitable. However, the reactivities of the prepared composites were observed to reduce to different extent depending on the type of fluoropolymers used. The reduced reactivity was ascribed to the subdued nanothermite reaction due to the consumption of copper oxide and aluminum by reacting with the fluoropolymers. Among the two fluoropolymers studied, the THV composite exhibited a relatively higher reactivity. The ignition delay and combustion temperature of the THV composite were further determined

Combustion of fluoropolymer coated Al and Al–Mg alloy powders

This work presents an experimental investigation of the combustion characteristics of micron scale aluminum and aluminum-magnesium alloy powders coated with a thin layer of fluoropolymer. Burn times of the coated powders ignited by CO₂ laser were estimated from the time resolved emission signals recorded by photomultiplier tubes. Both fluoropolymer coated powders recorded reduced burn times. This result is likely associated with the lowered diffusion barrier in the fluoropolymer coated particles due to the gasification of oxide shell in the presence of fluorinated species from the decomposing fluoropolymer. Combustion temperatures determined using two-color pyrometry and optical spectroscopy were consistently higher for the fluoropolymer coated powders in comparison with that of the pristine. The reactivity of Al and Al-Mg alloy powders as assessed by constant volume explosion experiments was improved due to the fluoropolymer coating. Dust clouds of fluoropolymer-coated samples could achieve higher burning velocity as estimated from the experimental pressure traces using a semi-empirical correlation for dust explosions. A plausible mechanism responsible for the improvement in metal combustion due to the incorporation of fluoropolymer was proposed

Combustion characteristics of fluoropolymer coated boron powders

The problem of sluggish combustion reactivity of Boron (B) can be overcome by inclusion of fluoropolymers. In this paper, three commercial fluoropolymers; PVDF (59 wt% F), Viton (66 wt% F) and THV (72 wt% F) coated (Ca. 4 wt%) B powders (Ca. 1-µm in size) were prepared and their combustion characteristics have been investigated. Among the fluoropolymer-coated powders, THV coated B provided the highest average improvement in heat of oxidation, reactivity in terms of pressure generation and combustion temperature followed by Viton and PVDF coated B. The sequence of reactivity enhancement has been explained by the variances in gasification efficiencies of the boric oxide shell, induced by the thermal decomposition of the respective fluoropolymer coatings. THV with higher fluorine and lower hydrogen content, supposedly promotes better gasification of the boric oxide shell by releasing more fluorine rich alkanes/alkenes during thermal decomposition. However, PVDF predominantly produces HF, which apparently less effective in gasification of the boric oxide shell, resulting in limited improvement of the measured properties

Nanothermite composites with a novel cast curable fluoropolymer

Structural nanoenergetic (SNE) composites consisting of nanothermites dispersed in a polymer binder are attractive for combustion applications due to their substantially higher volumetric energy, density, and tailorable energy release characteristics. Cast curing is an extensively utilized, safe, and economical method for processing energetic materials into structural composites possessing macroscale geometries. The use of cast curing as a route for processing SNE composites is limited by the lack of suitable binder systems. The widely used cast curable binder such as hydroxyl terminated polybutadiene (HTPB) if used for this purpose will render the SNE composites thermally non-ignitable. Fluoropolymers have emerged as energetic binders for aluminum containing nanothermites due to their capability to induce pre-ignition reaction and sustain the ignitability of composites. Herein, we report the synthesis of a novel cast curable fluoropolymer (FP) with 58 wt% fluorine content produced through the cationic ring opening polymerization of 3-Perfluorohexyl-1, 2-epoxypropane. We demonstrate the utility of the synthesized FP as a cast curable binder for aluminum/copper oxide (Al/CuO) nanothermite to prepare SNE composites. The SNE composites were cast cured as cylindrically shaped specimens with the FP content ranging from 30 to 50% by weight. All the SNE composites were observed to be thermally ignitable even under an inert environment, suggestive of the unique role of FP on promoting the ignitability of SNE composites. SNE composite containing 40 wt% FP recorded the highest energetic performance in terms of heat of reaction (880 cal/g) and pressure generation characteristics (pressurization rate = 3.81kPsi/s; time to maximum pressure = 340 ms). Additionally, the 40 wt% FP SNE composite generates time averaged combustion temperature of 2530 K as estimated by spectroscopic technique

Combustion characteristic and aging behavior of bimetal thermite powders

Nanothermites have been employed as fuel additives in energetic formulations due to their higher energy density over CHNO energetics. Nevertheless, sintering and degradation of nanoparticles significantly limit the practical use of nanothermites. In this work, combustion characteristic and aging behavior of aluminum/iron oxide (Al/Fe2O3) nanothermite mixtures were investigated in the presence of micron-scale nickel aimed to produce bimetal thermite powders. The results showed that the alumina content in the combustion residue increased from 88.3% for Al/Fe2O3 nanothermite to 96.5% for the nanothermite mixture containing 20 wt% nickel. Finer particle sizes of combustion residue were obtained for the nanothermite mixtures containing nickel, indicative of the reduced agglomeration. Both results suggested a more complete combustion in the bimetal thermite powders. Aging behavior of the nanothermite mixture was also assessed by measuring the heat of combustion of the mixture before and after aging process. The reduction in heat of combustion of nanothermite mixtures containing nickel was less severe as compared to a significant decrease for the nanothermite mixture without nickel, indicating better aging resistance of the bimetal thermite powders.Published versio

The effect of alumina as an interfacial layer on the reactivity of Al/PTFE energetic composites

The exothermic behaviors of metastable intermixed composites (MICs) composed of aluminum (Al) and fluoropolymer are highly dependent on the interfacial properties. To study the effect of alumina thickness on the reactivity of Al/PTFE composites, the composites with different particle sizes have been prepared and evaluated in terms of thermal reactivity, which has been realized by experiments and molecular dynamic (MD) simulations. The kinetic parameters obtained from thermal analysis experiments are in good agreement with the simulation results. The isothermal MD simulations reveal the initial reaction process of the composite containing nanosized Al can be divided into three zones. The existence of pre-reaction zone is responsible for the lower ignition delay observed for nanosized Al in presence of PTFE. However, the chemisorption dominates the reaction of the bulk alumina, with condensed products readily to aggregating on the surface. Self-diffusion coefficient analysis indicates reducing the thickness of alumina shell by one time will increase the peak diffusion rate of oxidant atoms by 1.6 times. The molten Al atoms crossing through the core-shell interface could reach a heat transfer rate of 150 J/ps, whereas the values for model representing micron composites and bulk alumina are only 51.5 J/ps and 37.0 J/ps, respectively

The Source of Fracture-Cave Mud Fillings of the Ordovician Yingshan Formation and Its Paleokarst Environment in the Northern Slope of the Tazhong Uplift, Tarim Basin, China: Based on Petrology and Geochemical Analysis

The karst fracture-cave oil and gas reservoirs of the Yingshan Formation in the northern slope of the Tazhong Uplift are well developed and have achieved good exploration results. However, the karst fracture-cave near the top of the Yingshan Formation is basically filled with mud fillings, which seriously affect the reservoir property, and the source and filling environment of the mud fillings have been unclear. Through the petrological and geochemical analysis of the fracture-cave fillings system in the typical wells of the Yingshan Formation, it has been found that (1) the fracture-cave fillings are mainly composed of a mixture of the bedrock dissolution dissociation particles, clay minerals, and calcite cements of the Yingshan Formation, and the content of each component in the different wells or in the cave interval is quite different. (2) Rare earth element analysis shows that the rare earth distribution pattern of the fracture-cave fillings is similar to the bottom marlstone of the Lianglitage Formation, indicating that the fracture-cave fillings should be mainly derived from the early seawater of the deposition during the Lianglitage Formation. (3) Cathodoluminescence, trace element analysis, and previous studies have shown that the formation and fillings of the fractures and caves mainly occurred in the hypergene period, which had the characteristics of an oxidized environment, and that there are two filling effects. First, the limestone of the Yingshan Formation experienced the formation of karst caves due to meteoric freshwater dissolution during the exposure period, and the limestone of the Yingshan Formation was dissolved, resulting in some insoluble clay and residual limestone gravel particles brought into the cave by the meteoric freshwater for filling. Second, the seawater transgression also played an important role during the deposition of the Lianglitage Formation. The clay content in the seawater was high during the early deposition of the Lianglitage Formation, which led to the clay being brought into the caves by the seawater during the deposition of the Lianglitage Formation for further filling; at the same time, calcite deposited into the caves with the clay. The above research promotes the study of the formation mechanism of the karst cave reservoir in the Yingshan Formation and has important theoretical significance for the guiding of the next oil and gas exploration in this area

Identification and Evolution of Different Genetic Types of Deep Karst Caves Controlled by Faults—A Case Study in Huanjiang Sag, Guangxi Province, South China

In recent years, it has become more and more common to drill deep karst caves as a part of deep shale gas resource exploration and engineering construction in South China. However, the amount of research on the genesis and development mechanism of deep karst caves is relatively low. Based on drilling core karst morphology analysis and two-dimensional (2D) seismic and wide-field geophysical exploration methods, it is revealed that the deep karst in the Huanjiang area is mainly composed of net cracks, holes expanding along cracks and dolomite honeycomb pores, and that large karst caves are also developed, which are related to NW-trending faults. The deep karst is developed in the hanging wall of the fault, with a width of 500 m and a height of 1500 m, and a linear distribution along NW faults in the region. Based on Th-U dating, inclusion testing and rare earth elements from cave fillings, it is revealed that the development of deep karst space is related to two deep karst genetic types: The first is the hypogene hydrothermal karst, which developed in the Yanshanian period and is related to regional magmatism. The second is the groundwater deep circulation karst, mainly developed after the Himalayan period, which is related to the deep circulation of meteoric water. The genesis of deep karst space is the result of multi-stage karst superposition and is mainly controlled by faults. It is difficult to determine the specific time points of these two types of deep karst transformation, but according to regional tectonic evolution, we speculate that the Yunnan-Guizhou Plateau has been uplifted since the Himalayan period (>3.54 Ma), and the Carboniferous carbonate rocks and early faults in the Huanjiang area have been exposed, leading to the change and evolution of deep karst. Through comprehensive analysis, a fault-controlled hypogene hydrothermal karst pattern and a meteoric water deep karst pattern are established. The genetic pattern of deep karst provides theoretical support for predicting this kind of karst in southern China and for avoiding drilling deep karst caves as a part of resource exploitation