Metal-based Energetic Materials: Experimental Investigation of the Effects of Aging and the Changes in Thermochemical Properties on Chemical Reaction Kinetics

Abstract

This thesis has conducted an in-depth study on aging of the metal-based energetic materials and identification of the fundamental cause of thermal runaway of the energy storage system (ESS). For the aging-related research, the energetic materials composed of titanium, zirconium, tungsten, and magnesium respectively were utilized while for the ESS research, the latent thermal energy storage (TES) materials and lithium-ion batteries (LIB) were adopted. The current study has enabled the expansion of their effective utilization via various experimental techniques. From conducted aging research, it was discovered the general aging effects on the chemical compositions of the energetic materials such as pre-oxidation of metals and a priori decomposition of oxidizer. Also, hygrothermal aging effects on the chemical reaction kinetics and the aging mechanism were described for the first time. For the ESS-related research, compatibility between the phase change materials (PCM) and the heat transfer fluids (HTF) was confirmed, and it was first suggested that a thermal reaction analogous to gunpowder can occur under a certain condition, resulting in thermal runaway. In addition, it was revealed that when high-nickel LIBs were heated to 50℃ during charging, the exothermic reaction between the anode materials and the electrolyte can act as a trigger reaction for thermal runaway.

Aging Analysis on Titanium-based Energetic Materials: Titanium Hydride Potassium Perchlorate (THPP) Titanium hydride potassium perchlorate (THPP), which is one of the most commonly utilized pyrotechnic initiators, can fail or deviate from the desired performance when subjected to aging. The aging process is known to change its composition as well as its thermochemical kinetics; however, the variation in the performance of THPP regarding its combustion behavior has not been addressed. This experimental thesis reports new correlations between the ignition and combustion characteristics of THPP. A thermal analysis, along with morphological observations, conducted on aged samples revealed that aging causes progressive oxidation of the fuels and subsequent decomposition of the oxidants. The reaction kinetics extracted using Friedman isoconversional method showed a decrease in the activation energy due to thermal aging while the opposite was noted under hygrothermal aging conditions. As a result, increased activation energy limited the range of ignition temperature and decreased the reactivity, thereby both the ignition delay time and burn time were extended. Such altered characteristics by aging could lead to inconsistent performance of the pyrotechnic initiators.

Aging Analysis on Zirconium-based Energetic Materials: Zirconium Potassium Perchlorate (ZPP) Aging of pyrotechnic substances, primarily fuel oxidization, can cause changes in composition that degrade their performance. This study investigates the effect of aging on zirconium potassium perchlorate (ZPP), a widely used NASA standard initiator. Although prior studies have investigated the effects of accelerated aging on ZPP, this is the first to conduct kinetic analyses at different relative humidity (RH) levels. Here, both thermal and kinetic analyses are conducted for a variety of hygrothermal aging cases in order to replicate the natural aging process. X-ray photoelectron spectroscopy (XPS) results reveal that oxidant levels drop and zirconium dioxide levels rise as ZPP ages. Lower heats of reaction and increases in activation energy were also observed under the RH conditions. Calculations using vant Hoff equation indicate that moisture shortened the lifespan of the unaged ZPP up to about 85% under extreme RH conditions, while significantly deteriorating the heat of reaction, and sensitivity, thus increasing the risk of a misfire.

Aging Analysis on Tungsten-based Energetic Materials: Tungsten Pyrotechnic Delay Mixtures Although exact predictions of the performance of aged pyrotechnic compositions intended to produce non-detonative self-sustaining exothermic chemical reactions remain unsolved, progress has been made to determine the reaction kinetics of aging compositions. The present study investigates the key factors that govern the reaction kinetics of hygrothermally aged mixtures that utilize metal powders of tungsten (W) as fuels and potassium perchlorate (KClO4) as a common oxidizer. It is shown that Gaussian distribution of activation energy for aged mixtures can be correlated to the thickness of the oxide layer on the metal particle as the mean value and the number of intermediate processes of reaction as the standard deviation. Here, the mean value of activation energy tended to increase in proportion to the oxide thickness as the oxide layer obstructs the reaction between metal fuels and oxidizer. Meanwhile, the changes in the standard deviation are directly correlated to the electronegativity between metal and oxygen. In other words, the metals with low percent ionic characters to the oxygen can give rise to decreased standard deviation as they can strongly combine with oxygen having a higher covalent bond, which then reduces the intermediate steps of a chemical reaction. Therefore, the key factors viz. the oxide thickness and the electronegativity of the metal component are shown responsible for defining the chemical reaction kinetics of the aging pyrotechnic compositions.

Aging Analysis on Magnesium Metal Particles as a Renewable Energy Carrier Magnesium is regarded as one of the promising metal energy carriers because it contains a considerable energy density comparable to fossil fuels. Also, the element is nontoxic and abundant on earth. With the advantages, Mg has been utilized for various energy-related fields such as the battery cathode, additives in propellants, pyrotechnics, hydrogen carriers, etc. The most appreciable point of Mg particles is that they can release intense heat energy via an oxidation process that never produces any carbon dioxide. The oxidation process of Mg, however, proceeds through very complex procedures as the element is known to be very reactive to moisture as well as oxygen. The analysis of chemical kinetics for Mg oxidation has been studied and it was found that the chemical reaction kinetics of Mg particles are highly dependent on morphological properties such as particle size and shape. The current study newly found that hygrothermal aging of Mg particles can give rise to severe changes in the shape of particles by agglomeration. In addition, the aging products were dependent on the aging duration: Mg(OH)2 (short-term aging) and MgO (long-term aging). The accompanied aging effects on Mg particles finally appeared as changed reaction kinetics. Thermal aging led to 55% decrease in activation energy and hygrothermal aging created additional intermediate sub-reactions involving products of pre-oxidation such as Mg(OH)2, H2O, and H2.

Thermal Runaway Reaction: Thermal Runaway Mechanism of a Thermal Energy Storage System based on KNO3/NaNO3/Graphite Exposed to a Heat Transfer Fluid An application of latent-type thermal energy storage (TES) system and its issues associated with the safe operation for TES composite materials (i.e., solar salts (KNO3/NaNO3) and expanded graphite (EG) are investigated. This work is aimed to present the compatibility of various compositions of TES materials with a HTF to provide a guideline for safe and reliable system usage. The thermochemical characteristics and chemical kinetic mechanism for HTF-diluted TES samples were measured using differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA). In addition, additional numerical analysis for validation of the constructed reaction kinetics was provided. Amongst the amount of HTF dilutions, 20 wt. % HTF diluted case showed the strongest exothermic runaway reaction. For instance, with a decrease in the onset temperature, the time to reach runaway was reduced, while the change in the heat of reaction was not significant. The numerical simulations calculated based on the results of chemical reaction kinetics revealed that the reaction runway characteristics of HTF-diluted TES materials show a similar detonative behavior as of gunpowder. Moreover, the function of HTF in TES system appeared to be identical to that of sulfur in gunpower as its specific percent composition was responsible for accelerating an exothermic chemical reaction.

Thermal Runaway Reaction: The Fundamental Cause of Thermal Runaway of High-nickel/silicon-graphite Lithium-ion Batteries During Charging Recently, the global Lithium (Li)-ion battery market has been focusing on improving energy density by increasing the nickel (Ni) content in the cathode material. High-nickel battery indicates a Li-ion battery in which the Ni content in the cathode active materials is increased to 70-90%, and has strong points such as an enhanced energy density and lightweight. However, Ni metals are highly reactive to oxygen and other solvents, so they can give rise to thermal runaway when the battery is overheated, increasing the risk of an explosion. Also, considerable cases of fire accidents in electric vehicles using Li-ion batteries have been reported during or after the charging process lately. To identify the fundamental cause of the unexpected exothermic events while charging of electric vehicles (EV), the current study has carried out the thermal analysis in detail to high-nickel LIBs. The combinations of five different states of charge (0%, 25%, 50%, 75%, and 100%) and mixtures of the battery components (i.e., anode, cathode, anode+electrolyte, cathode +electrolyte, anode+cathode+electrolyrte+separator, etc.) were employed in the DSC experiments. Comprehensively, it was confirmed that when the electrolyte coexists with the active materials of the cathode and the anode, the exothermic characteristics were remarkably intensified. Meanwhile, the solid electrolyte interface (SEI) layer decomposition from the anode materials and thermal reactions with the electrolyte occurred at around 50℃. The corresponding reaction can trigger a breakdown of the structure of the cathode materials which can release substantial heat of enthalpy. Eventually, the reaction can develop into a thermal runaway reaction by leading to the separator collapse and the internal short circuit in the battery cells after all.

본 논문은 다양한 금속 물질을 바탕으로 하는 고에너지 물질의 노화와 더불어 에너지저장물질의 열폭주 현상 원인 파악에 대한 심도있는 연구를 진행하였습니다. 우선, 노화관련 연구의 경우 티타늄, 지르코늄, 텅스텐, 마그네슘과 같이 다양한 금속 원료를 포함하는 고에너지 물질을 활용하였으며, 에너지저장물질의 경우 가정 및 산업환경에서 보편적으로 사용되는 잠열축열재와 리튬이온배터리에 집중하여 다양한 실험적 기법을 적용시켜 연구를 확장시켰습니다. 노화 연구를 통해 금속의 산화와 산화제의 사전분해라는 공통적인 노화 효과를 발견하였으며, 이를 바탕으로 수분 및 열이 각각의 고에너지 물질의 연소과정에 있어서 화학 반응 역학에 미치는 효과와 노화 메커니즘을 최초로 제공하였습니다. 그리고 에너지 저장물질의 경우 잠열축열재 시스템내 상변화물질과 열매유간의 호환성을 열분석 바탕의 실험을 통해 최초로 파악하였습니다. 그 결과, 특정 조건에서 축열재가 화약의 열폭주와 유사한 반응을 발현할 수 있음을 최초로 제시하였습니다. 또한, 하이니켈 리튬이온배터리의 경우 충전 중에 배터리 셀 내부가 약 50℃의 온도상승이 나타나게 되면, 음극재와 전해액간의 발열 반응이 열폭주의 촉발반응으로 작용할 수 있음을 파악하였습니다.