Thermal Reaction Characteristics Research Articles

Effect of heat treatment on the radial structure of coaxial polyacrylonitrile/mesophase pitch composite fiber

AbstractCoaxial composite fibers with a cortex of polyacrylonitrile (PAN) and a core layer of mesophase pitch (MP) and PAN were prepared by wet spinning, then heat‐treated in air. The effects of heat treatment on the thermal reaction characteristics, pre‐oxidation degree, radial structure, and thermal stability of coaxial composite fibers were studied by differential scanning calorometry, Raman, and thermogravimetric analysis. As the heat treatment temperature increases, the coaxial composite fibers only undergo cyclization without oxidation, the peak temperature of the thermal reaction increases from 258 to 270°C, and the enthalpy increases from 19.66 to 237.82 J g−1, the heat mainly arises from the PAN component. The Dr value of the coaxial fiber skin layer increases from 0.193 to 0.255 (330°C) and then decreases to 0.236; the core layer Dr value increases from 0.353 to 0.392 (310°C) and then decreases to 0.368. The pre‐oxidation reaction leads to an increase in sp2‐hybridized carbons and an increase in pre‐oxidation degree. When the heat treatment temperature <310°C, the pre‐oxidation degree of the composite fibers increases, leading to formation of a more stable structure compared with that at lower temperatures and an increase in the carbon residue rate.

Effects of electrolyte/cathode ratio on investigation of their thermal behaviors using differential scanning calorimetry

Battery materials with high-energy density and good safety performance have been regarded as the ultimate target for larger-scale applications of lithium-ion batteries in electric transportation and grid energy storage. Thermal safety of high energy density cathode materials has been a hot topic for battery safety research in recent years. Differential scanning calorimetry (DSC) is widely used to investigate thermal reaction characteristics of battery materials, but the results are always affected by multiple variables, e.g., sample pre-processing and testing conditions. The variety of DSC results reported in literature have been perplexing the research community. Herein, the cathode-electrolyte system is chosen to reveal the changes of reaction mechanisms with different electrolyte/cathode ratios, since this system has been widely adopted to characterize thermal stability of cathodes and electrolytes in previous studies. Lithium-rich manganese-based layered oxide cathode and ester-based electrolyte are selected as representative samples. Our results reveal that flooded, sufficient, insufficient, and lean electrolyte conditions may lead to distinct thermal behaviors from probably different reaction routines, which may hold different reference values for battery-level safety analysis. This study also suggests the importance of systematic investigations on test conditions in battery material thermal safety research.

Study on the thermal reaction characteristics and kinetics of coal and coal gangue coexisting spontaneous combustion

In the hope of exploring the effect of coal gangue left in goaf on coal spontaneous combustion, the variations of thermal effects and free radical parameters during coexisting spontaneous combustion of coal and coal gangue were tested, and kinetic at different heating stages were calculated. The following beneficial findings were obtained. For the mixed samples, the heat flow curve, ignition temperature, and free radical concentration all decline with the increase of coal gangue content, and the free radical parameters exhibit periodic changes with increasing temperature. Both the experimental value of thermal effect and free radical concentration with coal and coal gangue mixture during the spontaneous combustion process are higher than the calculated value. The calculation results of kinetic show that at the endothermic stage, the activation energies of the mixed samples decline with the increase of coal gangue, and the experimental value of activation energy is greater than the calculate value; at the exothermic stage, the case is exactly the opposite. Furthermore, due to the higher reactivity of coal gangue groups and stronger thermal conductivity, the coexistence of coal and coal gangue spontaneous combustion reaction is promoted. Resultantly, coal spontaneous combustion is normally more threatening in gangue-containing goafs.

Synergistic inhibitory effect of sodium bicarbonate and melamine cyanurate on the thermal reaction of ultrafine aluminium powder

In this study, the thermal reaction characteristics of ultrafine aluminium powder (UAP) and its composites containing different mass proportions of sodium bicarbonate (SBC), melamine cyanurate (MCA), or SBC and MCA were investigated through synchronous thermal analysis. The results indicated that SBC and MCA remarkably inhibited the thermal reaction of UAP. With an increase in the mass proportion of the inhibitor (i.e., SBC, MCA, or SBC and MCA), the thermogravimetry and differential thermogravimetry increased, and the heat release decreased, which strengthened the inhibition of a UAP explosion. For UAP + SBC + MCA powder with a mass proportion of 1:1:1, SBC and MCA had a strong explosion inhibition effect on UAP explosion near 660 °C. Finally, the synergistic inhibition effect of SBC and MCA on the thermal reaction of UAP was clarified, and physical and chemical inhibitory effects were observed. Inhibitor pyrolysis resulted in the absorption of the heat released through UAP oxidation and the generation of inert gas, which diluted the oxygen concentration; thus, the thermal reaction of UAP was inhibited.

Polydopamine as interfacial layer regulating micron-sized boron/polytetrafluoroethylene composite to improve the combustion performance

Boron powder has high bulk calorific value and mass calorific value, and is widely used in high-energy explosives and propellants. However, in practical application, there are issues such as poor ignition performance and low combustion efficiency, which affect its energy performance. In this work, firstly, polydopamine (PDA) was used as a cladding to coat the micron-sized boron (µB) and formed µB@PDA, and then the µB@PDA was connected with polytetrafluoroethylene (PTFE) to study the effect of fluoride and PDA interfacial structure on the combustion and agglomeration of boron powder in air. Results indicate that the PDA interfacial layer thickness on the µB surface increases with the concentration of the dopamine hydrochloride mixed solution. By slowing down the rate of PTFE decomposition, the PDA coating can control the thermal reaction characteristics of µB, which in turn has a considerable impact on the flame spread and the combustion intensity. In particular, the synergy of PDA and PTFE can considerably reduce µB agglomeration during combustion.

Thermal reaction characteristics and microstructure evolution of aluminium nano-powder in various mixtures of oxygen and nitrogen atmosphere

Aluminium nano-powder has a small particle size, large specific surface area, and high surface energy, therefore when using it, the risk of fires or explosions is high. In this study, synchronous thermal analyser was used to determine the thermal reaction characteristics of aluminium nano-powder for various oxygen and nitrogen mixtures. The results showed that the oxidation behaviour of aluminium nano-powder could be considered to occur in four stages, according to the characteristic temperature points and polymorphic phase changes. Under experimental heating rates of 5, 10, 15, and 20 K/min, the mass gain and thermal effects of the samples were tested, and also for different oxygen concentrations of 9, 15, and 21 vol%. Changes in the microstructure of the samples were analysed. Oxide layer thickness was found to increase as temperature increased, and this trend was the same for mass change. Surface morphology images of the aluminium particles exhibited agglomeration, melting deformation, rupture, and fragmentation at the different pretreated temperatures of 500, 650, 800, and 900 °C. The results of this study could be used as a basis for exploring oxidation kinetics, and had significance for explosion-proofing technology, particularly with respect to aluminium nano-powder.

Characteristics of lean oxygen combustion and dynamic microreaction process of water-soaked coal

Oxidative spontaneous combustion is likely to occur after long-term water immersion in the gob area of coal mines. Mostly, the oxidation process is slowly carried out under different air leakage conditions. This paper summarizes the four stages of the low-temperature oxidation of water-soaked coal samples through temperature-programmed experiments. Using infrared spectroscopy and thermal analysis experiments, the migration laws of the functional groups and the thermal reaction characteristics of the four stages were investigated. The results show that as the oxygen concentration increases, the gas product concentration increases, and the activation energy of the deep oxidation stage decreases. With the progress of the heating stage, when the oxygen concentration is higher than 10%, the change in each functional group is larger, and the conversion of the functional group and the intermediate complex is relatively closer at an oxygen concentration of 21%. The low-temperature oxidation reaction is a parallel process of the pyrolysis and micro-oxygen absorption, and the DTG size is not affected by the oxygen concentration at the critical temperature. The higher the oxygen concentration is, the lower the initial exothermic temperature is, which promotes the exothermic process in the deep oxidation and oxidation-limited stages. The microreaction mechanism among coal, oxygen and water molecular chemisorption is constructed. By controlling the coal oxidation reaction stages under different air leakage conditions, the spontaneous combustion of water-soaked coal in the goaf is avoided.

Thermal reaction characteristics of paraffin in the presence of combustion catalysts

Paraffin fuels are regarded to be the best solid fuel of hybrid rocket motor due to the high regression rate, low cost, and environment friendly products. However, the regression rate of the fuels still cannot completely meet the requirement of the motor. Catocene, copper chromite, and cobalt stearate have been used as the combustion catalysts in this study, where the simultaneous TG-FTIR-MS and TG experiments at different heating rates were carried to investigate the thermal behavior of paraffin and paraffin/combustion catalysts mixtures. The results showed that both paraffin and the mixtures could decompose or be oxidized into gases products almost completely, with the main decomposition product of C2H6 and oxidation products of C2H4O, CO, CO2, and H2O. There was only one stage for both decomposition and oxidation of paraffin, and the kinetic model of decomposition process are close to two-dimensional diffusion (D2 model); cobalt stearate favored the decomposition of paraffin owing to the higher pre-exponential factor and the lower activation energy, whereas only copper chromite played an obvious positive role in both oxidation and regression rate of paraffin.

Thermal Reaction Characteristics of Nano/Micron-Sized Aluminum Mixtures in Carbon Dioxide

Thermal reaction characteristics of nano/micron-sized aluminum mixtures in a carbon dioxide atmosphere were investigated by thermogravimetry that aimed to examine the interactions between nanosized aluminum powder and micron-sized aluminum powder. Thermal reaction characteristics of nano/micron-sized aluminum mixtures at different ratios were studied. The synergistic effect mechanism was discussed by comparing experiment result and theoretical calculation. The morphologies and compositions of products were obtained by scanning electron microscopy and X-ray diffraction technologies. Results indicated that there were three weight gain stages for nano/micron-sized aluminum mixtures at different ratios. Although the temperature range of synergistic effect of nano/micron-sized aluminum mixtures with various ratios had somewhat differences, there was a significant synergistic effect from about 900 to 1040°C. The products analyses indicated that the products morphologies of nano/micron-sized aluminum particles showed molten and stuck together, and products contained aluminum and α-Al2O3.

Thermal reaction characteristics of dioxins on cement kiln dust.

Cement kiln dust is commonly recycled back into the production process. This results in elevated concentrations of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the flue gases of cement plants. The present study investigated the effects the reaction temperature, oxygen content, and origin of kiln dust had on the thermal reaction characteristics of PCDD/Fs. The concentration of 2,3,7,8-PCDD/Fs that were desorbed from the kiln dust decreased as the reaction temperature was increased and the higher temperature facilitated the degradation of PCDD/Fs. However, the oxygen content, which ranged from 6–21%, had only a minor impact on the thermal reaction characteristics of PCDD/Fs. Finally, the thermal reaction characteristics of PCDD/Fs were largely affected by the origin of the kiln dust; 1.2 pg I-TEQ g−1 was desorbed from kiln dust originating from a cement plant that co-processed refuse-derived fuel (RDF) and 47.5 pg I-TEQ g−1 was desorbed from kiln dust originating from a cement plant that co-processed hazardous waste. The study also found that PCDD/F formation pathways were dependent on the origin of the kiln dust; precursor synthesis dominated PCDD/F formation on the kiln dust collected from a cement plant that co-processed RDF, while de novo synthesis dominated the formation of PCDD/Fs on the remaining samples of kiln dust.

The effect of hydrolysis on combustion characteristics of sewage sludge and leaching behavior of heavy metals

ABSTRACTThe aim of this paper is to present the effect of hydrolysis treatment on the thermal reaction characteristics of sludge and the leaching test of heavy metals contained in the sludge. Raw and hydrolysis-treated sludge (at temperature of 200°C and pressure 0.4 MPa) samples were collected from local municipal sewage sludge treatment plant. Thermogravimetric analysis was carried out between 25°C and 900°C at the heating rate of 10°C/min and 20°C/min under incineration and pyrolysis atmospheres. The leaching behavior of toxic heavy metals in the sludge was studied along with ash composition. The heating rate significantly changed the thermogravimetric and differential thermogravimetric curve profiles for studied fuels under different atmospheres. After hydrolysis treatment, the heavy metals of hydrolysis-treated sludge becomes more stable. The leaching concentration of Cr, Mn, Cu and Pb in hydrolysis-treated sludge were greatly reduced, when compared to raw sludge; however, no significant change was observed in Cd concentration. The X-ray diffraction pattern of bottom ash of hydrolysis-treated sludge was significantly reduced than raw sludge. This decrease is due to possible reduction in grain growth, which degrades the crystallinity of hydrolyzed sludge, implying that the hydrolysis treatment of sewage sludge could be a promising and beneficially safe disposal technology.

Thermal reaction characterization of micron-sized aluminum powders in CO2

The thermal reaction characterization of micron-sized aluminium powder in carbon dioxide were investigated by simultaneous thermal analysis technology (TG/DSC), using a series of heating rates (5, 10, 15, 20°C/min). The results showed that the reaction process of micron-sized aluminium powder in carbon dioxide was divided into three stages: the initial slow oxidation stage, the sharp oxidation stage and the last oxidation stage. The thermal performance was increased with the increase in the heating rates. Evolution of the samples was determined by collecting the products at the initial, sharp, and last oxidation stages of the process. The reaction products morphology was examined using scanning electron microscopy (SEM). The corresponding chemical changes were analysed by X-ray diffraction spectrometry (XRD). The effects of heating rate on the thermal reaction characteristics were discussed. A new reaction mechanism of micron-sized Al particle in CO2 with gradually increased temperature was proposed.

Thermal reaction mechanisms of nano- and micro-scale aluminum powders in carbon dioxide at low heating rate

Thermal reaction mechanisms of nano- and micro-scale aluminum powders with a mean particle diameter of 50 nm and 1–2 µm were investigated by a thermogravimetric analysis coupled with a differential scanning calorimetry. Thermal reaction characteristics of nano-scale aluminum powder are significantly different from that of micro-scale aluminum powder. The key reaction of nano-scale aluminum powder takes place in the low-temperature region, while for micro-scale aluminum powder, the key reaction takes place in the high-temperature region. Scanning electron microscopy and X-ray diffraction were also conducted to analyze the thermal reaction products. There is a striking difference for the thermal reaction products, which are the oxide shell disruption of micro-scale aluminum particles. X-ray diffraction patterns indicate that γ-Al2O3 is detected before aluminum melting point for nano-scale aluminum powder, but for micro-scale aluminum powder, γ-Al2O3 is detected after aluminum melting point. Alumina crystal structures of nano- and micro-scale aluminum powders are different at different mass gain stages. The sequence of the products of nano- and micro-scale aluminum powders is amorphous oxide → γ→α-Al2O3 and amorphous oxide → γ→θ → α-Al2O3, respectively. Furthermore, thermal reaction mechanisms of nano- and micro-scale aluminum powders are discussed at a low heating rate.

Ignition and combustion characteristics of compound of magnesium and boron

Compound of magnesium and boron (MB) is promising to be the ideal substitute of amorphous boron which is usually used as the raw material of boron-based fuel-rich propellants. In this study, the physical characteristics of MB and amorphous boron were studied by the scanning electron microscope, X-ray diffraction and X-ray photoelectron spectroscopy. The thermal reaction characteristics and the ignition and combustion characteristics were investigated through TG/DSC experiments and laser ignition experiments. The experimental results show that the MB particle is much more regular than amorphous boron, which favors for the preparation of boron-based fuel-rich propellants. Magnesium exists in the form of elementary substance, and boron oxide is produced during the preparation of MB which results in the longer ignition delay time of MB. The content of magnesium and the pressure have effects on the MB combustion performance. Although the combustion of magnesium can provide much heat for the combustion of boron, MB with moderate content of magnesium shows the best combustion performance. On the contrast, the ignition delay time of MB is independent on the content of magnesium and the pressure.

Thermal reaction characteristics of the boron used in the fuel-rich propellant

High pressure DSC and simultaneous TG/DSC were used to study the different kinds of boron that was used in the fuel-rich propellant and the amorphous boron in different gases and different pressure. Also, some of the samples before the experiment and after the experiment were analyzed by the SEM. The results show that: (1) there is a big exothermic peak between 550 °C and 850 °C for all the samples because the combustion heat of boron is very high, and the exothermic peak appears in advance when the pressure or the oxygen concentration increases. (2) Although the reaction process of all the samples with air or oxygen could be divided into five stages, the reaction characteristics are different from each other. Especially, amorphous boron is much more active than the boron used in the fuel-rich propellant. (3) The exothermic peak at about 700 °C appears in advance, and the percentage conversion of boron decreases when the content of magnesium increases and boron–magnesium compound is used as the raw materials. (4) Some samples start to lose their mass for the sake of the evaporation of the (BO)n.

장시간 연소용 초소형 저삭마 목삽입재 선정을 위한 내열성능 평가

고온/고압, 장시간 연소조건에서 비삭마에 가까운 삭마 특성을 갖는 초소형 목삽입재 선정을 위한 연구를 수행하였다. 평가 소재로서 C/SiC, CIT, W/<TEX>$Y_2O_3$</TEX>를 선정하였으며, 연소시간 20초의 내열성능평가모타로 시험하였다. 시험 결과 W/<TEX>$Y_2O_3$</TEX>가 가장 우수하였으나, 균열이 발생하였다. 본 연구를 통하여 각 소재들의 열반응 특성 및 내열 성능을 이해할 수 있었으며, W/<TEX>$Y_2O_3$</TEX>의 적용가능성을 확인하였다. This paper describes the research on the non-eroding throat insert materials under condition of high-temperature, high-pressure, and long-burn time. C/SiC, CIT and W/<TEX>$Y_2O_3$</TEX> were chosen and tested in thermal protection evaluation motor of burning time 20 seconds. From the test results, a heat resistance of W/<TEX>$Y_2O_3$</TEX> was the most excellent among them, but was happened crack on material surface. Thermal reaction characteristics and heat resistance of these materials and feasibility of W/<TEX>$Y_2O_3$</TEX> as throat material were ascertained.

Dynamics of infrared laser-induced pyrolysis of 1-chloro-1,1-difluoroethane

The dynamics of infrared (IR) laser-induced pyrolysis of CF 2ClCH 3 has been studied by using a TEA CO 2 laser radiation which consisted of a main pulse of 100 ns followed by a tail of 7 μs containing 70% of total energy. The minimum laser fluence for the effective decomposition of CF 2ClCH 3 is about 1.5 J cm −2 at 10P12 laser line (951.19 cm −1). Over a moderate range of laser fluence 1.5–7 J cm −2, the detected product was CF 2CH 2 generated via the elimination reaction of HCl from CF 2ClCH 3 and there was no evidence to support HF elimination from CF 2ClCH 3. The cross sections of photon absorption of CF 2ClCH 3 are measured as a function of laser fluence. The dependence of decomposition probability on pressure shows collisional dissociation at lower pressure and thermal reaction characteristics at high pressure. A model of rate equation has been used to calculate the population distributions of vibrational levels and decomposition probabilities for three kinds of laser pulse durations, which are in accord with the measured experimental results. The model calculation indicates that long laser pulse gives a wide population at the lower vibrational levels and leads to a relatively low decomposition probability.