Thermal Shock Process Research Articles

Refractory metal silicides reinforced by in-situ formed Nb2O5 fibers and mullite nanoclusters

There is keen interest in the use of refractory metal silicides as structural materials or thermal barrier coatings for a high temperature environment. However, a long-standing problem for these materials is their poor thermal shock property. To address this challenge, Nb-Al-SiC elements were introduced into the MoSi2 matrix and consolidated by in-situ hot pressing. We find that this treatment leads to improved performance of MoSi2 composites in high temperature thermal shock resistance and bending strength. After in-situ HPing, the Nb, Al2O3 particles, and SiC nanoclusters were uniformly dispersed in the MoSi2 matrix and inhibited the movement of dislocation, resulting in a strengthening effect. During the thermal shock process, the fragmentized oxide layer present in the surface of the pure MoSi2 alloy disappeared completely, and a dense multi-component oxide layer was formed in-situ on the surface of the MoSi2 composites. The dense multi-component oxide layer was composed of SiO2 glass, fiber-structured Nb2O5, and nano-sized mullite phases. The fiber structured and nano-sized oxide phases play an important role in strengthening the oxide layer.

High Temperature Thermal Shock Synthesis of Catalyst-Decorated Nanostructures for Energy Storage

An efficient and ultrafast method to synthesize well-dispersed functional nanoparticles with controlled size and distribution remains largely unanswered. Conventional synthesis methods (wet chemistry, spray pyrolysis, among others) tend to be less controllable, leading to nanoparticle aggregation as well as a wide particle size distribution. Therefore, a short synthesis duration is critical to control nanoparticle size and minimize aggregation. Here we report a rapid and universal high temperature thermal shock method to form and uniformly disperse metallic nanoparticles on a conductive carbon support, such as carbon nanofibers (CNF). First, the desired metal salt precursor is homogeneously mixed into an aqueous solution and loaded onto the conductive carbon support. After drying, the precursor-loaded CNF film is attached to electrical leads inside an argon-filled glovebox to facilitate the electrically triggered Joule heating procedure. An applied electrical pulse from an external power source induces the thermal shock process by heating the precursor-loaded CNF to ~2000 K for mere milliseconds at ultrafast heating/cooling rates (~105 K/s). Due to the short high temperature exposure and rapid cooling rate, nearly any precursor metal salt can decompose and nucleate into uniformly distributed metallic nanoparticles with ultrafine particle sizes without damaging the conductive support. Note that kinetic control can also be achieved by tuning the synthesis parameters (pulse duration, heating temperature, heating/cooling rates). Specifically, as the pulse duration increases to seconds, the temporal limitations for diffusion and migration broaden, allowing nanoparticles with larger diameters to form with a wider particle size distribution. In the field of energy storage, nanoparticle-decorated conductive supports are ideal battery electrodes. To demonstrate the proposed manufacturing method, lithium-oxygen (Li-O2) batteries were successfully assembled and tested using the nanocatalyst-decorated CNF films. Bifunctional ruthenium nanoparticles were synthesized by the rapid thermal shock method to increase the electrocatalytic activity of the CNF-based Li-O2 cathodes and improve oxygen reaction kinetics. The results indicate the potential of this rapid manufacturing method to synthesize an array of non-agglomerated nanoparticle-decorated supports for energy storage devices and beyond.

Char cracking of medium density fibreboard due to thermal shock effect induced pyrolysis shrinkage

Pyrolysis experiments were conducted on medium density fibreboard (MDF) in inert atmosphere and different ambient pressures, to investigate the char shrinkage and cracking. It is found that the char cracking under uniform heat flux is a typical thermal shock process induced by unbalance shrinkage along the sample thickness during pyrolysis. To predict the number of char fissures, the critical stress criterion and energy conservation theory are used to develop mathematical models under plane constitutive stress state, which reveal that under the same surface degradation the number of char fissures (blisters) strongly relates to the pyrolysis depth at cracking time. Increasing external heat flux decreases the pyrolysis depth and increases the number of char fissures. Both experiments and numerical modelling are used to validate the models. The experimental results show that the horizontal shrinkage is 11% of original length and the micro-structure of char fissures of MDF is less uniform compared to the one of natural wood with a cellular pattern. The surface stresses after cracking are found similarly close to the tensile strength under different heat fluxes, while the surface stresses are very different assuming no crack, which indicates the cracking process reduces the surface stress to lower than the tensile strength. The modelled cracking times are different from the observed cracking time as the fissures are hard to identify at its initial stage and only when they have expanded to certain size the fissures are visually observed. Using the modelled cracking time, the number of char blisters can be well correlated with the pyrolysis depth.

Thermomechanical Analysis of Transient Temperatures in a Radial Turbine Wheel

In turbomachinery design, the accurate prediction of the life cycle is one of the most challenging issues. Traditionally, life cycle calculations for radial turbine wheels of turbochargers focus on mechanical loads such as centrifugal and vibration forces. Due to the increase of exhaust gas temperatures in the last years, thermomechanical fatigue in the turbine wheel came more into focus. In order to account for the thermally induced stresses in the turbine wheel as a part of the standard design process, a fast method is required for predicting metal temperatures. In order to develop a suitable method, the mechanisms that cause the thermal stresses have to be understood. Thus, in a first step, a detailed analysis of the temperature fields is conducted in the present paper. Extensive numerical simulations of a thermal shock process are carried out and validated by experimental data from a test rig. Based on the results, the main heat transfer mechanisms are identified, which are crucial for the critical thermal stresses in transient operation. It is shown that these critical stresses mainly depend on local 3D flow structures. With this understanding, two fast methods to calculate the transient temperatures in a radial turbine were developed. The first method is based on a standard method for transient fluid/solid heat transfer. In this standard method, heat transfer coefficients are derived from steady-state computational fluid dynamics (CFD)/conjugate heat transfer (CHT) calculations and are linearly interpolated over the duration of the transient heating or cooling process. In the new method, this interpolation procedure was modified to achieve an exponential behavior of the heat transfer coefficients over the transient process in order to enable a sufficient accuracy. Additionally, a second method was developed. In this method, the specific heat capacity of the solid state is reduced by a “speed up factor” to shorten the duration of the transient heating or cooling process. With the shortened processes, the computing times can be reduced significantly. After the calculations, the resulting times are transferred into realistic heating or cooling times by multiplying them with the speed up factor. The results of both methods are evaluated against experimental data and against the results of a numerical method known from literature. The methods show a good agreement with those data.

Effect of magnesium aluminum silicate glass on the thermal shock resistance of BN matrix composite ceramics

AbstractThe effects of magnesium aluminum silicate (MAS) glass on the thermal shock resistance and the oxidation behavior of h‐BN matrix composites were systematically investigated at temperature differences from 600°C up to 1400°C. The retained strength rate of the composites rose with the increasing content of MAS showing a maximum value at the 60 wt% MAS. Compared with the original strength, the retained strength of the specimen after thermal shock increased to 77% (ΔT=1000°C). The strengthening effect of MAS and the surface microstructural evolution of composites are responsible for the improved thermal shock resistance. Surface oxidation of the composites during the thermal shock process plays a positive role in enhancing the retained strength by self‐healing cracks and the appearance of the compressive stress. The oxide layer also acted as a thermal barrier to decelerate the actual thermal stress. Furthermore, this dense layer also improved the oxidation resistance of h‐BN matrix composites by prevent diffusion of oxygen. These results indicated that short‐term surface oxidation during thermal shock process is favorable to the enhancement of the thermal shock resistance of BN‐MAS composite ceramics.

Porous graphene paper for supercapacitor applications

Graphene paper shows a great promise for the electrical energy storage. However, the high stability, purity and specific surface area have become stringent requirements for supercapacitor applications. Finding methods to tackle these problems is rather challenging. Here, we develop a facile method to prepare porous graphene papers with a thickness 0.5mm by a thermal shock to the layer-structure graphene paper self-assembled on Cu foil under nitrogen flowing. The as-prepared porous graphene paper exhibits a large specific capacitance of 100Fg−1 at the scan rate of 100mVs−1 with high stability and purity without any residual chemical reagents, showing a promising potential for supercapacitor applications. The high electrochemical properties are mainly attributed to the high-specific area and the improved conductivity of the porous graphene paper performed by the multieffect of reducing, cleaving and expanding to the layer-structure graphene paper by high-energy thermal heating during the thermal shock process. This work paves a pathway to the facile preparation of porous graphene paper for supercapacitor applications.

Thermal Shock Failure Mechanism of Nanostructured Zirconia Coating by Atmospheric Plasma Spraying

Nanostructured zirconia coatings has been prepared based on reasonable spraying technical parameters and the corresponding thermal shock property of the as-sprayed coating was examined at 1100 °C. The structure and the surface/interface morphology of the coatings have been analyzed using X-ray diffraction (XRD), metallographic microscope, scanning electron microscopy (SEM), energy dispersive spectrum (EDS) and transmission electron microscopy (TEM). Based on the detailed analysis of the structure and phase composition, a rational mechanism has been proposed for thermal shock failure of the coating. Compared with other nanoparticles, those particles close to pores and pre-existing microcracks would preferentially grow up during the thermal shock process due to a better growth space. The growth of these nanoparticles is conducive to the formation of new microcracks which would lead to the growth of the other nanoparticles. With the growth of most or all of the nanoparticles, the nanostructured zirconia coating correspondingly changes into the quasi-microstructured coating. The thermal shock failure mode of the as-sprayed coating is similar to that of traditional zirconia thermal barrier coatings (TBCs).

Produire des lames de hache en dolérite au Néolithique en Mayenne. La carrière-atelier de Beulin à Saint-Germain-le-Guillaume

Produire des lames de hache en dolérite au Néolithique en Mayenne. La carrière-atelier de Beulin à Saint-Germain-le-Guillaume

Structural model of InSb IRFPAs including underfill curing process

InSb infrared focal plane array(IRFPA) detector, active in 3-5 m range, has been widely used in military fields. Higher fracture probability appearing in InSb infrared focal plane arrays(IRFPAs) subjected to thermal shock test, restricts its final yield. In order to analyze and optimize the structure of InSb IRFPAs, it is necessary to create the three-dimensional structural model of InSb IRFPAs, which is employed to estimate its strain distribution appearing in the different fabricating processes. In this paper, the curing model of underfill is described by its volume contraction percentage combined with the elastic modulus of the completely cured underfill. Thus, both the von Mises stress and the Z-components of strain accumulated in the curing process of underfill are calculated. When InSb IRFPAs is naturally cooled to room temperature from the curing temperature of underfill, the Z-component of strain distribution appearing on the top surface of InSb IRFPAs is obtained with our structural model, which is identical to the deformation distribution on the top surface of InSb IRFPAs measured at room temperature. In the following thermal shock simulation, we find that the maximal von Mises stress appears at 100 K and the maximal Z-component of strain appears at 150 K, these two temperature points are located in the second half of the thermal shock process, these results indicate that the fracture of InSb chip happens more easily in liquid nitrogen shock test. This inference is consistent with the fact appearing in liquid nitrogen shock test. All these findings suggest that the proposed model is suitable to estimate the deformation distribution of InSb IRFPAs and its changing rule in its different fabricating stages.

Effect of thermal shock process in accelerated environment spectrum on the fatigue life of 7B04-T6 aluminum alloy

The effect of thermal shock, in an accelerated-corrosion environment spectrum, on the fatigue and corrosion behavior of 7B04-T6 aluminum alloy, was determined. The environment spectrum consists of two modules, namely: salt-spray corrosion and thermal shock. The effect of thermal shock on the mechanical properties was determined via tensile tests; SEM, DCS, and XRD were used to determine the effect of thermal shock on the corrosion products. In addition, the corrosion resistance of the products was ascertained through electrochemical testing. The results show that the mechanical properties and fatigue life of the aluminum alloy will decline with prolonged thermal shock time. The thermal shock process may result in denser surface corrosion products than those formed on the no thermal shock specimens, and transformation of some Al (OH)3 into AlOOH. AlOOH may have resulted in improved corrosion resistance and hence a lower decrease in the fatigue life after corrosion, compared with that of the no thermal shock specimen. Repeated corrosion/thermal shock may have delayed further decease in the fatigue life. Therefore, selection of an appropriate equivalent thermal shock temperature and time was essential for designing the environmental spectrum.

Rapid, in Situ Synthesis of High Capacity Battery Anodes through High Temperature Radiation-Based Thermal Shock.

High capacity battery electrodes require nanosized components to avoid pulverization associated with volume changes during the charge-discharge process. Additionally, these nanosized electrodes need an electronically conductive matrix to facilitate electron transport. Here, for the first time, we report a rapid thermal shock process using high-temperature radiative heating to fabricate a conductive reduced graphene oxide (RGO) composite with silicon nanoparticles. Silicon (Si) particles on the order of a few micrometers are initially embedded in the RGO host and in situ transformed into 10-15 nm nanoparticles in less than a minute through radiative heating. The as-prepared composites of ultrafine Si nanoparticles embedded in a RGO matrix show great performance as a Li-ion battery (LIB) anode. The in situ nanoparticle synthesis method can also be adopted for other high capacity battery anode materials including tin (Sn) and aluminum (Al). This method for synthesizing high capacity anodes in a RGO matrix can be envisioned for roll-to-roll nanomanufacturing due to the ease and scalability of this high-temperature radiative heating process.

Experimental Study of Deposits Removal in Thermal Desalination Plants by the Thermal Shock Technique

Deposition of salts on heat transfer surfaces in thermal desalination plants can lead to operational failure. Scale removal can occur by applying a thermal shock, which is a sudden decrease in the heating process. The difference in thermal expansion between the heat transfer surface and the deposit layer plays a key role in the thermal shock process. The objective of this research is to determine experimentally the minimum temperature of the heating surface in desalination units, such that the thermal shock is still applicable. The minimum heating temperature is important for minimization of heat losses. An experimental setup has been designed and developed, and it consists of an oil tank in which oil is heated by electrical heaters. The heated oil is circulated by a gear pump to the steam generator, which contains the water to be desalinated, that is, a CaSO4 solution, at atmospheric pressure. The water is heated and converted into steam by the hot oil leaving the salts behind, that is, the fouling layer, on the tubes of the steam generator. A thermal shock is applied when the asymptotic behavior is approached, such that the flow of the hot oil is suddenly stopped for a short period of time before resuming it again. The minimum heating temperature has been determined for two types of tubes: stainless steel and copper, and at a salt concentration of 2 g/L. The minimum heating oil temperature that allows the applicability of the thermal shock is 130°C when using copper tubes, and 140°C for stainless-steel tubes.

Study of the mechanical properties of Ti2AlC after thermal shock

The ternary carbides and nitrides, known as MAX phases combine the attractive properties of both ceramics and metals; for example the Ti2AlC ceramics examined in this work have attracted attention for aerospace applications. In the current work Ti2AlC ceramics were subjected to high level thermal shock (ΔT=1100°C) from a temperature of 1200°C with a cooling rate of more than 90°C/sec. After the thermal shock process the mechanical properties and the microstructure were investigated. Combined thermal shock and mechanical loading with an applied stress of 0, 50, 100 and 150MPa was examined and in all cases the samples maintained their microstructure without any micro-cracks or phase change as observed. Compression testing of all samples to failure indicated that their mechanical properties did not show any deterioration in the testing range examined. Such ceramic materials therefore show potential for high temperature and high load applications.

The Influence of Repeated Thermal Shock on the Mechanical Properties of Mongolia Scotch Pine and Moso Bamboo

The sharp temperature changes in nature (e.g., forest fires, ice, and snow) can cause mechanical damage to trees and bamboo. The mechanical properties of Mongolia Scotch pine (Pinus sylvestris L. var. mongolica) and Moso bamboo (Phyllostachys edulis) were investigated by a three-point bending test with a repeated thermal shock process (i.e., sudden changes of temperature). The experimental results indicated that the flexural modulus, flexural strength, and deformation work per volume decreased almost linearly with the increased repetition of thermal shock treatment for both Mongolia Scotch pine and Moso bamboo. The damage caused by repeated thermal shock was stronger for Mongolia Scotch pine, as compared to Moso bamboo, under the same thermal shock treatment. Thus, the experimental results provided basic data for engineering applications of Mongolia Scotch pine and Moso bamboo after natural disturbances.

Study of the effects of surface modification by thermal shock method on photocatalytic activity of TiO2 P25

In this study, we have modified TiO2 P25 photocatalyst by thermal shock method at different temperatures in order to study the effects of thermal shock process on the crystal structure, surface properties and photocatalytic activity. The thermal-shock-modified catalysts were respectively characterized by X ray diffraction (XRD), scanning electronic microscopy (SEM), X ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectra and compared with thermal-shock-fluorinated TiO2 catalysts. The photocatalytic activity was evaluated via the degradation of methylene blue under UV and visible light irradiation. The results show that the deficiency of oxygen on the surface of catalysts created by thermal shock at 500°C can enhance the photocatalysis under both UV and visible light. Moreover, when the thermal shock was combined with the fluorination of TiO2, the photocatalytic performance was further improved. However, the thermal shock from 700°C promoted the growth of particles and the phase transition from anatase to rutile, which resulted in two different effects: inhibit the photocatalysis under UV irradiation but improve it under visible irradiation.

Thermal Stress Development in Steam-Side Oxide Scales in High Temperature Components of Boilers under Thermal Shock

Failure of steam-side oxide scales in high temperature components of boilers such as superheater and reheater tubes has significant effect on the safety of thermal power plants. Finite element analysis is carried out to investigate the failure issues of steam-side oxide scales in boiler tubes during thermal shock processes. The effects of different steam temperature variation durations on oxide scale failure issues have been analyzed. According to the results, appropriately lengthening variation duration can contribute to relieving oxide scale failure issues, and are subsequently proposed.

Evolution of thermo-physical properties of diamond/Cu composite materials under thermal shock load

In this paper, the two-flume method was used to study the change laws of the thermal conductivity and thermal expansion coefficient of diamond/Cu composite materials with 100, 300, and 500 cycle numbers, under the action of thermal shock load between −196 and 85 °C; the X-ray diffraction method (XRD) was used to study the change of the residual stress in the thermal shock process of the diamond/Cu composite materials; and the evolution of the fracture microstructure with different thermal shock cycle numbers was observed through scanning electron microscopy (SEM). The results of the study show that the increase of the binder residue at the interface reduces the thermal shock stability of the diamond/Cu composite materials. In addition, under the thermal shock load between −196 and 85 °C, the residual stress of the diamond/Cu composite materials increases continuously with the increase of the cycle numbers, the increase of residual stress leads to a small amount of interface debonding, an increase of the interfacial thermal resistances, and a decrease of the constraints of low-expansion component on material deformation, thus the thermal conductivity decreases slightly and the thermal expansion coefficient increases slightly.

Synthesis of cuprous chloride and simultaneous recovery of Ag and Pd from waste printed circuit boards

A benign and effective process for cuprous chloride synthesis and simultaneously recovery of Ag and Pd from waste printed circuit boards (PCBs) was developed in the present study. The main merit of the process is that neither corrosive acid nor strong oxidant was used. The PCBs were firstly pretreated by thermal shock process to obtain metallic particles (MPs), then cuprous chloride was synthesized by reacting MPs with cupric sulfate in the presence of sodium chloride. The optimum VNacl/mCuSO4 (mL/g) ratio, [Cu]/[Cu2+] mole ratio, treatment time and operation temperature were 6, 0.95, 30min and 60°C, respectively. Approximately 98.5% of Cu could be recovered from the PCBs through two synthesis circles. During the synthesis process, Ag and Pd could be simultaneously recovered by forming a stable chloride complex since Cu2+ played the role of oxidant in the system. The recovery percentages of Ag and Pd were 93.9% and 95.3%, respectively. Accordingly, it is believed that the process developed in the current study is benign and practical for copper and precious metal (Ag, Pd) recovery from waste PCBs.

Experimental and Simulation Analysis of Thermal Shock with Rapid Heating Followed by Water Quenching for CuW70 Alloys

Abstract By means of the rapid heating followed by water quenching, thermal shock experiments were conducted for CuW70 alloys to study the variations of mechanical behaviour. On the other hand, the simulation with finite element method was implemented to analyze the variations and the distributions of the temperature and the thermal stresses of CuW70 specimens during thermal shock process. The results show that the strength of CuW70 alloys degenerates and the ductility is improved after repeated thermal shock, and the maximum thermal stress appears at the beginning of heating and quenching period. The alternated thermal stresses which vary between tensile and compression stress will be applied on the internal and the external of CuW70 alloys during repeated thermal shock.

Process Conditions of Exfoliated Single-Layer Graphite

The graphite was exfoliated by concentrated acid treatment, strong oxidants oxidation, high temperature rapid thermal shock process. Investigated in detail that the kind of oxidants, concentrated acid ratio, oxidation time affected on the graphite expansion rate. Experimental results (expansion, micro-structure observation and chemical structure analysis of graphite) showed that the optimum volume ratio of concentrated sulfuric acid and concentrated nitric acid was 4:1; the expansion rate was better that added oxidant KMnO4than added oxidant K2Cr2O7; The best oxidation time (concentrated acid treatment time) was 30~60 min.