Increase In Thermal Expansion Coefficient Research Articles

Thermomechanical behavior of alumina–magnesia castables and numerical modeling of the steel ladle thermal process

AbstractBy experimental characterization and numerical modeling, the dependence of thermophysical properties on time and temperature as well as its influence on the thermomechanical behavior of alumina–magnesia castables has been studied in this work. The thermal conductivity, thermal expansion coefficient, and Young's modulus are the most important parameters, which evolve with temperature and piecewise change with the thermal processes of drying, preheating, and service. It is mainly related to the sintering degree and amount of formed calcium aluminate and spinel phases. The sensitivity analysis of parallel simulations for multilayer steel ladle with different numbers of temperature‐dependent thermophysical parameters demonstrates that the thermal conductivity is the most influential parameter, as it dominates the temperature distribution and affects the succeeding thermal expansion as well as deformation. The increase of thermal expansion coefficient and decrease of Young's modulus with temperature counterbalance the deformation of linings under thermal process. Meanwhile, compared with the simulation using temperature‐independent parameters, the model with temperature‐dependent thermophysical parameters exhibits more severe damage in outer surface of both the working lining and the permanent lining. This gives new insight into the adoption of temperature‐dependent parameters for actual thermomechanical behavior evaluation of refractories.

Efficient statistical approach for thermo-electrical properties of gadolinia-doped ceria thin films

The thermo-electrical properties of gadolinia-doped ceria (GDC) thin films play a vital role in solid oxide fuel cell (SOFC) applications. Nonetheless, the effects of temperature, dopant concentration, and thickness on thermal expansion, and ionic conduction remain ambiguous. In the present study, we propose an efficient theoretical approach to emphasize the roles of surface layers and surface vicinity (called next-surface layers) on the thermo-electrical properties of GDC thin films. Using statistical moment method, thermal expansion coefficient, and ionic conductivity are calculated as functions of temperature, dopant concentration, and thickness. The lattice strain and ionic disorder in the surface and next-surface layers cause an increase in thermal expansion coefficient and ionic conductivity of thin films. Compared with bulk GDC, the thermal expansion coefficient is slightly larger while the ionic conductivity is more than one to two orders of magnitude enhancement and exhibits the maximum value at the higher concentration. The ionic conductivity depends non-linearly on the dopant concentration and the maximum conductivity is shifted toward a higher value as the temperature increases. The enhancement in thermal expansion coefficient and ionic conductivity offers new opportunities for SOFC electrolytes whose efficiency can be effectively controlled by tailoring the thickness. The theoretical calculations are compared to measured results from experiments.

Static strain aging in cold rolled stable austenitic stainless steel

The effects of cold rolling reduction ratio, initial grain size, and aging condition (temperature and time) on static strain aging (SSA) behavior were systematically investigated in stable austenitic stainless steel. The aging treatment increased the yield strength (YS) and ultimate tensile strength (UTS), and the SSA effect was more pronounced at higher reduction ratio or smaller initial grain size. The SSA effect occurred in the temperature range of 400∼600 °C, where the accumulated dislocations remained in the form of wavy boundaries. Quantifying the dislocation density clearly showed that the SSA effect was directly related to the dislocation density. The volume shrinkage and increase in thermal expansion coefficient by aging treatment suggested the correlation between SSA behavior and atomic distribution changes. 3D atom probe tomography analysis clearly exhibited the clustering and segregation of solute C atoms during aging treatment. These results suggest that the SSA effect may be attributed to the interaction between solute C atoms and dislocations. Through grain refinement and optimal aging treatment at 500 °C for 1 h, the YS and UTS were significantly increased by 319 MPa and 230 MPa, respectively, to obtain a non-magnetic stainless steel with ultrahigh YS of up to 1600 MPa.

Investigation of the glass forming regularity and thermal behavior of lead-free V2O5-TeO2-RO (R = Ca, Sr, Ba) low temperature sealing glass for electronics packaging

Lead-free V2O5-TeO2 glass systems are promising new sealant materials for electronics vacuum packaging, with low-melting temperature and unique light absorption. The glass forming regularity, structure and corresponding properties of the V2O5-TeO2-RO (R=Ca, Sr, Ba) glass system are systematically studied and analyzed in this paper. The results show that there is a wide glass forming region for the V2O5-TeO2-RO glass system, and with the increase of TeO2/V2O5 molar ratio, the thermal stability of V2O5-TeO2-SrO glass with the RO content of 20 mol% increases and the coefficient of thermal expansion increase from 11.2 to 15.86 × 10–6/°C due to transformation process of glass network structure from the [VO4] to transition state [TeO3 + 1] and [TeO3] group. The higher initial crystallization temperature of 40 mol%V2O5-40 mol%TeO2-20 mol%SrO glass is conducive to the flow of the glass slurry during the sealing process, obtaining a sealing device with good airtightness at extremely low temperatures of 350 °C.

Effect of Al2O3/SiO2 mass ratio on the structure and properties of medical neutral boroaluminosilicate glass based on XPS and infrared analysis

A new medical neutral boroaluminosilicate glass with thermal and chemical stabilities conforming to international standards has been successfully prepared by the traditional melt cooling method. Based on X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared spectroscopy (FTIR) characterizations, the relationship between the structure and physical and chemical properties of the glass was discussed. The increase of Al2O3 introduction led to the continuous increase of the octahedral proportion of [AlO6], and weakened the degree of glass network connection, which resulted in the increase of thermal expansion coefficient of the glass and the decrease of chemical stability. B3+ ions and Al3+ ions have a competition for free oxygen and basic metal ions, which caused obvious boron-aluminum anomaly. The research has certain guiding significance for the performance standard of medical glass materials and the control of industrial development cost.

Simulation study on uniaxial tensile mechanical and thermal properties of rhenium tungsten alloys

As an important potential first wall material in fusion reactor, tungsten will be irradiated by 14.1 MeV fusion neutron and then transmutated to rhenium, osmium, and tantalum during service. Due to rhenium is the main of transmutation products, which will affect the mechanical and thermal properties of the tungsten matrix. Based on this, in this work, the Young's Modulus and thermal properties of W-Re alloys with different rhenium contents were calculated by using the first principle calculations. Then, the uniaxial tensile processes in [100], [110], [111] and [211] directions were simulated by molecular dynamic simulation method. The results show that the tensile strength of W-Re alloy decreases gradually with the increase of rhenium content. With the increase of the rhenium content, the heat capacity of the material will decrease, and the thermal expansion coefficient increases.

Synthesis, structure and properties of PbO–PbF2–B2O3–SiO2 glasses

The structure, thermal and some physical properties of lead fluoroborate glasses containing 30 mol% SiO2 have been investigated by differential thermal analysis, X-ray diffraction and Fourier-transform infrared spectroscopy. The glasses were prepared by the conventional melt-quenching method. Fourier-transform infrared spectroscopy results showed that the network of these glasses consists mainly of BO3, BO4, SiO4, and PbO4 structural units. The thermal stability of the glass samples determined by differential thermal analysis was found to be about 80°C. Dilatometric measurements showed that the glass transition temperature and dilatometric softening temperature decrease with increasing lead content, whereas the coefficient of thermal expansion increases. The density and molar volume increased with the increase in lead content. The conductivity of the investigated glasses mainly depends on the mobility of F– and Pb2+ ions. The variation in volume resistance upon changing the composition has been correlated with the structural changes in the glass network. The results obtained in this study indicate that the investigated glasses can be potential candidates for advanced technologies as solder and sealing materials.

Development of early age autogenous and thermal strains of alkali-activated slag-fly ash pastes

Replacing ordinary Portland cement-based materials with alkali-activated industrial wastes is often limited because of significant volume changes occurring in these materials at early age. This experimental study aims to quantify the extent of the volume changes and explore the underlying mechanisms of pastes composed of slag and fly ash (ratio 50:50) which are activated by sodium hydroxide and sodium silicate. Eight compositions were tested, with silica modulus (Ms) varying between 1.04 and 1.58 and with solution-to-binder ratios (S/B) varying between 0.47 and 0.70. Specimen length changes in sealed conditions are monitored by applying repeated thermal variations in an adapted AutoShrink device and are accompanied by isothermal calorimetry, uniaxial compressive strength, and internal relative humidity (IRH) tests. This way, the temporal evolutions of autogenous strains, the coefficient of thermal expansion (CTE), the heat release, the apparent activation energy (Ea), the IRH and the strength are determined and compared to each other. Both the measured autogenous shrinkage and CTEs are rather large; they amount to 4,000–5,000 μm/m and roughly 40 μm/m/°C, respectively, at material ages of 2 weeks. An increase in S/B leads to a decrease in autogenous shrinkage and an increase in CTE. An increase in the Ms causes a decrease in both the autogenous shrinkage and the CTE. Most strikingly, autogenous shrinkage evolves linearly with the cumulative heat released by the binders. The IRH remains continuously above 94% during the first 2 weeks. The apparent activation energy amounts to roughly 74 kJ/mol and is virtually unaffected by S/B and Ms.

Effect of Fe-Doping on Thermal Expansion and Stability of Bismuth Magnesium Tantalate Pyrochlorere.

A continuous series of solid solutions (Bi1.5Mg0.75-xFexTa1.5O7±Δ (x = 0-0.75)) with the pyrochlore structure were synthesized with the solid-phase method. It was shown that iron, like magnesium, is concentrated in the structure in the octahedral position of tantalum. Doping with iron atoms led to an increase in the upper limit of the thermal stability interval of magnesium-containing pyrochlore from 1050 °C (x = 0) up to a temperature of 1140 °C (x = 1). The unit cell constant a and thermal expansion coefficient (TEC) increase uniformly slightly from 10.5018 Å up to 10.5761 Å and from 3.6 up to 9.3 × 10-6 °C-1 in the temperature range 30-1100 °C. The effect of iron(III) ions on the thermal stability and thermal expansion of solid solutions was revealed. It has been established that the thermal stability of iron-containing solid solutions correlates with the unit cell parameter, and the lower the parameter, the more stable the compound. The TEC value, on the contrary, is inversely proportional to the cell constant.

Сr-doped bismuth tantalate pyrochlore: Electrical and thermal properties, crystal structure and ESR, NEXAFS, XPS spectroscopy

The Bi1.6Сr0.8Ta1.6O7±Δ pyrochlore was synthesized by the solid-phase method (sp. gr. Fd-3m:2, 10.45523(3) Å, Z = 8). The thermal stability of the pyrochlore in air up to 1140 °C has been established. With increasing temperature (30–1200 °C), the unit cell parameter and the thermal expansion coefficient increase isotropically, although weakly from 10.4413 to 10.5255 Å and 4.6 × 10−6 to 9.2 × 10−6 °C − 1. The electron spin resonance spectrum of the Bi1.6Сr0.8Ta1.6O7±Δ contains an intense Lorentz-shaped band (g∼2.05) with a width of ΔBPP∼160 mT. According to X-ray absorption fine structure and X-ray photoelectron spectroscopy, the ions are in the charge states Bi(+3), Ta(+5-δ), Cr(+3). The sample contains shallow traps with a depth of 0.39 eV and deep traps with an energy of 1.7 eV. At temperatures above 400 °C, ion transfer begins to dominate, which is revealed in a decrease in the conductivity activation energy to 0.78 eV.

Fully coupled thermo-hydro-mechanical modeling and simulation of a fluid-saturated porous medium under local thermal non-equilibrium condition

• Fully coupled THM model of porous media under LTNE condition is presented. • T s is always higher than T f due to the LTNE effect. • Dependences of multi-physical fields on the pertinent parameters are investigated. This paper presents a fully coupled thermo-hydro-mechanical (THM) model of a fluid-saturated and compressible porous medium under local thermal non-equilibrium (LTNE) condition and carries out the corresponding numerical simulation. First, the coupling among the temperature field, the pore fluid pressure field, and the solid stress/displacement field is fulfilled by means of strong explicit coupling of governing equations in this study. Then, the presented model is applied to the THM coupling problem in an infinite porous medium with a cylindrical borehole subjected to uniform fluid pressure and temperature at the borehole boundary. Subsequently, the partial differential equations (PDEs) module in COMSOL Multiphysics software is employed to numerically solve the studied problem. Consequently, the finite element solutions of the pore fluid pressure field, the strain/stress fields and the two temperature fields of pore fluid and skeleton solid are obtained, and the spatial-temporal distribution characteristics of the above multi-physical fields are investigated. Finally, the corresponding parametric studies are carried out. The numerical results show that the stress decreases with the increase of the bulk volumetric thermal expansion coefficient of porous media. And the pore pressure decreases when the pore fluid mobility increases. Meanwhile, it is found in this work that the solid temperature is always higher than that of pore fluid due to the LTNE effect. Besides, with increasing the specific solid–fluid interfacial heat transfer coefficient, the solid temperature decreases, while the temperature of pore fluid increases. Accordingly, the temperature difference between the solid and the pore fluid diminishes. In addition, the thermal conductivity of solid grains has a greater impact on the solid temperature but has a limited impact on the temperature of pore fluid, and vice versa. The findings in this work are of benefit to obtain in-depth understanding of THM fully coupling mechanism and LTNE effect in a fluid-saturated porous medium.

Thermal properties and thermal cycling stability of graphite/copper composite fabricated by microwave sintering

Herein, graphite/copper (Gr/Cu) composites were fabricated by mechanical mixing and microwave sintering to investigate the effect of graphite volume fraction on the thermal conductivity (TC) and coefficient of thermal expansion (CTE) of the Gr/Cu composite. The TC and CTE of the composites were measured in the in-plane and through-plane directions. The obtained results revealed that the graphite volume fraction significantly affects the TC and CTE of Gr/Cu composites. A lower volume fraction led to higher thermal properties. The effect of thermal cycling on the thermal properties of metal matrix composites (MMCs) has remained largely unexplored. In this work, the effect of thermal cycling on the thermal properties of MMCs was investigated in a temperature range of −196°C/+200 °C. The thermal cycling resulted in a staged decrease of TC and a gradual increase in CTE. This could be mainly attributed to the CTE mismatch between the Cu matrix and the Gr phase. Observations by scanning electron microscopy (SEM) showed that thermal cycling created the micro-cracks and micro-voids in the composites, as well as the sliding and detachment of the Gr/Cu interface. The distribution of thermal stress and plastic strain during thermal cycling was simulated by the finite element method. The changes in TC and CTE are discussed regarding thermally induced stress.

Thermal and mechanical properties of the clathrate-II Na24Si136

Thermal expansion, lattice dynamics, heat capacity, compressibility, and pressure stability of the intermetallic clathrate ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ have been investigated by a combination of first-principles calculations and experimentation. Direct comparison of the properties of ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ with those of the low-density elemental modification ${\mathrm{Si}}_{136}$ provide insight into the effects of filling the silicon clathrate framework cages with Na on these properties. Calculations of the phonon dispersion only yield sensible results if the Na atoms in the large cages of the structure are displaced from the cage centers, but the exact nature of off-centering is difficult to elucidate conclusively. Pronounced peaks in the calculated phonon density of states for ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$, absent for ${\mathrm{Si}}_{136}$, reflect the presence of low-energy vibrational modes associated with the guest atoms, in agreement with prior inelastic neutron-scattering experiments and reflected in marked temperature dependence of the guest atom atomic displacement parameters determined by single-crystal x-ray diffraction. The bulk modulus is only weakly influenced by filling the Si framework cages with Na, whereas the phase stability under pressure is significantly enhanced. The room-temperature linear coefficient of thermal expansion (CTE) is nearly a factor of 3 greater for ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ compared to ${\mathrm{Si}}_{136}$. Negative thermal expansion (NTE), observed in ${\mathrm{Si}}_{136}$ below 100 K, is noticeably absent in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$. In contrast to ${\mathrm{Si}}_{136}$, the thermal expansion behavior in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$ is relatively well described by the conventional Gr\"uneisen-Debye model in the temperature range of 10--700 K. First-principles calculations in the quasiharmonic approximation correctly predict an increase in high-temperature CTE with Na loading, although the increase is less than observed in experiment. The calculations also fail to capture the absence of NTE in ${\mathrm{Na}}_{24}{\mathrm{Si}}_{136}$, perhaps due to anharmonic effects and/or inadequateness of the ordered structural model.

Investigation into structure and property of W and Ti co-doped SrFeO3 perovskite as electrode of symmetrical solid oxide fuel cell

SrFe0.9-xW0.1TixO3-δ (SFWT, x = 0.1 and 0.2) perovskite oxides are synthesized by combustion process. The experimental and computational analyses are conducted to reveal the effect of dopant W and Ti on the material properties and structure. The crystal structure transform from tetragonal (I4/m) to cubic(pm3¯m) when temperature is higher than 700 °C. Substitution of Fe with Ti can enhance the lattice structure. However, the Ti incorporation causes the increase in thermal expansion coefficient above 700 °C. The XPS and TG analyses show that the Ti incorporation also decreases oxygen activity. Thus, it is important to optimize the Ti content to design the SFWT materials to gain fast kinetics and high stability. The total conductivity (48 S cm−1), polarization resistances of symmetric single cell (0.28 Ω cm2), and durability (150 h, at 750 °C) reveal that the SrFe0.8W0.1Ti0.1O3-δ is a promising electrode materials for symmetric fuel cell. The first-principles calculation suggests that the Fe–O bond in SFWT dominates electric conductivity and can be suitably tailored by the dopant Ti and W.

Electrophoretic co-deposition of Mn1.5Co1.5O4, Fe2O3 and CuO: Unravelling the effect of simultaneous addition of Cu and Fe on the microstructural, thermo-mechanical and corrosion properties of in-situ modified spinel coatings for solid oxide cell interconnects

A systematic microstructural, thermo-mechanical and electrical characterization of simultaneous Fe–Cu doped Mn–Co spinel coatings processed by electrophoretic co-deposition on Crofer 22 APU is here reported and discussed. An innovative approach for the simultaneous electrophoretic deposition of three spinel precursors is designed, conceived and optimised, with the aim of outlining time- and energy-saving spinel modification routes. The effect of different levels of Cu and Fe co-doping is observed on the stability of the modified Mn–Co spinel phase, the coefficient of thermal expansion (CTE), the corrosion resistance and on the densification behaviour of the obtained coatings. Cu determines an increase of CTE, while Fe has the opposite behavior. The synergic effect of the simultaneous Fe and Cu co-doping results in an improved densification and the stabilization of the MnCo2O4 cubic phase. The most interesting results in terms of corrosion resistance are obtained for the Mn1.28Co1.28Fe0.15Cu0.29O4 spinel.

High-temperature annealing of porous anodic aluminium oxide prepared in selenic acid electrolyte

Selenic acid electrolyte provides porous anodic aluminium oxide (AAO) films with low porosity and high optical transparency. Here, the high-temperature behaviour of AAO obtained in a selenic acid electrolyte is reported for the first time. The crystallization of as-prepared amorphous AAO at about 800 °C is accompanied by the elimination of electrolyte impurities, an increase in pore diameter from 15 to 21 nm, and an increase in specific surface area to 55 m2·g−1 due to the mesopore formation inside the cell walls. The second crystallization stage at about 1150 °C results in the formation of coarse-grained corundum (α-Al2O3) films with a mean grain size of about 11 μm. Each of the grains consists of regularly arranged 27-nm-diameter pores, which are stable up to 1300 °C. AAO physicochemical properties depend on the annealing temperature. In particular, chemical stability is up to three orders of magnitude higher after two-step crystallization. Optical properties and thermal expansion of selenic-acid-made porous AAO films with different phase composition are quantified. The crystallization of as-prepared AAO to low-temperature Al2O3 polymorphs has a minor influence on optical transmittance and effective refractive index of AAO, whereas the coarse-grained structure of corundum causes an order of magnitude decrease in average transmittance in the visible region. The coefficient of thermal expansion increases by 25% to 8.5 × 10−6 K−1 for α-Al2O3 porous films compared to as-prepared AAO.

Упругость 3D- и 2D-соединений XC (X=Si, Ge, Sn): модели Китинга и Харрисона

For the bulk and monolayer IV group carbides in the scope of Keating and/or Harrison models analytical expressions are obtained for second order elastic constants , sound velocities , third order elastic constants , pressure dependencies of and , Grüneisen constant, thermal expansion coefficient and temperature dependence of bulk moduli. It is shown that in the row of SiC – GeC – SnC values of all considered characteristics decrease while thermal expansion coefficient increases. All the model estimations are compared with experimental data and calculating results of other authors.

Elasticity of 3D and 2D XC (X = Si, Ge, Sn) compounds: Keating and Harrison models

For the bulk and monolayer IV group carbides in the scope of Keating and/or Harrison models analytical expressions are obtained for second order elastic constants cij, sound velocities v, third order elastic constants cijk, pressure dependencies of cij and v, Grneisen constant, thermal expansion coefficient and temperature dependence of bulk moduli. It is shown that in the row of SiC-GeC-SnC values of all considered characteristics decrease while thermal expansion coefficient increases. All the model estimations are compared with experimental data and calculating results of other authors. Keywords: force constants, elastic constants, sound velocities, anharmonic characteristics.

A DFT study on the crystal stability, mechanical, electronic and thermodynamic properties of Ir3Nb under high pressure and temperature

The crystal stability, mechanical, electronic and thermodynamic properties of Ir3Nb at high pressures and high temperatures were systematically investigated by first-principles calculations based on density functional theory. The obtained lattice parameters are consistent with available experimental and theoretical results. The Ir3Nb crystal maintains its crystal stability in the range of 0–100 GPa and 0–1600 K. With the increase of pressure, the lattice parameter decreases, while the elastic constants and mechanical moduli (B, G, E) increase. The study shows that Ir3Nb exhibits intrinsic brittleness and mechanical anisotropy. There is no ductile-brittle transition occurred in the pressure range of 0–100 GPa according to the Pugh's ratio and Poisson's ratio analyses. The mechanical anisotropy of Ir3Nb is significantly enhanced with the increase of pressure. The electronic analyses show that the Ir-5d and Nb-4d electrons hybrid to form a pseudogap on the electronic density of states curves. The pseudogap is gradually widened with the increase of pressure, which means a stronger Ir-Nb bonding strength under high pressure. The study also shows that the elastic constants and mechanical moduli decrease with elevated temperature. Thermodynamic analysis indicates that the thermal expansion coefficient and heat capacity increase as temperature increases, especially at low temperatures. The sound velocity and Debye temperature increase with the increase of pressure.

Thermal and centrifugal stresses in curved double wall transpiration cooled components with temperature dependent thermoelastic properties

The integration of double wall transpiration cooling (DWTC) into gas turbines and hypersonic vehicles is key for addressing future needs in aerospace and power generation. This paper identifies the roles of geometric curvature and temperature dependence of thermoelastic properties on stresses in DWTC systems under combined thermal and centrifugal loading. An increase of thermal expansion coefficient, α, with temperature as exhibited by Ni alloys is shown to dominate over the decrease of Young’s modulus, E, in double walls, such that the thermal stresses are significantly higher than the case of temperature independent E and α. The effect is generally promoted by the thermal mismatch between the walls; in highly curved systems, the difference in wall circumferences also comes into play. The critical stresses at film holes, impingement holes and pedestals are affected differently for systems with convex and concave hot outer surfaces while the optimal wall thickness ratio is suggested to depend on the severity of thermal loading with respect to centrifugal loading. Our elastic solutions provide the essential information for evaluating the structural integrity of DWTC systems and can be generalised to multiwall systems.