Decrease In Coefficient Of Thermal Expansion Research Articles

The Effect of Graphite Flake Volume Fraction on the Thermophysical Proper Ties of Graphite Flake/Aluminum Composites

A vacuum hot‐press sintering process is utilized to manufacture graphite flake/aluminum composites, and the influence of graphite flake volume fraction on the microscopic structure, hardness, thermal conductivity (TC), and coefficient of thermal expansion (CTE) of the composites is investigated. The results indicate that the graphite flakes are neatly aligned along the in‐plane direction in the composites and exhibit a strong bond with the aluminum matrix interface. The in‐plane TC of the composite significantly increases with the addition of graphite flakes, whereas hardness, through‐plane TC, and CTE decrease. As the volume fraction of graphite flakes in the composites increases from 15% to 60%, in‐plane TC improves from 278.8 to 353.8 W m−1 K−1, through‐plane TC decreases from 164.4 to 33.2 W m−1 K−1, and the CTE decreases to 13.7 × 10−6 K−1, achieving high thermal conductivity and a low CTE.

Insight into the mechanical properties and thermal expansion behavior of epoxy nanocomposites reinforced with multi‐walled carbon nanotube solvent‐free nanofluids

AbstractEpoxy resin (EP) is a promising material that exhibits great potential in the fields of packaging and adhesion. However, reducing its brittleness and coefficient of thermal expansion (CTE) at low filler content is still a challenge. To address this problem, three types of MWCNTs nanofluids (MWCNTs NFs) with different shell structures were designed to modify epoxy resin. With the addition of 1 wt% MWCNTs‐M1, the compression, impact, and lap shear strength of the composites increased to 88.97 MPa, 81.39 kJ/m2, 12.69 MPa, respectively, which are 21.7%, 66.1%, and 58.0% higher than pure epoxy. Meanwhile, incorporating MWCNTs NFs also improved the CTE and thermal conductivity, with a 19.7% decrease in CTE and a 40% improvement in thermal conductivity at 2 wt% NFs content. Particularly, the effect of MWCNTs NFs structure on the composite properties was explored, which can be an available reference for designing specific NFs epoxy composites.Highlights Three types of MWCNTs nanofluids with different structures were designed. The MWCNTs NFs were used in packaging materials for the first time. The impact performance of composite has been improved by 66%. The CTE of composite is 19.7% lower than pure EP. The thermal conductivity of the composite increased by 40%.

Influence of temperature on the elastic properties of graphene and graphene-like nanosheets based on the asymptotic homogenization method

In this paper, the effective elastic properties of hexagonal monolayers such as graphene, boron nitride, silicon carbide, and aluminum nitride are evaluated using the asymptotic homogenization method taking into account temperature changes. The effective properties are determined analytically by considering temperature, force constants, bond lengths, and thickness, using atomic interactions within the context of the unit cell problem. The relationship between environmental temperature and the coefficient of thermal expansion is established by considering changes in the values ​​of bond stretching, bond angle bending, and torsion resistance constants at different temperatures from 0 to 1800 K. The results provided by the proposed homogeneous model are consistent with the valid findings of other researchers in the literature. It is found that these different hexagonal nanosheets exhibit isotropic temperature-dependent properties and their Young's and shear modulus decrease almost linearly as the temperature rises, considering a constant coefficient of thermal expansion. However, Poisson's ratio is found to be independent of temperature. Furthermore, the temperature-dependent elastic properties of these nanosheets become nonlinear when the variation of the coefficient of thermal expansion with temperature is taken into account. The Young and shear moduli of the nanosheets increase as the coefficient of thermal expansion decreases and vice versa. The highest elastic modulus values are obtained when the thermal expansion is close to its minimum threshold. However, it is recognized that the elastic modulus of graphene and boron nitride nanosheets is higher than that of silicon carbide and aluminum nitride at any temperature. As an alternative to lengthy computer modeling or rigorous testing, the homogeneous models provided can be used to simulate nanosheets.

In-depth analysis of the influence of bio-silica filler (Didymosphenia geminata frustules) on the properties of Mg matrix composites

A novel metal matrix composites (MMC) with Mg matrix reinforced with natural filler in the form of Didymosphenia geminata frustules (algae with distinctive siliceous shells) are presented in this work. Pulse plasma sintering (PPS) was used to manufacture Mg-based composites with 1, 5 and 10 vol.% ceramic filler. As a reference, pure Mg was sintered. The results show that the addition of 1 vol.% Didymosphenia geminata frustules to the Mg matrix increases its corrosion resistance by supporting passivation reactions, and do not affect the morphology of L929 fibroblasts. Addition of 5 vol.% the filler does not cause cytotoxic effects, but it supports microgalvanic reactions leading to the greater corrosion rate. Higher content than 5 vol.% the filler causes significant microgalvanic corrosion, as well as increases cytotoxicity due to the greater micro-galvanic effect of the composites containing 10 and 15 vol.% diatoms. The results of contact angle measurements show the hydrophilic character of the investigated materials, with slightly increase in numerical values with addition of amount of ceramic reinforcement. The addition of Didymosphenia geminata frustules causes changes in a thermo-elastic properties such as mean apparent value of coefficient of thermal expansion (CTE) and thermal conductivity (λ). The addition of siliceous reinforcement resulted in a linear decrease of CTE and reduction in thermal conductivity over the entire temperature range. With the increasing addition of Didymosphenia geminata frustules, an increase in strength with a decrease in compressive strain is observed. In all composites an increase in microhardness was attained.The results clearly indicate that filler in the form of Didymosphenia geminata frustules may significantly change the most important properties of pure Mg, indicating its wide potential in the application of Mg-based composites with a special focus on biomedical use.

Development of titanium-doped SmBaMn2O5+δ layered perovskite as a robust electrode for symmetrical solid oxide fuel cells

It is a big challenge ahead of finding a symmetric electrode material that optimally works as both anode and cathode, with excellent structural and chemical stability and high catalytic activity. Herein, we propose a high-performance symmetric electrode material, SmBaMn1.9Ti0.1O5+δ (SBMTi), with a single A-site layered perovskite phase in both reducing and oxidizing atmospheres. Owing to the high binding energy, titanium doping can enhance the structural stability and improve the catalytic activity to the hydrogen oxidation and oxygen reduction processes. The simple Ti-doping strategy enables a dramatic reduction in stoichiometric oxygen change upon oxidizing and reducing atmosphere alternation, and a considerable decrease in thermal expansion coefficient. The symmetrical cell with SBMTi electrode can provide a maximum power density of 603 mW cm−2 at 900 °C, and shows a relatively stable thermal-cycle and atmosphere alternation performance. The developed SmBaMn1.9Ti0.1O5+δ shows great potential as symmetric electrode in symmetrical solid oxide fuel cells.

Use of pumice aggregate in cementitious rheoplastic lightweight concrete

Rheoplastic lightweight concrete (RLC) is generally designed for pumping applications as fluid concrete free from segregation. Concrete is produced using polymeric admixtures to enhance concrete workability, strength, drying shrinkage, and durability. This research investigated the suitability of natural porous pumice aggregates in Turkey to obtain rheoplastic lightweight concrete with cement content in normal ranges. To produce and experience rheoplastic concrete mix design data, rheoplastic lightweight concrete mixes were tested with fine pumice aggregate (FPA) and coarse pumice aggregate (CPA) supplied from the Nevşehir region of Turkey. For rheoplastic lightweight concrete with cement contents in the 250 to 400 kg/m3 range, the percentage of fine pumice aggregates required was in the 73.6-81.0% range with complimentary water/cement ratios of between 0.53 and 0.68. The upper compressive strength limit was circa 30 N/mm2 . The research findings determined that the rheoplastic concrete samples with pumice aggregate met the design requirement of a slump value of 200 mm for fresh concrete predicted for fluid concrete forms. While technical properties of hardened concrete such as oven-dry density (1198-1362 kg/m3 ), strength values, static elasticity modulus (9236- 10756 MPa), thermal expansion coefficient (5.354 x10-6/°C - 6.929x10-6/°C) and thermal conductivity value (0.405-0.619 W/mK) decrease with increasing aggregate/cement ratios, they increase with increasing cement dosage. In addition, the high amount of fine pumice in concrete composition results in lower drying shrinkage and wetting expansion with decreasing cement dosage. The technical findings showed that RLC might be produced by using a superplasticizer and air-entraining admixtures and mixtures of different sizes of pumice aggregates.

Material characteristics of WO3/Bi2O3 substitution on the thermal, structural, and electrical properties of lithium calcium borate glasses

Glasses belonging to the xWO3-(30-x) Bi2O3–15Li2O–15CaO–40B2O3 (0 ≤ x ≤ 10) have been obtained by melt-quenching technique. With increasing the concentration of WO3, an increase in the measured glass transition temperature {T}_{mathrm{g}} and crystallization temperature {T}_{mathrm{c}} are observed, while the values of the thermal expansion coefficient decrease. Due to the unpaired pentavalent ions, the electron spin resonance spectra of glasses doped with {mathrm{WO}}_{3} them show unique signals. The results of Raman spectra show that the glass network is mainly constituted by [{mathrm{BiO}}_{6}], [{mathrm{BO}}_{3}], [{mathrm{BO}}_{4}], and [{mathrm{WO}}_{4}] basic structural units. The activation energies for all studied samples were observed to range from 0.02 to 0.08 eV, and the dielectric constant and the conductivities of the investigated glasses increased with increasing temperature. The electrical behavior of the prepared samples revealed their semiconducting character. The prepared samples are being investigated as potential semiconductor material candidates for use in electronic devices.

Enhancing low thermal expansion behavior and strength via induced Zr-rich intermetallic phase in Fe-36Ni Invar alloy

Nowadays, Invar alloy combining low thermal expansion behavior with improved mechanical properties is highly required. This study investigated the influence of Zr-rich intermetallic compounds (IMCs) on microstructure, thermal expansion behavior, and mechanical properties in Fe-36Ni Invar alloy. Zr-rich IMCs, namely Ni7Zr2 and Ni2Zr were introduced into Fe-36Ni Invar alloy by the addition of 0.22 wt% Zr. The microstructure of Zr-containing alloy was remarkably refined and accompanied by high dislocation density, which was associated with the resistance of recrystallization by the introduction of Zr-rich IMCs. The Invar alloy displayed a sufficiently low coefficient of thermal expansion (CTE) in the temperature interval of 20 °C ∼ 100 °C and 20 °C ∼ 200 °C after Zr treatment, with a CTE constant of 0.66 × 10-6 /°C and 2.07 × 10-6 /°C, respectively. The existence of residual stress and refinement of grain were responsible for an obvious decrease in CTE. Meanwhile, the yield strength increased from 256 MPa to 401 MPa after Zr treatment. The strengthening contribution mainly arose from dislocation strengthening and grain refinement strengthening.

Impact of Ni on the thermophysical and thermodynamic properties of Fe–C–Ni based alloys

Three model alloys based on Fe-C-Ni were studied containing carbon between 0.338 and 0.382 wt. % and nickel between 1.084 and 4.478 wt. %. Phase transition temperatures, heat capacity, enthalpy change, heat of fusion, coefficient of thermal expansion, and density were experimentally and theoretically determined in the high-temperature area from 1000 °C to 1595 °C. A number of techniques, namely differential thermal analysis (DTA), differential scanning calorimetry (DSC), and dilatometry, were used in this study, and the heat of fusion was determined by two approaches, that is, from the DSC peak area and from the enthalpy change. The experimental data were compared and discussed with the calculation results obtained using SW IDS, JMatPro, and Thermo-Calc operating with the commercially available TCFE8 thermodynamic database. The obtained experimental results show that the liquidus temperature and the coefficient of thermal expansion decrease with increasing nickel content. On the contrary, the density and heat of fusion values derived from the DSC peak increase with increasing nickel content. Furthermore, an ambiguous influence of nickel on the change in solidus temperature, heat capacity, enthalpy change, and heat of fusion obtained from the enthalpy change was observed.

Influence of ceramic particles additions on the properties of Ca3Co4O9

Ca3Co4O9 + x wt% B4C, AlN, TiC, TiB2, or TiN (x = 0.0, 0.25, 0.50, and 0.75) samples were prepared by the conventional solid-state route. In all samples, only the Ca3Co4O9 phase was identified by powder XRD. Nevertheless, microstructural studies have shown that most of the additives have reacted with air and Ca3Co4O9 phase on their surfaces, producing new phases. Moreover, it seemed that grain sizes were, at least, slightly reduced. On the other hand, while nearly no modification of the Seebeck coefficient has been observed, independently of the added compound and proportion, electrical resistivity decreased in all cases, when compared to the pristine sample. Consequently, the power factor of samples with additions was higher than the one determined for the pure sample. Linear thermal expansion also decreased with these additives, pointing out to the formation of relatively strong grain boundaries which can improve the carrier mobility and decrease the thermal expansion. The lowest thermal expansion value has been measured in 0.25 wt% B4C samples, being only around 20% higher than that of Al2O3, which can help to reduce the differential thermal expansion in thermoelectric modules working at high temperatures; these results may be very interesting for applications prospects.Article highlightsAddition of different ceramic additives to Ca3Co4O9 phase increases electrical conductivity, and power factor.All additive ceramics react with oxygen and the Ca3Co4O9 phase modifying the properties of pristine compound.The decrease of thermal expansion coefficient will allow reducing the differential thermal expansion in modules.

Utilizing a Long Short-Term Memory Algorithm Modified by Dwarf Mongoose Optimization to Predict Thermal Expansion of Cu-Al2O3 Nanocomposites

This paper presents a machine learning model to predict the effect of Al2O3 nanoparticle content on the coefficient of thermal expansion in Cu-Al2O3 nanocomposites prepared using an in situ chemical technique. The model developed is a modification of Long Short-Term Memory (LSTM) using dwarf mongoose optimization (DMO), which mimics the behavior of DMO to find its food for predicting the behavior of the composite. The swarm of DMO consists of three groups, namely the alpha group, scouts, and babysitters. Each group has its own behavior to capture the food. The preparation of the nanocomposite was performed using aluminum nitrate that was added to a solution containing scattered copper nitrate. After that, the powders of CuO and Al2O3 were obtained, and the leftover liquid was removed using thermal treatment at 850 °C for 1 h. The powders were consolidated using compaction and sintering processes. The impact of Al2O3 contents on the thermal properties of the Cu-Al2O3 nanocomposite was investigated. The results showed that the Thermal Expansion Coefficient (TEC) decreases with increasing Al2O3 content due to the increased precipitation of Al2O3 nanoparticles at the grain boundaries of the Cu matrix. Moreover, the good interfacial bonding between Al2O3 and the Cu may participate in this decrease in TEC. The proposed machine learning model was able to predict the TEC of all the produced composites with different Al2O3 content and was tested at different temperatures with very good accuracy, reaching 99%.

Studies on phase evolution, microstructure and crystallization kinetics in Nd2O3 doped Li2O-Al2O3-SiO2 glass system

Nd2O3 doped Lithium Alumino Silicate (LAS) glass of composition (wt.%) 57SiO2–19Al2O3–3.5Li2O-7P2O5–11.5X–2Nd2O3 (X = B2O3, MgO, ZnO, TiO2, ZrO2) was synthesized and transformed into glass ceramic. X Ray Diffraction (XRD) showed formation of TiZrO4 nano crystalline phase at 948 K and major phase β-spodumene (β-LiAlSi2O6) along with some minor phases at temperatures >1073 K. In the intermediate temperature range, γ-spodumene /LixAlxSi3-xO6 (0<x<1)/Virgilite was identified as a major phase which was shown to get converted into stable β-spodumene phase at elevated temperature. Micrographs of heat treated samples revealed nanocrystals of size (<25 nm) of Titanium Zirconate phase at 948 K and shell type microstructure of β-spodumene phase at temperature >1073 K. Decrease in thermal expansion coefficient (α) from 5.5×10−6/K to 2.98×10−6/K (303–573 K) was observed for sample crystallized at around 1073 K. Non isothermal DTA data showed two exothermic peaks, indicating crystallization of two different phases in temperature range of 993 to 1223 K. Activation energy for crystallization (Ea) of first peak was found to be 168±20 kJmol−1 and 187±20 kJmol−1 using Kissinger and Marotta methods, respectively. Model free kinetics was used to find more accurate kinetic parameters such as Ea and predict isothermal reaction progress.

Influence of cerium content on the microstructure and thermal expansion properties of suction cast Al-Cu-Fe alloys.

Low-expansion alloys are of great importance and can be used for the development of new aerospace materials. Herein, we report diverse rare earth quasicrystal alloys fabricated by the vacuum suction casting process. The effects of the addition of cerium (Ce) on the microstructure, thermal expansion properties and microhardness of the Al-Cu-Fe alloy were systematically investigated. This study discovered the tiny Al-Cu-Fe-Ce microstructure. A uniform distribution could be achieved after Ce addition amount is elevated. At the Ce addition amount of 1 at%, the lowest alloy thermal expansion coefficient was obtained. The alloy exhibited the maximum microhardness under these conditions. The microhardness of alloys containing 1 at% of Ce was approximately 2.4 times higher than the microhardness exhibited by alloys devoid of Ce additives. The coefficient of thermal expansion decreases by approximately 20%. The use of the suction casting process and the addition of an appropriate amount of Ce can potentially help design and develop Al-Cu-Fe-Ce alloys.

Preparation of NdBaCo2O5+δ–Ce0.9Gd0.1O1.95 composite cathode by in situ sol-mixing method and its high-temperature electrochemical properties

NdBaCo2O5+δ-xCe0.9Gd0.1O1.95 (NBCO-xCGO, x = 0 ~ 8 wt%) composite materials are prepared by novel sol-mixing method and evaluated as cathodes for intermediate-temperature solid oxide fuel cells. The chemical compatibility, thermal expansion behavior and electrochemical performance of NBCO-xCGO composite cathodes are studied. There is no chemical reaction between NBCO and CGO after sintering at 1100 °C for 12 h. Introducing CGO leads to the decrease of both thermal expansion coefficient and conductivity. Among all the composites, NBCO-5CGO shows the lowest polarization resistance of 0.034 Ω cm2 at 700 °C. The anode-supported single-cell with NBCO-5CGO cathode exhibits a maximum power density of 1.0 W cm−2 at 700 °C. The electrochemical impedance spectrum measurement combining with the distribution of relaxation times analysis proves that the addition of CGO nanoparticles significantly improves the surface oxygen dissociation process, and the charge transfer process is identified to be the reaction limiting step to control the oxygen reduction kinetics on NBCO-5CGO composite cathode.

Effect of ball milling process on the mechanical and thermal properties of the nanodiamond/2024Al composites

In this study, 5 wt.% nanodiamond (ND) reinforced 2024Al matrix composites (ND/2024Al) were fabricated with various ball milling processes. The microstructure, compressive yield strength (σyc) and coefficient of thermal expansion (CTE) of ND/2024Al composites were investigated. It is found that the addition of 5 wt.% NDs significantly increases the σyc and decreases the CTE of 2024Al. High energy ball milling further increases σyc and decreases CTEs of ND/2024Al composite due to it enhances the fine grain strengthening of α-Al matrix and the dispersion strengthening of NDs, decreases the thermal mismatch stress at the ND/α-Al matrix interface, and increases the constraint of NDs on α-Al. The composites fabricated by the combinations of ball milling speeds and times of (5−7) h×(200−250)rpm and 9 h×300 rpm have the highest σyc and the lowest CTE respectively. Considering the different influence of ball milling parameters on σyc and CTEs, a evaluation coefficient α=σycC⋅εkcCσycM⋅εkcM⋅CTEMCTEC is proposed to evaluate the synergistic influence of mechanical properties and CTEs on the dimensional stability of ND/2024Al composites. Large value of α may lead to high dimensional stability of material, hence the α can be used to determine the ball milling parameters.The ball milling process of 250−300 rpm ball milling speeds and 5−7 h ball milling times are recommended based on α, which causes a 95–100 % increase in σyc and a 30–35 % decrease in CTE compared with 2024Al alloy, respectively.

Density and thermal expansion of borate and borosilicate glass-forming melts

Density, molar volume and thermal expansion of borate and borosilicate melts are properties that significantly affect the process of forming composite coatings by slip-firing technology. Based on experimental data on the density of glass-forming melts, adequate generalizing mathematical models have been developed by using the method of multiple correlation; these models describe the dependence of the molar volume (VT) of alkaline borate, alkaline silicate and multicomponent melts on their oxide composition and temperature with a standard deviation of (0.22–0.27) cm3 mol–1. The regression equation has the following general form: VT=A(xi)–B(xi)/T, where xi is the content of an oxide, T is the temperature, subscript "i" stands for the number of an oxide concerned. The changes in volumetric coefficient of thermal expansion (CTE) of the molten glasses are analyzed as functions of both temperature and chemical composition. It should be noted that borate melts in comparison with silicate ones have much higher values of CTE. The highest value of CTE of experimental melts is observed at glass transition temperatures (Tg). An increase in the temperature above Tg contributes to a significant decrease in CTE. An increase in the content of alkali metal oxides in most glass-forming melts results in an increase in the values of CTE.

Effects of Ni Additions on the High Temperature Expansion, Melting and Oxidation Behaviors of Cobalt-Based Superalloys

Nickel is often added to cobalt-based superalloys to stabilize their austenitic structure. In this work the effects of Ni on several high temperature properties of a chromium-rich cobalt-based alloy reinforced by high fraction of TaC carbides are investigated. Different thermal analysis techniques are used: differential scanning calorimetry (DSC), thermo-mechanical analysis (TMA) and thermogravimetry (TG). Results show that the progressive addition of nickel did not induce great modifications of microstructure, refractoriness or thermal expansion. However, minor beneficial effects were noted, including reduction of the melting temperature range and slight decrease in thermal expansion coefficient. The most important improvement induced by Ni addition concerns the hot oxidation behavior. In this way, introducing several tens wt % Ni in this type of cobalt-based alloy may be recommended.

Improving dimensional stability at high temperature and toughness of polyimide films via adjustable entanglement density

Polyimide (PI) films with ultralow coefficient of thermal expansion (CTE) and high glass transition temperature (Tg) are in great demand for applications as substrates in flexible displays. Furthermore, trade-off between CTE and toughness is highly concerned due to strategies for reducing CTE always result in weakness of toughness. In this work, we developed a facile method to improve dimension stability and toughness of PI containing benzimidazole units via regulating entanglement density based on controlling molecular weight and solid content of precursor polyamic acid (PAA) solution. As a result, the Tg of PI films increased from 394.3 to 441.4 °C and the in-plane CTE (40–350 °C) decreased from 65.7 to 3.9 ppm/K with entanglement density increasing from 253.3 to 2868.6 mol/m3. Moreover, mechanical properties especially toughness of PI films were enhanced significantly (from 7% to 18%) as entanglement density increased. In short, we delineated the effects of entanglement on the properties of PI films and found that the decrease of CTE mainly arises from the increase of entanglement density. Our studies provide new insights into the future design of flexible PI substrates with low CTE.

Influence of processing parameters on the porosity, thermal expansion, and oxidation behavior of consolidated Fe22Cr stainless steel powder

The objective of this study was to determine the effects of powder metallurgy process parameters on the porosity, coefficient of thermal expansion, and oxidation behaviors of porous ferritic Fe22Cr stainless steel compacts. Optimum values of the process were also investigated. The results showed that, porosity and the coefficient of thermal expansion decrease with increasing process parameters; and changing powder shape from spherical to flaky at a certain extent. The minimum and maximum porosity values of the samples were recorded as 7% and 30%, respectively. While those for the samples produced from milled powders were noted as 3%, and 19%, respectively. The coefficient of thermal expansion values were measured at 800 °C and found to be in the range of 12–14.5 × 10−6 K−1 for spherical powders and flaky powders. From oxidation kinetics point of view, both process parameters and powder shape influenced the oxidation behavior.

Theoretical analysis of thermophysical properties of nanomaterials

In the present study, a theoretical comparative work is done to calculate the variation of melting temperature in nanomaterials by using two quantum models i.e. Lu model and Jiang model and the expression of melting temperature calculated with the help of best model is extended to calculate the variation of Debye temperature and thermal expansion coefficient with size and shape of nanomaterials. The melting temperature and Debye temperature are observed to increase as the diameter of nanomaterials is increased and thermal expansion coefficient decrease with increase in the diameter of nanosolid. The results obtained from the given models are compared with the experimental values to judge the best-suited model for the study of the thermodynamic properties of nanosolids. The available experimental results are found very close to the results from the Jiang model and then Debye temperature and thermal expansion coefficient are calculated with the help of Jiang model.