Showing papers on "Thermal expansion published in 2021"

Dual-phase rare-earth-zirconate high-entropy ceramics with glass-like thermal conductivity

Abstract: A series of rare earth zirconates (RE2Zr2O7) high-entropy ceramics with single- and dual-phase structure were prepared. Compared with La2Zr2O7 and Yb2Zr2O7, the smaller “rattling” ions (Yb3+, Er3+, Y3+) have been incorporated into pyrochlore lattice in (La0.2Nd0.2Y0.2Er0.2Yb0.2)2Zr2O7 (LNYEY) while larger ions (La3+, Nd3+, Sm3+, Eu3+) incorporated into fluorite lattice in (La0.2Nd0.2Sm0.2Gd0.2Yb0.2)2Zr2O7 (LNSGY). Due to high-entropy lattice distortion and resonant scattering derived from smaller ions Yb3+, Er3+, and Y3+, LNYEY shows a lower glass-like thermal conductivity (1.62-1.59 W m-1 K-1, 100-600℃) than LNSGY (1.74-1.75 W m-1 K-1, 100-600℃). Moreover, LNYEY and LNSGY exhibit enhanced Vickers’ hardness (LNYEY, Hv = 11.47 ± 0.41 GPa; LNSGY, Hv = 10.96 ± 0.26 GPa) and thermal expansion coefficients (LNYEY, 10.45 × 10-6 K-1, 1000℃; LNSGY, 11.02 × 10-6 K-1, 1000℃). These results indicate that dual-phase rare-earth-zirconate high-entropy ceramics could be desirable for thermal barrier coatings.

Preparation and properties of CMAS resistant bixbyite structured high-entropy oxides RE2O3 (RE = Sm, Eu, Er, Lu, Y, and Yb): Promising environmental barrier coating materials for Al2O3f/Al2O3 composites

Abstract: Y2O3 is regarded as one of the potential environmental barrier coating (EBC) materials for Al2O3f/Al2O3 ceramic matrix composites owing to its high melting point and close thermal expansion coefficient to Al2O3. However, the relatively high thermal conductivity and unsatisfactory calcium-magnesium-aluminosilicate (CMAS) resistance are the main obstacles for the practical application of Y2O3. In order to reduce the thermal conductivity and increase the CMAS resistance, four cubic bixbyite structured high-entropy oxides RE2O3, including (Eu0.2Er0.2Lu0.2Y0.2Yb0.2)2O3, (Sm0.2Er0.2Lu0.2Y0.2Yb0.2)2O3, (Sm0.2Eu0.2Er0.2Y0.2Yb0.2)2O3, and (Sm0.2Eu0.2Lu0.2Y0.2Yb0.2)2O3 were designed and synthesized, among which (Eu0.2Er0.2Lu0.2Y0.2Yb0.2)2O3 and (Sm0.2Er0.2Lu0.2Y0.2Yb0.2)2O3 bulks were prepared by spark plasma sintering (SPS) to investigate their mechanical and thermal properties as well as CMAS resistance. The mechanical properties of (Eu0.2Er0.2Lu0.2Y0.2Yb0.2)2O3 and (Sm0.2Er0.2Lu0.2Y0.2Yb0.2)2O3 are close to those of Y2O3 but become more brittle than Y2O3. The thermal conductivities of (Eu0.2Er0.2Lu0.2Y0.2Yb0.2)2O3 and (Sm0.2Er0.2Lu0.2Y0.2Yb0.2)2O3 (5.1 and 4.6 W·m−1·K−1) are only 23.8% and 21.5% respectively of that of Y2O3 (21.4 W·m−1·K−1), while their thermal expansion coefficients are close to those of Y2O3 and Al2O3. Most importantly, HE RE2O3 ceramics exhibit good CMAS resistance. After being attacked by CMAS at 1350 °C for 4 h, the HE RE2O3 ceramics maintain their original morphologies without forming pores or cracks, making them promising as EBC materials for Al2O3f/Al2O3 composites.

Investigation of the effect of adding nano-encapsulated phase change material to water in natural convection inside a rectangular cavity

Abstract: The present simulation aims to investigate adding NEPCM nanoparticles to water in the natural convection inside a cavity by using FVM method and SIMPLE algorithm Nano-encapsulated phase change material (NEPCM) consists of a shell and core with phase change property The NEPCM particles in base fluid have the ability to transfer heat by absorbing and dissipating heat in the liquid-solid phase change state In this study, the energy wall phenomenon due to the phase change of NEPCM core has appeared that the whose energy transfer strength is proportional to the latent heat of NEPCM core and the thickness of the energy wall Moreover, the relationship between the energy wall and the heat transfer rate is payed attention, and the effects of the energy wall parameters including strength, thickness, and event location of energy wall and volume fraction are studied on the energy wall and heat transfer rate According to the obtained results, adding NEPCM to the water enhances its heat transfer up to 48% in order to increase heat capacity of water-NEPCM mixture Also, best heat transfer rate happens when the energy wall is at the center of the cavity Moreover, a relation is presented for the thermal expansion coefficient of NEPCM, which considers the effects of the thermal expansion coefficient of the core and shell material

Influence of carbon nanotubes on thermal expansion coefficient and thermal buckling of polymer composite plates: experimental and numerical investigations

Abstract: The first aim of this article is to experimentally explore the effect of multi-walled carbon nanotubes (MWCNTs) on the coefficient of thermal expansion (CTE) of epoxy-based composites. Focusing on ...

Changes in the thermodynamic properties of alkaline granite after cyclic quenching following high temperature action

Abstract: During the development of hot dry rock, the research on thermal fatigue damage caused by thermal shock of cold and heat cycles is the basis that ensures the long-term utilization of geothermal resources, but there are not enough relevant studies at present. Based on this, the thermal damage tests of granite at different temperatures (250, 350, 450 °C) and quenching cycles (1, 5, 10, 15 cycles) were carried out. Preliminary reveals the damage mechanism and heat transfer law of the quenching cycle effect on hot dry rock. The results show that with the increase of temperature and cycles, the uneven thermal expansion of minerals and the thermal shock caused by quenching promote the crack development of granite, resulting in the decrease of P-wave velocity, thermal conductivity and uniaxial compressive strength of granite. Meanwhile, the COMSOL was used to simulate the heat transfer of hot dry rock under different heat treatment conditions. It concluded that the increase in the number of quenching cycles reduced the heat transfer capacity of the granite, especially more than 10 quenching cycles, which also reflects that the thermal fatigue damage leads to a longer time for the temperature recovery of the hot dry rock mass. In addition, the three-dimensional nonlinear fitting relationship among thermal conductivity, temperature and cycle number was established for the first time, which can better reveal the change rule of thermal conductivity after quenching thermal fatigue effect of hot dry rock. The research results provide theoretical support for hot dry rock reservoir reconstruction and production efficiency evaluation.

Process optimization, microstructure characterization and thermal properties of mesophase pitch-based carbon fiber reinforced aluminum matrix composites fabricated by vacuum hot pressing

Abstract: Continuous mesophase pitch-based carbon fiber (MPCF) reinforced aluminum (Al) matrix composites were fabricated by vacuum hot pressing to meet the requirements of good thermal conductivity (TC), low coefficient of thermal expansion (CTE) and resistance to deformation for thermal management materials. The effect of process parameters on the microstructures and TC of 40 vol % MPCF/Al composites and the influence of MPCF volume fraction on the thermal-mechanical properties of MPCF/Al composites were studied. The results reveal that the composites with 20–50 vol % MPCF fabricated at 650 °C/45 MPa/60 min have longitudinal TC of 230.3–288.3 W/(m⋅K), over 90% of the predictions by rule of mixture, longitudinal CTE of 2.28−-0.22 ppm/K and elastic modulus of 147.5–324 GPa. The interface is mainly composed of an amorphous interface layer of 2–5 nm and a very small amount of carbide crystals. Furthermore, these composites have moderate TC, lower CTE and higher specific modulus compared with other Al matrix composites and Al alloys, which indicates they are expected to become unidirectional thermal management materials with integrated structure and function.

Combining Alumina Particles with Three-Dimensional Alumina Foam for High Thermally Conductive Epoxy Composites

Abstract: Ceramic/polymer composite with a high thermal conductivity is a candidate of insulating materials for electronic packaging. However, traditional polymer composites filled with alumina (Al2O3) powde...

Spark plasma sintering of B4C and B4C-TiB2 composites: Deformation and failure mechanisms under quasistatic and dynamic loading

Abstract: Spark plasma sintering (SPS) was employed to consolidate powder specimens consisting of B4C and various B4C-TiB2 compositions. SPS allowed for consolidation of pure B4C, B4C-13 vol.%TiB2, and B4C-23 vol.%TiB2 composites achieving ≥99 % theoretical density without sintering additives, residual phases (e.g., graphite), and excessive grain growth due to long sintering times. Electron and x-ray microscopies determined homogeneous microstructures along with excellent distribution of TiB2 phase in both small and larger-scaled composites. An optimized B4C-23 vol.%TiB2 composite with a targeted low density of ∼3.0 g/cm3 exhibited 30–35 % increased hardness, fracture toughness, and flexural bend strength compared to several commercial armor-grade ceramics, with the flexural strength being strain rate insensitive under quasistatic and dynamic loading. Mechanistic studies determined that the improvements are a result of a) no residual graphitic carbon in the composites, b) interfacial microcrack toughening due to thermal expansion coefficient differences placing the B4C matrix in compression and TiB2 phase in tension, and c) TiB2 phase aids in crack deflection thereby increasing the amount of intergranular fracture. Collectively, the addition of TiB2 serves as a toughening and strengthening phase, and scaling of SPS samples show promise for the manufacture of ceramic composites for body armor.

Multicomponent rare-earth cerate and zirconocerate ceramics for thermal barrier coating materials

Abstract: Thermal barrier coatings can improve the energy efficiency of industrial or aircraft gas turbines by increasing the operation temperature. In this study, new TBC materials, namely multicomponent rare-earth cerate (Sm0.2Eu0.2Tb0.2Dy0.2 Lu0.2)2Ce2O7 (5RC) and zirconocerate (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2ZrCeO7 (5RZC) ceramics were synthesized by solid-state reaction sintering. 5RC and 5RZC had a homogeneous rare-earth element distribution and a pure fluorite structure up to at least 1400 °C, hence, showing good phase stability. The coefficient of thermal expansion of 5RC appeared to have a linear increase, reaching 12.60 × 10−6 K–1 at 1200 °C without a sharp increase at low temperatures, as observed for several single-component rare earth cerates. The thermal conductivity was also reduced in multicomponent 5RC and 5RZC. Moreover, 5RZC exhibited a better sintering resistance than 5RC. The doped Zr reduces the volatilization of ceria and enhances the stability of 5RC. As conclusion, the multicomponent rare-earth cerates 5RC and 5RZC are potential thermal barrier coating materials.

Near-zero thermal expansion of ZrW2O8/Al–Si composites with three dimensional interpenetrating network structure

Abstract: Negative thermal expansion (NTE) materials can compensate high thermal expansion of metal materials, showing great potential use in precision instruments and microelectronics field. We developed a near-zero thermal expansion ZrW2O8/Al–Si composites with three dimensional mutually interpenetrated structure. Pressure infiltration ensured dense structure and mild interface reaction. The resulting composite has ultra-low thermal expansion (1.09 × 10−6 K−1) in a broad temperature range of −50 to 120 °C (ΔT = 170 °C). The content of high pressure phase (γ-ZrW2O8) drops dramatically from 71.4% for the particle reinforced composite to 18.7% for the interpenetrating composite. Raman tests and numerical computational results indicated that the residual stress can be effectively released in the interpenetrating composites. This low dense and ultralow thermal expansion results in ideal materials for precision instruments and microelectronics component such as optical communication and gyroscope.

Reversible Thermosalience in a One-Dimensional Coordination Polymer Preceded by Anisotropic Thermal Expansion and the Shape Memory Effect.

Abstract: Thermally responsive crystals hold great potential for their use as actuating materials by acting as energy transducers to convert heat energy to mechanical work. Control over defined phase transition temperature with rapid reconfiguration is of great advantage for actuation. The thermosalient (TS) effect is a rarely observed phenomenon in coordination polymers (CPs), let alone the reversibility of thermosalience in CPs. Herein, we report the reversible TS effect in a one-dimensional CP due to the martensitic phase transition during both heating and cooling cycles. The TS effect was preceded by anisotropic thermal expansion showing high expansion coefficients. In addition, the nonmolecular crystals show reversible contraction and recovery during multiple heating-cooling cycles due to the self-restorative shape memory effect. The reversible actuation of the CP could be repeated for 20 heating-cooling cycles in differential scanning calorimetry experiments, suggesting its great potential as a multicyclic actuator. Such thermal responsive behavior is unique in metal-organic materials.

Effect of Carbon Nanotube (CNT) Content on the Hardness, Wear Resistance and Thermal Expansion of In-Situ Reduced Graphene Oxide (rGO)-Reinforced Aluminum Matrix Composites

Abstract: Aluminum matrix composites reinforced with reduced graphene oxide (rGO) and hybrid of carbon nanotube (CNT) and rGO are fabricated by solution coating powder metallurgy process. The hardness, wear resistance and coefficient of thermal expansion (CTE) of the reinforced aluminum composites and the associated microstructural changes with rGO range (0.2–0.6 wt%) and hybrids of 0.2 wt% CNT–rGO at different ratios have been investigated. The intensive microstructural observations show that rGO is adsorbed on Al particles and uniformly distributed in the Al matrix composites. The hardness values of the composites increase significantly with rGO reinforcement exhibiting the maximum hardness at 0.4 wt% rGO. Compared with the hybrid composites CNT–rGO/Al counterparts fabricated by the same route and wt. percent of 0.2, the hardness values in the hybrid CNT–rGO increase considerably. Similar to the hardness, the results of wear tests also exhibit corresponding variation in the values of the wear rates. The improvement in the wear resistance of the hybrid CNT–rGO /Al composite is pronounced in this work. Whereas the rGO reinforcements decrease significantly the wear rate of the aluminum-base by 98%, the wear resistance of the corresponding hybrid CNT–rGO is significantly higher than that of the preceding composites. Maximum CTE reduction of 28% was recorded for hybrid CNT–rGO (1:1) reinforced composite.

A review of graphene reinforced Cu matrix composites for thermal management of smart electronics

Abstract: Heat dissipation remains a key challenge to be addressed, determining the performance and durability of smart electronic devices. Graphene reinforced metal matrix composites have been extensively studied as a thermal management material due to their high thermal conductivity and low coefficient of thermal expansion. The emphasis of this review is pivoted on the thermal conductivity enhancement of graphene reinforced Cu matrix composites developed in the recent literature. An overview of factors affecting thermal conductivity of composite namely defect processing route, density, graphene derivative, lateral size, concentration, alignment, graphene/matrix interfacial bonding and graphene modification are discussed. An extensive weightage is given to the processing route as it is the most influential factor in determining the enhancement efficiency. Furthermore, graphene based functional products such as heat spreader and heat sink developed for heat dissipation of electronic devices are also reviewed. Finally, the development and outlook for graphene based Cu composites are presented.

New class of high-entropy defect fluorite oxides RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Y, Ho, Er, or Yb) as promising thermal barrier coatings

Abstract: High-entropy ceramics exhibit great application potential as thermal barrier coating (TBC) materials. Herein, a series of novel high-entropy ceramics with RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE2HE2O7, RE = Y, Ho, Er, or Yb) compositions were fabricated via a solid-state reaction. X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) mapping analyses confirmed that RE2HE2O7 formed a single defect fluorite structure with uniform elemental distribution. The thermophysical properties of the RE2HE2O7 ceramics were investigated systematically. The results show that RE2HE2O7 ceramics have excellent high-temperature phase stability, high thermal expansion coefficients (10.3–11.7 × 10−6 K-1, 1200 ℃), and low thermal conductivities (1.10-1.37 W m-1 K-1, 25 ℃). In addition, RE2HE2O7 ceramics have a high Vickers hardness (13.7–15.0 GPa) and relatively low fracture toughness (1.14-1.27 MPa m0.5). The outstanding properties of the RE2HE2O7 ceramics indicate that they could be candidates for the next generation of TBC materials.

In-situ monitoring and detection of spatter agglomeration and delamination during laser-based powder bed fusion of Invar 36

Abstract: Invar 36 (FeNi36) is a weldable nickel alloy, known for its low thermal expansion properties, that contains 36% of nickel and 64% iron. The laser-based powder bed fusion process is often associated with defects and flaws that should be controlled for production quality. Spatter formation and delamination are two phenomena that could be observed in thermal images taken during the laser melting process. This paper presents an in-situ monitoring setup for detecting spatters and delamination of Invar 36. The size, shape, count, and cooling rate of spatter particles are evaluated using a combination of infrared pyrometer and high-speed infrared thermography. Thermograms of Invar 36 are processed to determine the spatter characteristics. The influence of processing conditions on the spatter count, spatter size, and size of melt track is discussed. The effect of spatters on the surface morphology of Invar 36 parts is also investigated. The processing conditions that cause melt pool instabilities, excessive spattering, and delamination are presented. This paper concludes recommendations for avoiding delamination and excessive spatter formation during the laser-based powder bed fusion of Invar 36.

Enhanced Thermal Conductivity of Epoxy Composites Filled with Al 2 O 3 /Boron Nitride Hybrids for Underfill Encapsulation Materials.

Abstract: In this study, a thermal conductivity of 0.22 W·m−1·K−1 was obtained for pristine epoxy (EP), and the impact of a hybrid filler composed of two-dimensional (2D) flake-like boron nitride (BN) and zero-dimensional (0D) spherical micro-sized aluminum oxide (Al2O3) on the thermal conductivity of epoxy resin was investigated. With 80 wt.% hybrid Al2O3–BN filler contents, the thermal conductivity of the EP composite reached 1.72 W·m−1·K−1, increasing approximately 7.8-fold with respect to the pure epoxy matrix. Furthermore, different important properties for the application were analyzed, such as Fourier-transform infrared (FTIR) spectra, viscosity, morphology, coefficient of thermal expansion (CTE), glass transition temperature (Tg), decomposition temperature (Td), dielectric properties, and thermal infrared images. The obtained thermal performance is suitable for specific electronic applications such as flip-chip underfill packaging.

A new class of high-entropy fluorite oxides with tunable expansion coefficients, low thermal conductivity and exceptional sintering resistance

Abstract: High-temperature thermal barrier coating (TBC) materials are desired for the development of high-efficient gas turbines and diesel engines. Herein, to meet up with this requirement, a new class of high-entropy fluorite-type oxides (HEFOs) has been synthesized via a solid-state reaction method. Comparing to La2Ce2O7, a promising TBC material, the HEFOs exhibit similar high thermal expansion coefficients (TECs) of 11.92×10−6∼12.11×10−6 K-1 at temperatures above 673 K but a better TEC matching performance at the temperature range of 473–673 K. It is also found that through tuning the average A-site cation radius, the TEC of the HEFOs could be tailored efficiently. The HEFOs also possess low thermal conductivities of 1.52-1.55 W∙m-1∙K-1 at room temperature, which is much lower than that of La2Ce2O7 and comparable to pyrochlores as Gd2Zr2O7. Moreover, the HEFOs display good sintering resistance and phase stability even at temperatures as high as 1873 K. The combination of these fascinating properties makes the HEFOs good candidates for thermal barrier coating and thermal insulating materials.