Thermal expansion performance and intrinsic lattice thermal conductivity of ferroelastic RETaO4 ceramics

Ferroelastic RETaO₄ ceramics are promising thermal barrier coatings (TBCs) because of their attractive thermomechanical properties. The influence of crystal structure distortion degree on thermomechanical properties of RETaO₄ is estimated in this work. The relationship between Young's modulus and TECs is determined. The highest TECs (10.7 × 10⁻⁶ K⁻¹, 1200°C) of RETaO₄ are detected in ErTaO₄ ceramics and are ascribed to its small Young's modulus and low Debye temperature. The intrinsic lattice thermal conductivity (3.94-1.26 W m⁻¹ K⁻¹, 100-900°C) of RETaO₄ deceases with increasing of temperature due to an elimination in thermal radiation effects. The theoretical minimum thermal conductivity (1.00 W m⁻¹ K⁻¹) of RETaO₄ indicates that the experimental value is able to be reduced further. We have delved deeply into the thermomechanical properties of ferroelastic RETaO₄ ceramics and have emphasized their high-temperature applications as TBCs.     

Elastic constants and thermal expansivity of gel-spun polyethylene fiber and its composites

The five independent stiffness constants, C₁₁, C₃₃, C₄₄, C₆₆, and C₁₃, and the axial and transverse thermal expansivity of unidirectional gel-spun polyethylene fiber reinforced composites have been measured as functions of fiber volume fraction V_f. The axial extensional modulus C₃₃ and axial Poisson's ratio v₁₃ follow the rule of mixtures, while the axial shear modulus C₄₄, transverse shear modulus C₆₆, and transverse plane-strain bulk modulus C_t ( = C₁₁ − C₆₆) obey the Halpin-Tsai equation. Extrapolation to V_f = 1 gives the five stiffness constants of gel-spun polyethylene fiber. The tensile property of the fiber is highly anisotropic, with the axial Young's modulus about 40 times higher than the transverse Young's modulus. In contrast, the axial shear modulus exceeds the transverse shear modulus by only 5%. A similar treatment of the thermal expansivity data in terms of the Schapery equations gives an axial thermal expansivity of −1.25 × 10⁻⁵ K⁻¹ and a transverse thermal expansivity of 11.7 × 10⁻⁵ K⁻¹ for the fiber.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

High interfacial thermal resistance induced low thermal conductivity in porous SiC-SiO2 composites with hierarchical porosity

The incorporation of a thermally insulating secondary phase can significantly increase the interfacial thermal resistance attributed to its low intrinsic thermal conductivity and the creation of multiple phonon scattering interfaces between adjacent SiC particles. The newly developed porous SiC-33 wt% SiO₂ composites with SiO₂ as a thermally insulating secondary phase exhibited a very low thermal conductivity (0.047 Wm⁻¹ K⁻¹, 72.4 % porous), which is an order of magnitude lower than the previously reported lowest thermal conductivity (0.14 Wm⁻¹ K⁻¹, 76.3 % porous) for powder processed porous SiC ceramics and is even lower than the thermal conductivity (0.060 Wm⁻¹ K⁻¹, 87.9% porous) of SiO₂ aerogel. The porous SiC-(16–73 wt%) SiO₂ composites processed from nano β-SiC and a 40 wt% carbon template exhibited a hierarchical (meso-/macro-porous) pore structure that transformed to a trimodal (micro-/meso-/macro-porous) porous structure when polysiloxane was added and sintering was performed at 600–1000 °C in air.     

Fiber Reinforced Polyimide Aerogel Composites with High Mechanical Strength for High Temperature Insulation

Glass fiber/polyimide aerogel composites are prepared by adding glass fiber mat to a polyimide sol derived from diamine, 4,4′‐oxydianiline, p‐phenylene diamine, and dianhydride, 3,3′,4,4′‐biphenyltetracarboxylic dianhydride. The fiber felt acts as a skeleton for support and shaping, reduces aerogel shrinkage during the preparation process, and improves the mechanical strength and thermal stability of the composite materials. These composites possess a mesoporous structure with densities as low as 0.143–0.177 g cm^?3, with the glass fiber functioning to improve the overall mechanical properties of the polyimide aerogel, which results in its Young's modulus increasing from 42.7 to 113.5 MPa. These composites are found to retain their structure after heating at 500 °C, in contrast to pure aerogels which decompose into shrunken ball‐like structures. These composites maintain their thermal stability in air and N₂ atmospheres, exhibiting a low thermal conductivity range of 0.023 to 0.029 W m^?1 K^?1 at room temperature and 0.057to 0.082 W m^?1 K^?1 at 500 °C. The high mechanical strengths, excellent thermal stabilities, and low thermal conductivities of these aerogel composites should ensure that they are potentially useful materials for insulation applications at high temperature.     

First-principles study of thermophysical properties of polymorphous YTaO4 ceramics

Yttrium tantalate ceramics with ferroelasticity are potential candidates for thermal barrier coating (TBC) ceramics. During the phase transition process, there are three main phases with monoclinic (I2/a), monoclinic-prime (P2/a), and tetragonal structures (I₄₁/a), and a comprehensive understanding of their thermophysical properties is required. In this study, the thermal and mechanical properties of polymorphous yttrium tantalate (YTaO₄) ceramics are systematically investigated under finite temperature by performing first-principles calculations combined with quasi-harmonic approximation. The first-principle study results show that the volume change from M' to T phase is 12.85 Å³ to 12.95 Å³ per atom, whereas the T to M is 12.95 Å³ to 12.84 Å³ per atom, and the change is less than 1%, showing that this process produces almost no volume change. However, the thermal expansion coefficients (TECs) and Young's modulus vary greatly, the TECs value of M YTaO₄ is about 11.13 × 10⁻⁶ K⁻¹, which is smaller than T YTaO₄ as the value 12.01 × 10⁻⁶ K⁻¹, and the Young's modulus values of M, M', and T phases are 140.34, 156.68, and 123.29 GPa, respectively. Lastly, the calculated O–Ta bond is stronger than the O–O and O–Y bonds according to the mean bond population and average bond length, resulting in a higher modulus. This work will not only expand the internal mechanism of the thermophysical properties of YTaO₄, but also provides support for the design and application of TBC systems.     

Impact of ZrO2 alloying on thermo-mechanical properties of Gd3NbO7

The ZrO₂ alloying effect is widely used to optimize the thermo-mechanical properties of potential thermal barrier coatings. In this study, dense x mol% ZrO₂-Gd₃NbO₇ with C222₁ space group were manufactured via a solid-state reaction. The crystalline structure was determined through X-ray diffraction and Raman spectroscopy, when the surface morphology was observed by scanning electron microscopy. ZrO₂-Gd₃NbO₇ had identical orthorhombic crystal structures, and there was no second phase. The crystalline structure of ZrO₂-Gd₃NbO₇ shrunk with the increasing ZrO₂ content as indicated by XRD and Raman results. The heat capacity and thermal diffusivity of ZrO₂-Gd₃NbO₇ were 0.31–0.43 J g⁻¹ K⁻¹ (25–900 °C) and 0.25–0.70 mm²/s (25–900 °C), respectively. It was found that ZrO₂-Gd₃NbO₇ had much lower thermal conductivity (1.21–1.82 W m⁻¹ K⁻¹, 25–900 °C) than YSZ (2.50–3.00 W m⁻¹ K⁻¹) and La₂Zr₂O₇ (1.50–2.00 W m⁻¹ K⁻¹). The thermal expansion coefficients (TECs) were higher than 10.60 × 10⁻⁶ K⁻¹ (1200 °C), which were better than that of YSZ (10.00 × 10⁻⁶ K⁻¹) and La₂Zr₂O₇ (9.00 × 10⁻⁶ K⁻¹). The mechanical properties of Gd₃NbO₇ change little with the increasing ZrO₂ content, Vickers hardness was about 10 GPa, and Young's modulus was about 190 GPa, which was lower than YSZ (240 GPa). Compared with previous work about alloying effects, much lower thermal conductivity was obtained. Due to the high melting point, high hardness, low Young's modulus, ultralow thermal conductivity and high TECs, it is believed that ZrO₂-Gd₃NbO₇ is promising TBCs candidate.     

Process induced alignment of carbon nanotube decreases longitudinal thermal conductivity of Al2O3 based porous composites

Alumina (Al₂O₃) based porous composites, reinforced with zirconia (ZrO₂), 3 and 8 mol% Y₂O₃ stabilized ZrO₂ (YSZ) and 4 wt% carbon nanotube (CNT) are processed via spark plasma sintering. The normalized linear shrinkage during sintering process of Al₂O₃-based composite shows minimum value (19.2–20.4%) for CNT reinforced composites at the temperature between 1650 °C and 575 °C. Further, the combined effect of porosity, phase-content and its crystallite size in sintered Al₂O₃-based porous composite have elicited lowest thermal conductivity of 1.2 Wm⁻¹K⁻¹ (Al₂O₃-8YSZ composite) at 900 °C. Despite high thermal conductivity of CNT (∼3000 Wm⁻¹K⁻¹), only a marginal thermal conductivity increase (∼1.4 times) to 7.3–13.4 Wm⁻¹K⁻¹ was observed for CNT reinforced composite along the longitudinal direction at 25 °C. The conventional models overestimated the thermal conductivity of CNT reinforced composites by up to ∼6.7 times, which include the crystallite size, porosity, and interfacial thermal resistance of Al₂O₃, YSZ and, CNT. But, incorporation of a new process induced CNT-alignment factor, the estimated thermal conductivity (of <6.6 Wm⁻¹K⁻¹) closely matched with the experimental values. Moreover, the high thermal conductivity (<76.1 Wm⁻¹K⁻¹) of the CNT reinforced porous composites along transverse direction confirms the process induced alignment of CNT in the spark plasma sintered composites.     

Crystal structure,mechanical and thermal properties of Yb4Al2O9: A combination of experimental and theoretical investigations

The key requirements for a successful thermal and environmental barrier coating (T/EBC) material include stability in high temperature water vapor, low Young's modulus, close thermal expansion coefficient (TEC) with mullite, low thermal conductivity and weak mechanical anisotropy. The current prime candidates for top coat are ytterbium silicates (Yb₂SiO₅ and Yb₂Si₂O₇). A major weakness of these two silicates is the severe anisotropy in mechanical properties and thermal expansion that would lead to cracking of the coating. Thus, searching for new materials with weak mechanical and thermal anisotropy is of signification. In this work, the crystal structure, mechanical and thermal properties of a promising T/EBC candidate, Yb₄Al₂O₉, are investigated theoretically and experimentally. Good ductility, low shear deformation resistance, low Young's modulus (151 GPa) and low thermal conductivity (0.78 W m⁻¹ K⁻¹) is underpinned by heterogeneous bonding characteristic and distortion of the structure. Close TEC (6.27 × 10⁻⁶ K⁻¹) with mullite and weak mechanical anisotropy highlight the suitability of Yb₄Al₂O₉ as a prospective T/EBC.     

Conductive and radiative heat transfer inhibition in YSZ photonic glass

A high temperature stable ceramic photonic structure is demonstrated with low thermal conductivity and suppressed external radiative heat transfer. The structure is based on a disordered arrangement of yttria-stabilized zirconia (YSZ) microparticles, called photonic glass (PhG). The prepared YSZ-PhG film exhibits low thermal conductivity of 0.03 Wm⁻¹K⁻¹ comparable to that of the air. The small point contacts of the adjacent YSZ particles are the main cause of such low thermal conductivity. After annealing at 1400 °C for 5 h, the solid thermal conductivity increased to 0.3 Wm⁻¹K⁻¹ at room temperature due to the thermally induced neck formation, associated with an increased contact area between adjacent particles. This thermal conductivity is still much lower than that of conventional YSZ thermal barrier coatings (TBCs) with approximately 1 Wm⁻¹K⁻¹. At the same time, the PhG structure is an efficient scatterer for thermal radiation in the wavelength range between 1 and 6 μm. In an only 100 μm thick structure an average reflection of 84% was obtained. At 1400 °C, the effective thermal conductivity is 0.2 Wm⁻¹K⁻¹. The presented structure is applicable to other oxides with even lower bulk thermal conductivity and can be considered for future TBCs.     

Effect of lattice defects on the thermal conductivity of β-Si3N4

Two types of β-Si₃N₄ were sintered at 1900 °C one for 8 h and the other for 36 h by using Yb₂O₃ and ZrO₂ as sintering additives. The latter specimen was further annealed at 1700 °C for 100 h to promote grain growth. The microstructures of the sintered materials were investigated by SEM, TEM, and EDS. The thermal conductivities of the specimens were 110 and 150 Wm⁻¹K⁻¹, respectively. The sintered material which possessed 110 Wm⁻¹K⁻¹ had numerous small precipitates that consisted of Yb, O and N elements and internal dislocations in the β-Si₃N₄ grains. In the sintered material with 150 Wm⁻¹K⁻¹ neither precipitates nor dislocations were observed in the grains. The microscopic evidence indicates that the improvement in the thermal conductivity of the β-Si₃N₄ was attributable to the reduction of internal defects of the β-Si₃N₄ grains with sintering and annealing time as the grains grew.     

The thermo-mechanical properties and ferroelastic phase transition of RENbO4 (RE = Y,La, Nd,Sm, Gd,Dy, Yb) ceramics

In this work, RENbO₄ (RE = Y, La, Nd, Sm, Gd, Dy, Yb) ceramics with low density, low Young's modulus, low thermal conductivity, and high thermal expansion have been systematically investigated, the excellent thermo-mechanical properties indicate that RENbO₄ ceramics possess the potential as the new generation of thermal barrier coatings (TBCs) materials. X-ray diffraction and Raman spectroscopy phase structure identification reveal that all dense bulk specimens obtained by high-temperature solid-state reaction belonged to the monoclinic (m) phase with C12/c1 space group. The ferroelastic domains are detected in the specimens, revealing the ferroelastic transformation between tetragonal (t) and monoclinic (m) phases of RENbO₄ ceramics. The Young's modulus and hardness of the RENbO₄ ceramics measured by the NanoBlitz 3D nanoindentation method are discussed in details, and the lower Young's modulus (60-170 GPa) and higher hardness (the maximum value reaches 11.48 GPa) indicating that higher resistance of RENbO₄ ceramics to failure and damage. Lower thermal conductivity (1.42-2.21 W [m k]⁻¹ at 500°C-900°C) and lower density (5.330-7.400 g/cm³) than other typical TBCs materials give RENbO₄ ceramics the unique advantage of being new TBCs materials. Meanwhile, the thermal expansion coefficients of RENbO₄ ceramics reach 9.8-11.6 × 10⁻⁶ k⁻¹ and are comparable or higher than other typical TBCs materials. According to the first-order derivative of the thermal expansion rate, the temperature of the ferroelastic transformation of RENbO₄ ceramics can be observed.     

Improving tribological properties of phenolic resin/carbon fiber composites using m-Si3N4@PANI core–shell particles

Phenolic resin/carbon fiber (PF/CF) composites have good tribological properties; however, their extensive applications are limited because of the poor thermal conductivity of the phenolic resins. In this work, core‑shell particles of polyaniline-coated (3-aminopropyl) triethoxysilane-modified β-Si₃N₄ (m-SiN@PANI) were used to enhance the tribological, electrical, and thermal conductivity properties of a PF/CF composite. A core‑shell particle, consisting of m-SiN@PANI, was characterized by Fourier Transform Infrared Spectrometry, X-Ray Diffraction, Scanning Electron Microscope, and Transmission Electron Microscope. The friction, thermal, and electrical properties of the composites were characterized by multifunctional vertical friction testing, wear measurement testing, thermogravimetric analysis, thermal constant analysis, and electrical conductivity testing. Remarkably, the test results showed that compared with the wear surface of the PF/CF composite, that of the phenolic resin/(2.0 wt % m-SiN@PANI)/carbon fiber composite exhibited a smoother morphology. The results indicated that the addition of m-SiN@PANI effectively improved the thermal conductivity, electrical conductivity, friction coefficient, and wear rate of the composites, which were 3.164 Wm⁻¹ K⁻¹, 5.33 × 10⁻⁶ S/m, 0.1681 and 1.13 × 10⁻⁸ mm³/Nm, respectively. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47785.     

Direct Printing of Thermal Management Device Using Low‐Cost Composite Ink

This work presents a new method for fabricating thermal devices, such as heat sinks, using a 3D printing technique and lightweight composite ink. The method focuses on formulating composite inks with desired properties and direct ink writing for manufacturing. The ink undergoes two phases: phase one uses low viscosity epoxy to provide viscoelastic properties and phase two provides the fillers consisting of carbon fiber and graphite nanoplatelets to provide high thermal conductivity and structural properties. By combining these functional materials, 3D structures with a high thermal conductivity (≈2 W m⁻¹ K⁻¹) are printed for thermal management applications with the storage modulus of 3000 MPa and a density only 1.24 g cm⁻³. The results show that by carefully tailoring functional properties of the ink, net‐shape multifunctional structures can be directly printed for thermal management device applications, such as heat sinks.     

The effect of pre-carbonization on the properties of PAN-based carbon fibers

A continuous stabilization and two-stage carbonization process was used to prepare polyacrylonitrile (PAN)-based carbon fibers, The effect of pre-carbonization (300 to 550°C) on the final properties and microstructure of carbon fibers was measured. Experimental results using an X-ray diffractometer indicated the presence of a less ordered structure at 2Θ from 5 to 18° in the pre-carbonized fibers and the final carbon fibers. This study found that the pre-carbonization process strongly affects the microstructure of the resulting carbon fibers. The results also showed that a suitable pre-carbonization was very conducive to improvement in tensile strength or in Young's modulus of the final carbon fibers. When the final carbon fiber was pre-carbonized at 300 and 550°C, respectively, these fibers had a higher tensile strength and higher Young's modulus than carbon fibers pre-carbonized at other conditions.     

Influence of processing parameters on the microstructure of the eco-friendly glass foam

Eco-friendly glass foam (GF) with a specific mechanical strength of 5.96 MPa g⁻¹ cm³ and thermal conductivities between 0.131 and 0.282 Wm⁻¹ K⁻¹ was produced using fluorescent lamp glass residue (FLGR) and white eggshell as foaming agent (FA). The influence of the FLGR average diameter particle, the amount of FA, and the heat treatment on the GF final properties were evaluated. The highest expansion (500%) and the lowest density (0.24 g cm⁻³) were achieved using 32.90 μm mean diameter FLGR particles, 5 wt% FA, and foaming at 700°C. The produced GF showed promising application as porous building materials with load-bearing function.     

Multilayered ultrahigh molecular weight polyethylene/natural graphite/boron nitride composites with enhanced thermal conductivity and electrical insulation by hot compression

A scalable strategy to fabricate thermally conductive but electrically insulating polymer composites was urgently required in various applications including heat exchangers and electronic packages. In this work, multilayered ultrahigh molecular weight polyethylene (UHMWPE)/natural graphite (NG)/boron nitride (BN) composites were prepared by hot compressing the UHMWPE/NG layers and UHMWPE/BN layers alternately. Taking advantage of the internal properties of NG and BN fillers, the UHMWPE/NG layers played a decisive role in enhancing thermal conductivity (TC), while the UHMWPE/BN layers effectively blocked the electrically conductive pathways without affecting the thermal conductive pathways. The in-plane TC, electrical insulation, and heat spreading ability of multilayered UHMWPE/NG/BN composites increased with the increasing layer numbers. At the total fillers loading of 40 wt%, the in-plane TC of multilayered UHMWPE/NG/BN composites with nine layers was markedly improved to 6.319 Wm⁻¹ K⁻¹, outperforming UHMWPE/BN (4.735 Wm⁻¹ K⁻¹) and pure UHMWPE (0.305 Wm⁻¹ K⁻¹) by 33.45% and 1971.80%, respectively. Meanwhile, the UHMWPE/NG/BN composites still maintained an excellent electrically insulating property (volume resistance~5.40×10¹⁴ Ω cm ; breakdown voltage~1.52 kV/mm). Moreover, the multilayered UHMWPE/NG/BN composites also exhibited surpassing heat dissipation capability and mechanical properties. Our results provided an effective method to fabricate highly thermal conductive and electrical insulating composites.     

Mechanical and Thermal Properties of Yb2SiO5: A Promising Material for T/EBCs Applications

Yb₂SiO₅ is a promising material for thermal/environmental barrier coatings (T/EBCs), and its mechanical and thermal properties, which are essential to the coating design and applications, are investigated in this work. Yb₂SiO₅ has relatively high fracture toughness, bending and compressive strength, but low Young's modulus. It is also tolerant to damage, which is underpinned by grain delamination and cleavage along {100}, {001}, and {040} planes. The average linear coefficient of thermal expansion (CTE) is 6.3 × 10^?6 K^?1 (473–1673 K) and the anisotropic CTEs are: α_a = (2.98 ± 0.16) × 10^?6 K^?1,α_b = (6.51 ± 0.19) × 10^?6 K^?1, and α_c = (9.08 ± 0.16) × 10^?6 K^?1. The thermal conductivities are 2.3 and 1.5 W (m·K)^?1 at 300 and 1200 K, respectively. The unique combination of these properties warrants Yb₂SiO₅ promising for T/EBCs applications.     

Moisture-Resistant and Mechanically Strong Polyimide-Polymethylsilsesquioxane Hybrid Aerogels with Tunable Microstructure

Moisture-resistant and mechanically strong polyimide (PI)-polymethylsilsesquioxane hybrid aerogels with doubly cross-linked structures are synthesized through sol–gel technology and supercritical CO₂ fluid drying. By using bis(trimethoxysilylpropyl) amine as a cross-linker, the end-capped polyamide acid derived from biphenyl-3,3′,4,4′-tetracarboxylic dianhydride and 4,4′-oxydianiline is cross-linked with a silica network using methyltrimethoxysilane as the silica source precursor. The resultant hybrid aerogels show low density (0.12–0.15 g cm⁻³), low thermal conductivity (0.032–0.049 W m⁻¹ K⁻¹), high hydrophobicity (125–140°) and good thermal stability (above 435 °C) with tunable microstructure. With the increase of silica sol volume, the microstructure of hybrid aerogels transforms from fibrous network to hierarchical microstructure. Aerogels with high content of silica sol exhibit good moisture resistance, high Young's modulus (Max. 19.6 MPa), and high specific modulus (Max. 131 kN m kg⁻¹), which are attributed to their unique hierarchical microstructure with a sheet skeleton. These hybrid aerogels are promising in the fields of thermal insulation, aerospace applications and so on.     

Theoretical investigation of anisotropic mechanical and thermal properties of ABO3 (A=Sr,Ba; B=Ti,Zr, Hf) perovskites

As promising TBC (thermal barrier coating) candidates, perovskite oxides own designable properties for their various options of cations and structural diversity, but limited comprehensions of structure‐property relationship delay their engineering applications. In this work, mechanical/thermal properties of ABO₃ (A=Sr, Ba; B=Ti, Zr, Hf) perovskites and their anisotropic nature are predicted employing density functional theory. Their theoretical minimum thermal conductivities range from 1.09 to 1.74 W·m^?1·K^?1, being lower than Y₂O₃ partially stabilized ZrO₂. Reduced thermal conductivities up to 16% along particular directions are reached after considering thermal conductivity anisotropy. All compounds own high hardness while SrZrO₃, SrHfO₃, and BaHfO₃ possess well damage tolerance. We found that small electronegativity discrepancy leads to big anisotropy of chemical bond, Young's/shear moduli and thermal conductivities, together with good damage tolerance. These results suggest that the next generation TBCs with extra low thermal conductivity should be achieved through combining material design and orientation‐growth tailoring.