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1.
Al2Mo3O12 is a typical negative thermal expansion (NTE) material, whose thermal expansion behavior depends on its crystal phase. The thermal shock caused by temperature-induced phase transition limits its wide application. The two series of Al2. xScxMo3O12 (0 ≤ x ≤ 1) and Al2Mo3-xWxO12 (0 ≤ x ≤ 2.5) solid solutions with controllable phase transition temperature were synthesized via single cation substitution at the A or B position. The problem of thermal shock caused by the change of temperature is effectively solved in the synthesized Al1.6Sc0.4Mo3O12 and Al2Mo0.5W2.5O12, showing stable NTE performance above room temperature, and the coefficients of thermal expansion of which are ?2.19 × 10?6 °C?1 in 100–550 °C and ?4.25 × 10?6 °C?1 in 85–500 °C, respectively. A-site cation substitution is a more effective way to tune the thermal expansion properties of Al2Mo3O12, which is attributed to the fact that the bond strength of A-O is weaker than that of B–O in the compound.  相似文献   

2.
Thermal barrier coatings (TBCs) are one of the most important materials in gas turbine to protect the high temperature components. RETa3O9 compounds have a defect‐perovskite structure, indicating that they have low thermal conductivity, which is the critical property of TBCs. Herein, dense RETa3O9 bulk ceramics were fabricated via solid‐state reaction. The crystal structure was characterized by X‐ray diffraction (XRD) and Raman Spectroscope. Scanning electron microscope (SEM) was used to observe the microstructure. The thermophysical properties of RETa3O9 were studied systematically, including specific heat, thermal diffusivity, thermal conductivity, thermal expansion coefficients, and high‐temperature phase stability. The thermal conductivities of RETa3O9 are very low (1.33‐2.37 W/m·K, 373‐1073 K), which are much lower than YSZ and La2Zr2O7; and the thermal expansion coefficients range from 4.0 × 10?6 K?1 to 10.2×10?6 K?1 (1273 K), which is close to La2Zr2O7 and YSZ. According to the differential scanning calorimetry (DSC) curve there is not phase transition at the test temperature. Due to the high melting point and excellent high‐temperature phase stability with these oxides, RETa3O9 ceramics were promising candidate materials for TBCs.  相似文献   

3.
《Ceramics International》2022,48(9):12065-12073
In this work, by focusing on widespread problem of thermal mismatch caused by different coefficients of thermal expansion (CTE) in electronic packaging materials, low-temperature co-fired ceramic (LTCC) materials with tunable CTE values were designed. By substituting Ba2+ with Sr2+ and replacing quartz with alumina and zirconia, respectively, BaO–Al2O3–SiO2–B2O3/quartz LTCC composites with CTE of 7.05–9.52 × 10?6/°C were developed. Results show that major crystalline phases of LTCC composite materials are quartz and hexacelsian. By replacing quartz with alumina or zirconia, sintering behavior and subsequently thermal expansion and dielectric properties were modulated. On the other hand, substituting Ba2+ with Sr2+ can be beneficial to the densification of composite materials. The introduction of Sr2+ triggered mixed alkali effect and hindered the crystallization of hexacelsian phase, which can further improve mechanical properties. Finally, sandwich structure module of BaO–Al2O3–SiO2–B2O3/quartz with gradient CTE values was obtained, which showed potential for electronic packaging with sustained thermal compatibility under cyclic temperatures.  相似文献   

4.
《Ceramics International》2023,49(4):6401-6408
Dense TaTiP3O12 ceramics were synthesized by the solid-state method and spark plasma sintering (SPS) with 6 wt% V2O5 as a sintering aid, and their phase, microstructure, thermal conductivity, hardness, compressive strength, and expansion property and mechanism were investigated. Results show that the pure phase can be achieved by the two methods. In particular, the sample prepared by SPS possesses a relative density of 97.62% and a porosity of 3.07%, and has better properties than that prepared by the solid-state method. The SPS sample has a thermal conductivity at room temperature of 2.03 w/(m· °C), a Vickers hardness of 4.34 GPa and a compressive strength of 175.98 MPa, which are 0.95, 1.49 and 1.59 times greater than those of the sample prepared by the solid-state method, respectively. In addition, the TaTiP3O12 ceramic prepared by SPS exhibits a linear ultralow negative thermal expansion property with a coefficient of thermal expansion of ?0.74 × 10?6 °C ?1 (-100–400 °C). The negative thermal expansion in TaTiP3O12 is induced by the coupling effect of [Ta(Ti)O6] octahedron and [PO4] tetrahedron caused by the transverse vibration of bridging oxygen atoms.  相似文献   

5.
Coefficient of thermal expansion (CTE) of a solid material plays a critical role for a variety of high temperature applications such as thermal barrier coating (TBC) systems during the thermal cycling process. Ceramics contain ionic bonds; hence they tend to exhibit lower CTE values than alloys/metals. Developing new ceramic thermal barrier materials using promising dopants and compositions that have higher CTE values than the conventional 6-8 wt% Y2O3 stabilized ZrO2(8YSZ) will contribute to the decrease in thermal expansion mismatch between a typical ceramic 8YSZ (10 ~ 11 × 10−6°C−1) top coat and a metal alloy based bond coat such as NiCrAlY (14 ~ 17×10−6°C−1, Padture et al., Science, 2002;296:280–4; Liang et al., J Mater Sci Technol, 2011;27(5):408–14), which is highly desirable. This work reports design, modeling, synthesis, and characterization of promising new compositions based on Dy3+, Al3+, and Ce4+-doped YSZ that consist of the tetragonal structure and have an enhanced thermal expansion than 8YSZ. The intrinsic CTE at the atomic level has been investigated via molecular dynamics (MD) simulation. The atomic scale analysis provides new insights into the enhanced doping effects of multiple trivalent and tetravalent cations on the lattice structure, lattice energy, and thermal expansion in ZrO2. The calculated lattice energy becomes smaller with the incorporation of Dy3+, Al3+, and Ce4+ions, which corresponds strongly to the increase in CTE. The crystalline size is reduced due to the incorporation of the Al3+ and Ce4+, whereas the sintering resistance is enhanced ascribed to the addition of Dy3+ and Al3+. Doping Dy3+, Al3+, and Ce4+ cations to YSZ increased the CTE value of YSZ and for Dy0.03Y0.075Zr0.895O1.948, the CTE is 12.494 × 10−6°C−1 at 900°C, which has an 11% increase, as compared with that of 8YSZ.  相似文献   

6.
Yttria-stabilized zirconia (YSZ) has been considered as state-of-the-art material for high-temperature thermal barrier coatings, which provide thermal insulation to the superalloy components in gas turbines and jet engines. Oxygen vacancies induced by yttria substitutions are believed to be mainly responsible for the low thermal conductivity of YSZ due to their phonon scattering effect. However, high mobility of oxygen vacancies in YSZ leads to a rapid oxygen diffusion at high temperatures, therefore accelerates the failure of coatings by grain coarsening, sintering, and simultaneous oxidation of the underlying metallic bondcoat. In the present research, we further explored in the ZrO2–Ln2O3 binary phase diagram and synthesized a series of ceramic materials with the chemical formula of Zr3Ln4O12 (Ln = La, Gd, Y, Er, and Yb), in which more oxygen vacancies were involved and extremely low phonon thermal conductivities (1.3-1.6 W/m·K) were obtained, even approaching to the theoretical minimum. In addition, the mobility of these oxygen vacancies was remarkably suppressed by the lattice ordering with the decrease of Ln3+ radius, as confirmed by X-ray diffraction, Raman and transmission electron microscopy. Thus, the oxygen barrier property and sintering resistance were significantly enhanced accordingly, which makes Zr3Ln4O12 compounds promising thermal barrier coating materials for next generation gas turbines and jet engines.  相似文献   

7.
Polycrystalline Mo4Y2Al3B6 ceramic (92.84 wt% Mo4Y2Al3B6 and 7.16 wt% MoB) was prepared by spark plasma sintering at 1250 ℃ under 30 MPa using Mo, Y, Al, and B as starting materials. The dense sample obtained has a high relative density of 96.6 %. The average thermal expansion coefficient is 8.38 × 10?6 K?1 in the range of 25–1000 ℃. The thermal diffusivity decreases from 6.50 mm2/s at 25 °C to 4.33 mm2/s at 800 °C. The heat capacity, thermal conductivity, and electrical conductivity are 0.30 J·g?1·K?1, 11.73 W·m?1·K?1, and 0.66 × 106 Ω?1·m?1 at 25 °C, respectively. Vickers hardness with increasing load in the range of 10–300 N at room temperature decreases from 10.82 to 9.49 GPa, and the fracture toughness, compressive strength, and flexural strength are 5.14 MPa·m1/2, 1255.14 MPa, and 384.82 MPa, respectively, showing the promising applications as structural-functional ceramics.  相似文献   

8.
《Ceramics International》2023,49(6):9173-9184
The effects of Al2O3 content on the sintering behaviour, microstructure, and physical properties of Al2O3/vitrified bonds (SiO2–Al2O3–B2O3–BaO–Na2O–Li2O–ZnO–MgO) and Al2O3/vitrified bond cubic boron nitride (CBN) composites were systematically investigated using X-ray diffraction, differential scanning calorimetry, dilatometry, scanning electron microscopy, and X-ray photoelectron spectroscopy. Various amounts of Al2O3 promoted the formation of BaAl2Si2O8 and γ-LiAlSi2O6, increasing the relative crystallinity of the Al2O3/vitrified composite from 85.0 to 93.2%, resulting in residual compressive stress on BaAl2Si2O8, thereby influencing the thermal behaviour and mechanical properties of the Al2O3/vitrified composite. The bulk density, porosity, flexural strength, hardness, and thermal conductivity of 57.5 wt% Al2O3 sintered at 950 °C were 3.12 g/cm3, 6.1%, 169 MPa, 90.5 HRC, and 4.17 W/(m·K), respectively. The coefficient of thermal expansion of the bonding material was 3.83 × 10?6 °C?1, which was comparable to that of CBN, and the number of N–Al bonds were increased, which boosted the flexural strength of the Al2O3/vitrified CBN composite to 81 MPa. The excellent mechanical properties, compact structure, and suitable interfacial bonding state with the CBN grains of the Al2O3/vitrified composite make it a promising high-performance bonding material for superhard abrasive tools.  相似文献   

9.
The ceramics of dysprosium tantalate (DyTaO4) doping with Al3+ (the doping content are 0, 2, 4, and 6 mol%, respectively) are successfully synthesized in this work. The results of transmission electron microscopy (TEM) reveal that the DyTaO4 has the domains within ferroelasticity. The thermal properties indicate that the (AlxDy1?x)TaO4 ceramics have much lower thermal conductivity and better high‐temperature phase stability than that of 8YSZ (yttria‐stabilized zirconia). The elastic properties imply that the (AlxDy1?x)TaO4 have lower elastic properties than that of DyTaO4. The values of Young's modulus of (AlxDy1?x)TaO4 range from 82 to 135 GPa, and the thermal expansion coefficients (TEC) of (AlxDy1?x)TaO4 vary in the range of (6‐10) × 10?6 K?1 when the temperature is below 1200°C.  相似文献   

10.
Dense oxygen permeable ceramic membranes are promising materials for separating oxygen from air and for syngas production. The crystal structure, oxygen vacancy, microstructure, thermal expansion behavior, electrical conductivity, and oxygen permeability of two mixed‐conducting ceramics: 5 mol% SrAl2O4‐doped SrCo0.8Fe0.2O3 ? δ (SCF‐SA) and SrCo0.76Fe0.19Al0.1Ox (SCFA), with the same overall cation composition, were systematically investigated in this work. The main phases of SCFA and SCF‐SA are cubic perovskite and no orthorhombic brownmillerite appears from 700°C to 900°C in air, while SCF‐SA presents minor unidentified phases. SCF‐SA shows lower electrical conductivity than SCFA. SCFA membranes exhibit lower oxygen permeation flux than SCF‐SA. At 850°C, the oxygen permeation fluxes of SCFA and SCF‐SA are 1.23 and 1.46 mL/cm2/min, respectively, under the oxygen partial pressure gradient of 0.21 × 105/1 × 102 Pa.  相似文献   

11.
《Ceramics International》2023,49(5):7842-7852
Thermal barrier coatings with excellent thermal performance and corrosion resistance are essential for improving the performance of aero-engines. In this paper, (Y3-xYbx)(Al5-xScx)O12 (x = 0, 0.1, 0.2, 0.3) thermal barrier coating materials were synthesized by a combination of sol-gel method and ball milling refinement method. The thermal properties of the (Y3-xYbx)(Al5-xScx)O12 ceramics were significantly improved by increasing Yb and Sc doping content. Among designed ceramics, (Y2.8Yb0.2)(Al4.8Sc0.2)O12 (YS-YAG) showed the lowest thermal conductivity (1.58 Wm?1K?1, at 800 °C) and the highest thermal expansion coefficient (10.7 × 10?6 K?1, at 1000 °C). In addition, calcium-magnesium- aluminum -silicate (CMAS) corrosion resistance of YS-YAG was further investigated. It was observed that YS-YAG ceramic effectively prevented CMAS corrosion due to its chemical inertness to CMAS as well as its unique and complex structure. Due to the excellent thermal properties and CMAS corrosion resistance, YS-YAG is considered to be prospective material for thermal barrier coatings.  相似文献   

12.
Development of microporous magnesia based aggregates serving as working-line refractories have great significance in reducing energy loss and saving resource. Microporous magnesia-based aggregates were fabricated at 1780 °C by in-situ decomposition of magnesite with addition of nano-sized Al2O3. Intergranular MgAl2O4 phases formed in situ decreased the closed-pore size, thermal conductivity and improved the ceramic bonding and thermal shock resistance. Furthermore, the results suggested that pore size distribution was the dominate factor affecting thermal conductivity. Thermal contact resistance owing to networks of intergranular spinel in magnesia could improve thermal insulation performance effectively. The mismatch of thermal expansion coefficient between spinel and magnesia and the micro-scale closed pores enhanced thermal shock resistance by accommodating thermal stress and suppressing crack propagation. Microporous magnesia-based aggregates with 3 wt% nano-sized Al2O3 presented a mean pore size of 3.42 μm, thermal conductivity of 5.76 W m?1 k?1 (800 °C), a cold compressive strength of ~285 MPa, and a residual strength retention rate of 65.0% after thermal shock cycles. The low-conductivity microporous magnesia-based aggregates with excellent thermal shock resistance show promise for future application in working-lining lightweight refractories.  相似文献   

13.
In this work, RE3NbO7 ceramics are synthesized via solid‐state reaction and the phase structure is characterized by X‐ray diffraction and Raman spectroscopy. The relationship between crystal structure and thermophysical properties is determined. Except Sm3NbO7, each RE3NbO7 exhibits excellent high‐temperature phase stability. The thermal expansion coefficients increase with the decreasing RE3+ ionic radius, which depends on the decreasing crystal lattice energy and the maximum value reaches 11.0 × 10?6 K?1 at 1200°C. The minimum thermal conductivity of RE3NbO7 reaches 1.0 W m?1 K?1 and the glass‐like thermal conductivity of Dy3NbO7 is dominant by the high concentration of oxygen vacancy and the local structural order. The outstanding thermophysical properties pronounce that RE3NbO7 ceramics are potential thermal barrier coating materials.  相似文献   

14.
《Ceramics International》2022,48(9):12193-12208
Magneto-optical materials exhibiting low/no temperature-dependent Faraday rotation, high magnetization and good thermal/mechanical stability are required in various magneto-optical sensing and photonic applications. For the first time, we employed the rare earth Lu3+ and Sc3+ ions to strengthen the structure and enhance magnetic& magneto-optical properties of heavy metal oxide diamagnetic glasses. Experimental results revealed that the doping of Lu2O3 and Sc2O3 not only significantly increased the UV-VIS transmittance, improved the thermal stability, Vicker's hardness and tensile strength, but also remarkably increased the magnetic susceptibility and Verdet constant. The reasons were analyzed through the XRD, FT-IR, Raman spectra, oxygen chemical bonds, EPR of Lu2O3 and Sc2O3 crystal fields and affinity to oxygen. Promising Faraday rotating glass with Verdet constant of 76.32 rad/Tm at 633 nm, thermal stability KH = 0.817, very low thermal expansion coefficient (14.04 × 10?6 K?1) and high diamagnetic susceptibility of 9.25 × 10?6 emu/g has obtained which has potential merits for photonics and MO devices applications.  相似文献   

15.
In(HfMg)0.5Mo3O12, which can be considered as a 1:1 mole ratio solid solution of the low‐positive thermal expansion material HfMgMo3O12 and the low‐negative thermal expansion (NTE) material In2Mo3O12 was prepared. From DSC and XRPD results, we show that In(HfMg)0.5Mo3O12 exists in a monoclinic (P21/a) structure at low temperature and undergoes a phase transition at ~425 K to an orthorhombic phase (Pnma), with an associated enthalpy change of 0.89 kJ mol?1. Thermal expansion is large and positive in the low‐temperature monoclinic phase (average α? = 16 × 10?6 K?1 and 20 × 10?6 K?1, from dilatometry and XRPD, respectively). Remarkably, this material has a near‐zero thermal expansion (ZTE) coefficient over the temperature range ~500 to ~900 K in the high‐temperature orthorhombic phase, both intrinsically and for the bulk sample. The average linear intrinsic (XRPD) value is α? = ?0.4 × 10?6 K?1, and the average bulk (dilatometric) value is α? = 0.4 × 10?6 K?1 with an uncertainty of ± 0.2 × 10?6 K?1. The slight difference between intrinsic and bulk thermal expansion is attributed to microstructural effects. XRPD results show that the thermal expansion is more isotropic than for the parent compounds HfMgMo3O12 and In2Mo3O12.  相似文献   

16.
Orthorhombic Sc2Mo3O12 films have been successfully prepared via spin coating technique followed by annealing at 500–750 °C. The phase composition, microstructure, morphology and negative thermal behavior of the synthesized Sc2Mo3O12 films were investigated. XRD and XPS analysis indicate that as-deposited film is amorphous. Orthorhombic Sc2Mo3O12 films can be prepared after post-annealing at 500–750 °C for 1 h. The crystallinity of Sc2Mo3O12 films gradually improved with the increase of post-annealing temperature. SEM analysis shows as-deposited film is smooth and compact, and the grain size of Sc2Mo3O12 film grows up as the post-annealing temperature increases. Variable temperature XRD analysis demonstrates that the synthesized orthorhombic Sc2Mo3O12 films show stable thermo-chemical and anisotropic NTE property in 25–700 °C. The corresponding coefficients of thermal expansion (CTEs) of the orthorhombic Sc2Mo3O12 film in a, b and c directions are ?6.68 × 10?6 °C?1, 5.08 × 10?6 °C?1 and ?4.76 × 10?6 °C?1, respectively. The whole unit cell of the orthorhombic Sc2Mo3O12 film shrinks and the volumetric CTE of the Sc2Mo3O12 thin film is ?6.36 × 10?6 °C?1, and the linear CTE is about ?2.12 × 10?6 °C?1 (αv = 3αl).  相似文献   

17.
Rare-earth (RE) hafnates are promising thermal and environmental barrier coating (TEBC) materials for SiCf/SiC ceramic matrix composites. In this study, pure-phase and dense δ-RE4Hf3O12 (RE = Yb, Lu) bulk ceramics have been fabricated via a hot-pressing method. The crystal structure, microstructure, mechanical, and thermal properties of δ-RE4Hf3O12 were systematically investigated in order to probe their potential application as TEBCs. The high-temperature elastic moduli of δ-Yb4Hf3O12 and δ-Lu4Hf3O12 are measured to be 185 and 188 GPa at 1673 K, respectively, which are over 85% values of room temperature. The coefficients of thermal expansion are 7.64 × 10−6 and 7.46 × 10−6 K−1 for δ-Yb4Hf3O12 and δ-Lu4Hf3O12, respectively. The relatively low coefficient of thermal expansion and thermal conductivity as well as their excellent high-temperature stability endow these hafnates as potential TEBC candidates.  相似文献   

18.
《Ceramics International》2022,48(8):11124-11133
A series of rare-earth-tantalate high-entropy ceramics ((5RE0.2)Ta3O9, where RE = five elements chosen from La, Ce, Nd, Sm, Eu and Gd) were prepared by conventional sintering in air at 1500 °C for 10 h. The (5RE0.2)Ta3O9 high-entropy ceramics exhibit an orthogonal structure and sluggish grain growth. No phase transition occurs in the test temperature of 25–1200 °C. The thermal conductivities of all (5RE0.2)Ta3O9 ceramics are in the range of 1.14–1.98 W m?1 K?1 at a test temperature of 25–500 °C, approximately half of that of YSZ. The sample of (Gd0.2Ce0.2Nd0.2Sm0.2Eu0.2)Ta3O9 exhibits a low glass-like thermal conductivity with a value of 1.14 W m?1 K?1 at 25 °C. The thermal expansion coefficient of (5RE0.2)Ta3O9 ceramics ranges from 5.6 × 10?6 to 7.8 × 10?6 K?1 at 25–800 °C, and their fracture toughness is high (3.09–6.78 MPa·m1/2). The results above show that (5RE0.2)Ta3O9 ceramics could be a promising candidate for thermal barrier coatings.  相似文献   

19.
A mayenite-based electride is being considered for field emitter applications where it will be critical to have a good understanding of processing repeatability and thermal properties. This paper focuses on the details of processing of mayenite compounds in a strongly reducing graphite environment to form an electride solid-solution phase as Ca12Al14O32(C2,O2)1-x(e2)x. The work reports, in addition to processing details, several thermal properties. The electride phase was found to have coefficient of thermal expansion (CTE) of 7e-6 K−1, thermal diffusivity between 0.0063 and 0.0102 cm2/s, and thermal conductivity between 1.9 and 3.1 W m−1 K−1. Rattling motion of O2− or C22− ions in cage structure has been suggested to explain low thermal conductivity of the electride phase.  相似文献   

20.
The thermal conductivity of stoichiometric CeO2 was determined through measurement of thermal expansion from 313 to 1723 K, thermal diffusivity from 298 to 1473 K, and specific heat capacity from 313 to 1373 K. The thermal conductivity was then calculated as the product of the density, thermal diffusivity, and specific heat capacity. The thermal conductivity was found to obey an (A + BT)?1 relationship with A = 6.776×10?2 m·K·W?1 and B = 2.793 × 10?4 m·W?1. Extrapolations of applied models were made to provide suggested data for the specific heat capacity, thermal diffusivity, and thermal conductivity data up to 1723 K. Results of thermal expansion and heat capacity measurements agreed well with the limited low‐temperature data available in the literature. The thermal conductivity values provided in the current study are significantly higher than the only high‐temperature data located for CeO2. This is attributed to the tendency of CeO2 to rapidly reduce at elevated temperatures given the available partial pressure of O2 in air at ambient pressure. The CeO2 data are compared to literature values for UO2 and PuO2 to evaluate its suitability as a surrogate in nuclear fuel systems where thermal transport is a primary criterion for performance  相似文献   

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