Direct ink writing of cordierite ceramics with low thermal expansion coefficient

Since the application of cordierite ceramics is limited by the disadvantages of traditional preparation techniques, 3D printing technology provides the only choice for the rapid preparation of cordierite ceramics with highly complex structures. In this work, the fabrication of cordierite ceramics with complex structures was achieved by direct ink writing. The near-net-shape of cordierite ceramics was realized by the volume expansion caused by the phase transformation. A cordierite ceramic with an average shrinkage rate of 1.58 % was obtained at 1400 °C. The low shrinkage avoids design and manufacturing procedures carried out for dimensional and alignment errors. In addition, the coefficient of thermal expansion was as low as 1.69 × 10^?6 °C^?1. The effect of configuration on the thermal behavior of cordierite ceramics is understood by analyzing the phase composition and microstructure. The cordierites ink reported in this work offers additional possibilities for the production of novel complex structures.     

Preparation of porous cordierite ceramic by gel-casting method

Porous cordierite ceramics were synthesised by gel-casting method, using talcum powder, kaolin and alumina as raw materials. Organic monomers and cross-linker were used as additives. The phase composition and microstructure were investigated by X-ray diffraction and scanning electron microscope. The open porosity, compressive strength and thermal expansion coefficient were tested by the Archimedes method, universal testing machine and thermal expansion instrument, respectively. The results indicate that sintering temperature and holding time have a great influence on the cordierite properties. We obtain the good performance of porous cordierite ceramic sintering at 1350°C for 3?h. The cordierite phase content in the sample is higher and the crystallinity is better. At this point, the porosity is 58.53%, the compressive strength is 22.44?MPa and thermal expansion coefficient reaches 1.69?×?10^?6?°C^?1.     

Potential thermal barrier coating materials: RE3NbO7 (RE=La, Nd,Sm, Eu,Gd, Dy) ceramics

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.     

The microstructure,coefficient of thermal expansion and flexural strength of cordierite ceramics prepared from alumina with different particle sizes

Cordierite ceramics were produced from alumina with 5 and 0.65 μm particle sizes or AlOOH and talc, clays and feldspar, to determine the influence of the alumina particle size on the microstructure, coefficient of thermal expansion (CTE) and flexural strength (FS) of the ceramics. After sintering at 1300 °C the ceramics made from 5-μm-sized alumina consisted of cordierite, glass, quartz, mullite and alumina, and had the highest density, FS and CTE. The alumina grains act as inclusions, from which the trajectories of the cracks were deflected or terminated, which increases the FS and CTE. The ceramics from sub-micrometre-sized alumina or AlOOH contained a negligable amount and no alumina, respectively, together with other phases. This is reflected in the low CTE and FS. The cordierite ceramic with the lowest CTE of ∼2.0 × 10⁻⁶ K⁻¹ and a high FS of 100 MPa was prepared from the 0.65-μm-sized alumina particles.     

Environmental-friendly economical cordierite-mullite-based ceramics for kiln furniture production and supports for CO2 hydrogenation towards C5+ fuels

In this study, lightweight cordierite-mullite ceramics with high strength and high thermal-shock resistance were successfully synthesized by solid-state method with the usage of hollow ceramic microspheres. After careful physico-chemical and mechanical characterization, we gained an economical cordierite material with a low bulk density of 1.40 g/cm³ with an apparent porosity of 44.78%, a flexural strength of 20.17 MPa and a coefficient of thermal expansion of 2.26 × 10^{−6 o}C⁻¹ compared to the bulk counterpart with a bulk density of 2.00 g/cm³ with an apparent porosity of 25.75%, a flexural strength of 23.69 MPa and a coefficient of thermal expansion of 2.47 × 10^{−6 o}C⁻¹. As a catalyst support of Na-FeO_x, the economical cordierite has proved the same stability and activity in CO₂ hydrogenation towards C₅₊ fuels as bulk cordierite-based catalyst counterparts.     

Rare-earth-tantalate high-entropy ceramics with sluggish grain growth and low thermal conductivity

A series of rare-earth-tantalate high-entropy ceramics ((5RE_0.2)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 (5RE_0.2)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 (5RE_0.2)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 (Gd_0.2Ce_0.2Nd_0.2Sm_0.2Eu_0.2)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 (5RE_0.2)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·m^1/2). The results above show that (5RE_0.2)Ta₃O₉ ceramics could be a promising candidate for thermal barrier coatings.     

High-entropy titanate pyrochlore as newly low-thermal conductivity ceramics

Low-thermal conductivity ceramics play an indispensable role in maximizing the efficiency and durability of hot end components. Pyrochlore, particularly zirconate pyrochlore, is currently a highly promising and widely studied candidate for its extremely low thermal conductivity. However, there are still few pyrochlores that offer both stiffness, insulation, and good thermal expansion properties. In this work, the solidification method was innovatively introduced into the preparation of titanate pyrochlore, and combined it with the compositional design of high-entropy. Through careful composition design and solidification control, the high-density and uniform elements distributed high-entropy titanate pyrochlore ceramics were successfully prepared. These samples possess high hardness (15.88 GPa) and Young’s modulus (295.5 GPa), low thermal conductivity (0.947 W·m^?1·K^?1), excellent thermal expansion coefficient (11.6 ×10^?6/K) and an exquisite balance between stiffness and insulation (E/κ, 312.1 GPa·W^?1·m·K), in which the E/κ exhibits the highest value among the current reported works.     

Thermal expansion behavior of various aromatic polyimides

The thermal expansion behavior for various aromatic polymides was investigated. Usally polymers, including polyimides, have high thermal expansion coefficients (3–6 × 10^?5 K^?1), compared with metals and ceramics. However, there are some polyimides which have very low thermal expansion coefficients below 1 × 10^?5 K^?1. This property was observed for the polymides obtained from pyromellitic dianhydride or 3,3′ 4,4′-biphenyltetracarboxylic dianhydride and aromatic diamines which were constituted of only benzene rings fused at para positions. It was proposed that their low thermal expansion coefficient related to the linearity in their polymer molecular skeltons.     

3D printing of cordierite materials from raw reactive mixtures

Porous cordierite materials are 3D printed by robocasting from two kaolin containing raw materials mixtures. Water suspensions of both mixtures at variable solid concentrations (40–67 wt%) are characterized by rheological measurements, showing good printability for concentrations >60 wt% without the need of any printing additive. The mixtures react during sintering (at 1250 °C) giving indialite as the main phase in the structures, which differ in minor phases. Three types of lattices are printed for both compositions with a logpile inner structure. Properties of interest like the coefficient of thermal expansion (CTE), the thermal conductivity (K_T) and the compression strength (σ) of the printed cordierites are determined and compared with published data. Results evidence that printing of clay containing reactive mixtures is a straightforward and cost-effective way to achieve porous complex shaped cordierite with CTE～ 2–3 x10^?6 K^-1, K_T ～ 0.4–0.6 W m^?1 K^?1 and maximum σ of 24 MPa.     

Formation,conversion, and elimination of intermediate spinel phase in MgO/kaolin mixture firing for cordierite

Cordierite ceramic is usually used for diesel particulate filter owing to its excellent low thermal expansion coefficient and high thermal shock resistance properties. However, the co-exited intermediate spinel phase can deteriorate the thermal and mechanical performances of cordierite ceramic product, because the spinel phase has much higher thermal expansion coefficient comparing to that of cordierite. In this study, two methods are utilized to reduce the spinel impurity in the cordierite ceramic. On the one hand, rational reaction resources were introduced to decrease spinel production. The formation of intermediate spinel phase is systematically researched by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman characterizations and the results clarified the preference of path “Enstatite + Mullite → Cordierite” for less spinel production in comparison to path “Enstatite + Al₂O₃ → Cordierite + Spinel.” Additionally, MgO was introduced as fluxing agent to promote liquid-phase sintering, thus facilitating the conversion of spinel. On the other hand, the sintering schedule was improved by introducing a holding temperature gradient to promote the diffusion of Si⁴⁺ and further promote the conversion of spinel into cordierite. With these methods, the residual spinel phase is minimized, the resulting high-purity cordierite has a 47% reduction in the thermal expansion coefficient from 3.07 × 10⁻⁶/K to 1.63 × 10⁻⁶/K compared to the original cordierite sample.     

Theoretical Prediction and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β‐Yb2Si2O7

The thermal and mechanical properties of β‐Yb₂Si₂O₇ were investigated using a combination of first‐principles calculations and experimental investigations. Theoretically, anisotropic chemical bonding and elastic properties, weak interatomic (010) and (001) planes in the crystal structure, damage tolerance, and low thermal conductivity are predicted. Experimentally, preferred orientation, superior mechanical properties, and damage tolerant behavior for hot‐pressed bulk β‐Yb₂Si₂O₇ are approved. Slipping along the weakly bonded {010}, {001}, or {100} planes, grain delamination, buckling, and kinking of nanolaminated grains are identified as main mechanisms for damage tolerance. The anisotropic linear thermal expansion coefficients (CTEs) are: α_a = (3.57 ± 0.18) × 10^?6 K^?1, α_b = (2.49 ± 0.14) × 10⁶ K^?1, and α_c = (1.48 ± 0.22) × 10^?6 K^?1 (673–1273 K). A low thermal conductivity of ~2.1 W (m·K)^?1 at 1273 K has been confirmed. The unique combination of these properties endow it a potential candidate for thermal barrier coating (TBC)/environmental barrier coating of silicon‐based ceramics.     

First principles investigation on mechanical and thermal properties of α‐ and β‐YAlB4 ultra‐high temperature ceramics

Ultra‐high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties that makes them promising candidates for applications in extreme environments. Various UHTCs are needed due to diverse harsh conditions that UHTCs are faced with in different applications. Due to structural similarity to ZrB₂, possible high melting point and possible protective oxide scale formed in oxygen rich and water vapor environments, REAlB₄ (RE: rare‐earth) is suggested a good candidate for UHTCs. In the present work, temperature‐dependent mechanical and thermal properties of both α‐YAlB₄ (YCrB₄ type, space group Pbam) and β‐YAlB₄ (ThMoB₄ type, space group Cmmm) were investigated by first principles calculations in combination with quasi‐harmonic approach. Due to the structural similarity between α‐YAlB₄ and β‐YAlB₄, their properties are very similar to each other, which are approximately transverse isotropic with properties in (001) plane being almost the same and differing from properties out of (001) plane. The results reveal that resistance to normal strain in (001) plane (~460 GPa) is higher than that along [001] direction (~320 GPa) and thermal expansion in (001) plane (~10 × 10^?6 K^?1) is lower than that along [001] direction (~17 × 10^?6 K^?1), which is because the stiff boron networks are parallel to (001) plane. The average thermal expansion coefficient is around 12 × 10^?6 K^?1, which is fairly high among UHTCs and compatible with metallic frameworks. The combination of high thermal expansion coefficient and protective oxidation scale forming ability suggest that REAlB₄ is promising for practical applications not only as high‐temperature structural ceramic but also as oxidation resistant coating for alloys.     

Synthesis and thermophysics properties of ferroelastic SmNb1-XTaXO4 ceramics

Monoclinic phase SmNb_1-XTa_XO₄ ceramics are synthesized via solid-state reaction. X-ray diffraction and Raman spectra are applied to characterize the crystal structure. The ferroelasticity of SmNb_1-XTa_XO₄ ceramics is confirmed by the domain structure observed via scanning electron microscopy. The band gap of SmNb_1-XTa_XO₄ ceramics ranges from 4.3 to 5.0?eV and they exhibit excellent absorption for UV light. Thermophysics properties including specific heat, thermal diffusivity, thermal conductivity and thermal expansion coefficients of SmNb_1-XTa_XO₄ ceramics are investigated systematically. The result shows that the thermal conductivity of SmNb_1-XTa_XO₄ ceramics is as low as 1.33?W?m^?1 K^?1 (900?°C) and the thermal expansion coefficients are as high as 11.7?×?10^?6 K^?1 (1200?°C). The unique ferroelasticity and outstanding thermophysics properties indicate that SmNb_1-XTa_XO₄ ceramics are promising thermal barrier coating materials.     

Determinations of Ce solid solution mechanism and limit thermal conductivity of YTaO4 ceramics

x mol% CeO₂-YTaO₄ (x = 0, 3, 6, 9, 12) ceramics have been synthesized by the spark plasma sintering (SPS) technique. We focus on the changes in lattice distortion, bonding length, thermal conductivity, thermal expansion, and phase stability of the prepared samples. XRD, Raman, and XPS are used to determine the chemical valence and solid solution mechanism of Ce in the lattice of YTaO₄, while its effects on thermal/mechanical properties are elucidated from microstructures. Y³⁺ is substituted via Ce³⁺, and all samples maintain a monoclinic phase. The limit thermal conductivity (1.2 W?m^?1?K^?1, 900 °C) is realized in 9 mol% CeO₂-YTaO₄, and the thermal expansion coefficients are increased to 10.2 × 10^?6 K^?1 at 1200 °C. Furthermore, the exceptional phase stability and mechanical properties of all samples indicate that they can provide good thermal insulation at high temperatures, and have higher working temperatures than the current YSZ thermal barrier coatings.     

Effects of Yb and Sc co-doping on the thermal properties and CMAS corrosion of garnet-type (Y3-xYbx)(Al5-xScx)O12 ceramics

Thermal barrier coatings with excellent thermal performance and corrosion resistance are essential for improving the performance of aero-engines. In this paper, (Y_3-xYb_x)(Al_5-xSc_x)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 (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ ceramics were significantly improved by increasing Yb and Sc doping content. Among designed ceramics, (Y_2.8Yb_0.2)(Al_4.8Sc_0.2)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.     

Synthesis,microstructure and property characterization of Mo4Y2Al3B6 ceramic fabricated by spark plasma sintering

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·m^1/2, 1255.14 MPa, and 384.82 MPa, respectively, showing the promising applications as structural-functional ceramics.     

Synthesis and thermophysical properties of RETa3O9 (RE = Ce,Nd, Sm,Eu, Gd,Dy, Er) as promising thermal barrier coatings

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.     

Zero thermal expansion in cubic MgZrF6

Isotropic zero thermal expansion (ZTE) is rare but intriguing physical property in materials. Here, we report an isotropic ZTE property in a double ReO₃‐type compound of MgZrF₆, which exhibits a negligible value of coefficient of thermal expansion (α_l = ?7.94 × 10^?7 K^?1 (XRD), α_l = ?4.22 × 10^?7 K^?1 (dilatometry), 300‐675 K). The ZTE mechanism of MgZrF₆ is understood by the joint studies of temperature dependence of crystal structure and lattice dynamics. Interestingly, different magnitudes of atomic displacement parameters (ADPs) for the fluorine atoms in MZrF₆ (M = Ca, Ni, Mg) are found. The strong temperature sensitivity of ADPs demonstrates intensive transverse thermal vibration of fluorine atoms, which contributes essentially to the negative thermal expansion of CaZrF₆. By contrast, for NiZrF₆ with positive thermal expansion, the temperature response of ADPs is weak. Moderate transverse thermal vibration takes place in MgZrF₆, and ZTE appears. Furthermore, lattice dynamics of MgZrF₆ is studied by temperature‐dependent Raman spectroscopy, which reveals the ZTE mechanism. In particular, the F_2g and A_g modes, corresponding to the bending and stretching vibrations of fluorine atoms, respectively, neither soften nor harden over the whole temperature range, which is correlated with the isotropic ZTE property of MgZrF₆.     

Near‐Zero Thermal Expansion in In(HfMg)0.5Mo3O12

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.     

Preparation and characterization of cordierite glass–ceramics for bonding solar thermal transmission pipelines

TiO₂ was employed to develop cordierite glass–ceramics for thermal transmission pipeline binders by a melt-quenching method. The effects of TiO₂ on the phase composition, microstructure, and physical properties of glass–ceramics were studied. In addition, the thermal shock resistance of the glass–ceramics based binder was investigated. The results showed the formation of α cordierite could be increased by adding 1.0 wt% TiO₂, thereby improving bending strength and decreasing the coefficient of thermal expansion. However, a 3-5 wt% TiO₂ additive resulted in massive generation of µ cordierite, which exhibited a negative effect on the above performances. After crystallization at 1000°C for 2 h, sample B1 (1 wt% TiO₂ additional) displayed the best overall properties. It was demonstrated that cordierite glass–ceramics were satisfactory materials as heat transmission pipeline binders when the C2 binder (40 wt% frit, 60 wt% as-prepared sample B1) was applied, which had a good thermal shock resistance.