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.     

Effect of Microstructure on the Thermal Expansion Coefficient of Sintered Cordierite Prepared from Sol Mixtures

Dense cordierite ceramics were prepared from a sol mixture of alumina, silica, and magnesia, and the relationship between microstructure and thermal expansion was clarified for sinters with relative density greater than 97%. In the dense cordierite ceramics, submicrometer‐sized primary cordierite crystals aligned in the same crystal orientation and constituted the domain structure. We discovered that these domain structures could be easily observed by optical polarizing microscopy and quantified by digital image analysis of the photographs. The occurrence of microcracks between domains larger than 40 μm was induced by the thermal expansion anisotropy of the cordierite crystal axes. As a result, the mean thermal expansion coefficient of the cordierite ceramics decreased to 0.4 × 10^?6 K^?1 from the average value of the crystal axes of 1.7 × 10^?6 K^?1. This lower thermal expansion coefficient could be theoretically explained by partial microcracking.     

Preparation of refractory cordierite using amorphous rice husk silica for thermal insulation purposes

This study aims to investigate the effect of sintering temperatures on the phase formation and physical characteristics of refractory cordierite prepared from rice husk silica, Al₂O₃, and MgO powders. The samples were subjected to sintering temperatures of 1050–1350 °C, and development of structures was characterized using Fourier Transform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD) coupled with Rietveld analysis, scanning electron microscopy (SEM) and dilatometry. The results obtained indicated the significant role of sintering temperatures on phase transformation of spinel and cristobalite into cordierite, in which at sintering temperatures of 1230–1350 °C the cordierite emerges as a dominant phase, while spinel and cristobalite are practically undetected. Formation of cordierite was followed by decrease in density, porosity, and thermal expansion coefficient, while for hardness and bending strength the opposite was true. Thermal expansion coefficient of the sintered sample at 1350 °C is 3.3×10⁻⁶/°C and the XRD analysis demonstrated that the main crystalline phase is cordierite. Based on these characteristics, the samples are considered as insulator, suggesting their potential use in refractory devices.     

Fabrication of cordierite foam ceramics using direct foaming and slip casting method with plaster moulds

The cordierite foam ceramics were successfully fabricated using direct foaming and slip casting method with plaster moulds. Kaolin, attapulgite and magnesium oxide were used as starting materials with Arabic gum added as the dispersant. The samples were sintered at 1200°C, and then the microstructure, porosity, bulk density and thermal conductivity were characterised. The results show that the cordierite foam ceramics had a porous structure of open cells and the struts had abundant small pores. The maximum open porosity achieved 87·65% with a bulk density of 329 kg m^??3, and the thermal conductivity was as low as 0·095 W (m K)^??1. Therefore, these cordierite foam ceramics show promise for use as the thermal insulator.     

The effect of β-eucryptite on cordierite ceramic materials prepared using a temperature-induced forming technique

The microstructural evolution and sintering behavior of powder compacts composed of β-eucryptite–cordierite with different particle size distributions were prepared by a temperature-induced forming (TIF) technique. Grinding conditions and milling time were varied to obtain coarse or fine cordierite particles before mixing with β-eucryptite. TIF mixes were formed with varying eucryptite and sintering temperatures, and the physical and mechanical properties, microstructure, and thermal expansion of the mixes were examined. Results indicate that CB1350 mixes exhibited high densification and good mechanical properties. The addition of 30%wt. beta eucryptite to the CB1350 mix and sintering at 1300 °C was free of cracks and exhibited good physical and mechanical properties. However, the thermal expansion of this mix was low (?1.31 × 10^?6 °C^?1) up to 20%wt. β-eucryptite. Thermal shock resistance tests also indicated that the composite resisted crack propagation over 25 cycles.     

Preparation and performance study of cordierite/mullite composite ceramics for solar thermal energy storage

The employment of solar energy in recent years has reached a remarkable edge. It has become even more popular as the cost of fossil fuel continues to rise. Energy storage system improves an adjustability and marketability of solar thermal and allowing it to produce electricity in demand. This study attempted to prepare cordierite/mullite composite ceramics used as solar thermal storage material from calcined bauxite, talcum, soda feldspar, potassium feldspar, quartz, and mullite. The thermal physical performances were evaluated and characterized by XRD, SEM, EPMA, and EDS. It was found that the optimum sintering temperature was 1280°C for preparing, and the corresponding water adsorption was 11.25%, apparent porosity was 23.59%, bulk density was 2.10 mg·cm^?3, bending strength was 88.52 MPa. The residual bending strength of specimen sintered at 1280°C after thermal shock of 30 times decreased to be 57 MPa that was 36% lower than that before. The thermal conductivity of samples sintered at 1280°C was tested to be 2.20 W·(m·K)^?1 (26°C), and after wrapped a PCM (phase change materials) of K₂SO₄, the thermal storage density was 933 kJ·kg^?1 with the temperature difference (ΔT) ranged in 0‐800°C. The prepared cordierite/mullite composite ceramic was proved to be a promising material for solar thermal energy storage.     

Refractory glass–ceramics based on alkaline earth aluminosilicates

Glass–ceramics that can be used at temperatures of 1200–1500 °C are found in the alkaline earth aluminosilicate field, and are generally nucleated internally with titania. These glass–ceramics have good strength (>100 MPa, abraded), can be tailored to produce high fracture toughness (2–5 MPa m^1/2), and have good dielectric properties. Coefficients of thermal expansion (CTEs) are low to moderate ((25–45) × 10^?7 °C^?1, from 25 to 1000 °C).The major crystalline phase in the glass–ceramics exhibiting the lowest CTEs is hexagonal cordierite (indialite), while important toughening accessory phases are enstatite and acicular magnesium dititanate.The most refractory glass–ceramics that are easily melted at 1650 °C, yet when crystallized do not deform at 1450 °C, are based on strontium and barium monoclinic feldspars of the celsian type. CTEs range from 35 to 45 × 10^?7 °C^?1. Acicular mullite is an important accessory phase aiding fracture toughness in these materials.Mullite glass–ceramics which contain considerable siliceous residual glass are probably the most refractory of these glass–ceramics, but they require melting above 1700 °C. Nevertheless, they can be used at temperatures near 1600 °C.Potential applications for refractory glass–ceramics include improved radomes, engine components, substrates for semiconductors and precision metallurgical molds.     

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.     

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.     

Glass‐Ceramics in the System BaO–SrO–ZnO–SiO2 with Adjustable Coefficients of Thermal Expansion

Glasses from the system BaO–SrO–ZnO–SiO₂ with different Ba/Sr ratios were characterized regarding crystallization behavior as well as the thermal expansion of almost fully crystallized glasses. Depending on the SrO concentration, different crystalline phases precipitate from the glasses. Those with low SrO concentrations precipitate crystals with the structure of low‐temperature BaZn₂Si₂O₇ as one of the major phases. Higher SrO concentrations cause the formation of Ba_1?xSr_xZn₂Si₂O₇ solid solutions with the structure of high‐temperature BaZn₂Si₂O₇. Both, the low‐ as well as the high‐temperature phase exhibit very different thermal expansion behaviors ranging from a very high coefficient of thermal expansion in the case of the low‐temperature phase to a very low coefficient of thermal expansion in the case of the high‐temperature phase. The glass‐ceramics with the highest and that with the lowest coefficient of thermal expansion measured between 100°C and 800°C show a difference of 7.9 × 10^?6 K^?1, which is caused solely by a substitution of BaO with SrO. In contrast, the maximum variation in the thermal expansion of the glasses was only 1.5 × 10^?6 K^?1. The microstructure of sintered and afterward crystallized glass powders was analyzed via scanning electron microscopy and showed crack‐free samples with low porosity.     

Fabrication and characterization of porous cordierite ceramics prepared from fly ash and natural minerals

Low cost, single-phase porous cordierite ceramics are successfully synthesized by in-situ solid-state reactions from fly ash, quartz, and magnesite. The effects of sintering temperature and magnesite content on phase transformation, open porosity, bulk density, mechanical properties, and microstructure are carefully investigated. Factsage analyses are carried out to calculate the isopleth diagrams, and the results agree well with the experimental outcomes. Thermal gravimetric and differential scanning calorimeter analyses (TG-DSC) are performed to characterize the weight loss and transient behaviors of the raw materials. Linear thermal expansion properties are also studied. The α-cordierite phase is the only phase observed in S-3 (magnesite content 25%) sintered at 1300 °C for 2 h, and its linear thermal expansion coefficient (2.71 × 10⁻⁶ K⁻¹) is close to that of typical cordierite. Both the compressive strength (72.64 MPa) and flexural strength (23.92 MPa) of the as-synthesized samples are high with an open porosity of 33.16% and a bulk density of 1.61 g/cm³.     

Utilization of granite sludge for production of cordierite ceramics by direct coagulation casting

In this work, an attempt to produce cordierite ceramics from granite sludge waste, talc and alumina was performed by direct coagulation casting process. To optimize the conditions for cordierite formation, three mix-compositions were firstly prepared by processing the starting materials in different conditions. The first mix was prepared by firing the mix of granite sludge, talc and alumina up to 1300 °C while the second and third mixes were fabricated by firing alumina and talc at 1300 °C or 1350 °C, respectively, then the granite sludge was added. Both batches were fired at different temperatures. According to the percentage of formed cordierite, the third mix was selected to be solidified by direct coagulation casting method followed by sintering at different temperatures. The casted cordierite was examined by thermal analysis while the sintered bodies were tested for their physical, mechanical and electrical properties. The results indicated that the pre-heating of alumina and talc at 1350 °C (third mix) enhanced the formation of cordierite and some amounts of spinel. For the casted sintered specimens, the porosity was decreased with increasing the sintering temperature. Also, there was an increase in compressive strength for the samples sintered up to 1250 °C. The dielectric constant values were varied between 4.5 and 5.89 while the dielectric loss was varied between 2 × 10^?3 and 7 × 10^?3, at room temperature.     

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.     

Structural origin of negative thermal expansion of cordierite honeycomb ceramics and crystal phase evolution with sintering temperature

The structural origin of the negative coefficient of thermal expansion (CTE) along the c-axis in cordierite is investigated using high temperature X-ray diffraction. The evolution of the lattice parameters, bond length, covalent character, and electron density distributions are analyzed with increasing temperature. The increase of covalent character of the tetrahedral bonds in the six-membered hexagonal ring is considered as the possible origin for the negative expansion. In addition, the bridging oxygen can act as a buffer absorbing thermal energy and prevents the increase in M1-O-M2 bond. The electron density distribution indicates that the linking oxygen strongly vibrates transversely across two coordinate M1-O-M2 linkages. The CTE and pore size and distribution change drastically at near 1300 °C due to the substantial increase in cordierite phase proportion and the formation of a liquid phase. Raw materials are consumed completely at 1450 °C and the CTE decreases to 1.3 ppm K^?1.     

ZrO2 and MxOy (M= La,Ce, and Nb) synergistically reinforced porous cordierite ceramics synthesized via a facile solid-state reaction

In an attempt to enhance the sintering and mechanical performance of porous cordierite ceramics (Mg₂Al₄Si₅O₁₈) as support materials for vehicle exhaust catalysts, ZrO₂ and M_xO_y (M = La, Ce, and Nb) were simultaneously introduced into cordierite sintered at 1350 °C for 4 h. Then, the reinforced function of ZrO₂ and M_xO_y on the properties of porous cordierite ceramics was systematically evaluated, especially for the sample co-doped with ZrO₂ and La₂O₃. The results displayed a distinct enhancement in mechanical and thermal performance, and the cold compressive strength increased from 72.74 to 158.59 MPa as well as thermal conductivity ranged from 1.66 to 2.01 W m^?1 K^?1, respectively. It is found that ZrO₂ facilitated activation sintering introduced via lattice distortion in cordierite and La₂O₃ accelerated the formation of cordierite and ZrSiO₄ microcrystalline through the low-temperature liquid phase. The synergic effect between ZrO₂ and La₂O₃, therefore, had a significant role in the reinforced mechanical and thermal performance of porous cordierite ceramics. This work not only offers a feasible strengthening strategy, but also expands the possibilities for building high-performance structural and functional ceramics.     

Synthesis and characterization of low CTE value La2O3-B2O3-CaO-P2O5 glass/cordierite composites for LTCC application

Low-softening-point La₂O₃-B₂O₃-CaO-P₂O₅ (LBCP) glass-ceramic/cordierite composite systems have been prepared in this work. Influence of the ratio of La₂O₃ to B₂O₃ and the content of cordierite on the sintering behavior, microstructure, sintering quality, thermal properties and dielectric properties of composites are studied. The results show that high La₂O₃/B₂O₃ ratio improves the crystalline quality of LBCP glass-ceramic, but also narrows its process window. The increase of cordierite content reduces the coefficient of thermal expansion (CTE) value of composites obviously. However, excess cordierite is detrimental to the densification of the composite microstructure, and too low cordierite content causes serious foaming. Sample containing 30?wt% LBCP1 glass-ceramic and 70?wt% cordierite sintered at 850?°C shows excellent properties: relative density of 95.26%, CTE value of 4.12?ppm/°C, dielectric constant of 4.78 (1?MHz)/4.52 (12.8?GHz), dielectric loss of 2.3?×?10^?3 (1?MHz)/2.5?×?10^?3 (12.8?GHz) and the ability to co-fire with silver, which suggests that LBCP glass/cordierite composite system has potential to meet the requirements of LTCC substrate material.     

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.     

Preparation study for the low thermal expansion spodumene/mullite composites

In this work, spodumene/mullite ceramics with low thermal expansion were successfully prepared from spodumene, quartz, talc, and clay. The effects of spodumene content and sintering temperature on the mechanical properties of spodumene/mullite ceramics were investigated. The formed phases were then detected by X-ray diffraction analysis and the microstructures of the sintered bodies were determined by scanning electron microscopy. The interaction effects of the spodumene content and sintering temperature on the apparent porosity and bulk density were studied by response surface methodology. The results demonstrate that an appropriate sintering temperature and spodumene content can promote densification, improve the mechanical properties, and reduce the coefficient of thermal expansion (CTE) of spodumene/mullite ceramics. At the spodumene content of 40 wt.%, the sintering temperature of 1270°C, and the holding time of 90 min, the bending strength was 60.45 MPa, the CTE was 1.73 × 10^–6/°C (α_{[25–650°C]} < 2 × 10^–6/°C), the bulk density was 2.28 g cm^-3, and the apparent porosity was 0.43%. Therefore, this study was of guiding significance for reducing the production cost of spodumene low thermal expansion ceramics and improving product quality.     

Thermo-mechanical and high-temperature dielectric properties of cordierite-mullite-alumina ceramics

Heterogeneous ceramics made of cordierite (55–56 wt%), mullite (22–33 wt%) and alumina (23–11 wt%) were prepared by sintering non-standard raw materials containing corundum, talc, α-quartz, K-feldspar, kaolinite and mullite with small amounts of calcite, cristobalite and glass phases. The green specimens prepared by PVA assisted dry-pressing were sintered within the temperature range of 950–1500 °C for different dwelling times (2–8 h). The effects of sintering schedule on crystalline phase assemblage and thermomechanical properties were investigated. The sintered ceramics exhibited low coefficients of thermal expansion (CTE) (3.2–4.2×10⁻⁶ °C⁻¹), high flexural strength (90−120 MPa and high Young modulus (100 GPa). The specimens sintered at 1250 °C exhibited the best thermal shock resistance (∆T~350 °C). The thermal expansion coefficients and thermal shock resistance were studied using Schapery model, the modelling results implying the occurrence of non-negligible mechanical interactions between the phases in bulk. The dielectric properties characterized from room to high temperature (RT– HT, up to 600 °C) revealed: (i) noticeable effects of sintering schedule on dielectric constant (5–10) and dielectric loss factor (~0.02–0.04); (ii) stable dielectric properties until the failure of the electrode material. The thermomechanical properties coupled with desirable dielectric properties make the materials suitable for high density integrated circuitry or high temperature low-dielectric materials engineering.     

Effects of Li Substitution on the Microstructure and Thermal Expansion Behavior of Pollucite Derived from Geopolymer

The purpose of this work was to study the role of lithium in cesium‐based geopolymers and the thermal evolution during heat treatment together with thermal expansion behavior of the resulting geopolymer ceramic. A series of lithium‐substituted cesium‐based geopolymers, Cs_(1?x)Li_xGP (where x = 0, 0.1, 0.2, and 0.3), were prepared. All the geopolymer samples were heated at 1300°C for 2 h and thermal evolution on heating was studied by a variety of techniques. Phase composition, microstructure evolution, and thermal expansion behaviors of the ceramics derived from the geopolymers were characterized. All the geopolymer specimens exhibited similar thermal evolutionary trends. With increases in lithium content, overall mass loss increased gradually due to the higher hydration energy of Li⁺ than Cs⁺. Thermal shrinkage of these specimens can be divided into four stages, i.e., structural resilience, dehydration, dehydroxylation, and sintering, according to the dilatometer results. The introduction of Li results in two‐step sintering behavior for the lithium‐substituted cesium‐based geopolymers. The average thermal expansion coefficient (CTE) of Cs_(1?x)Li_xGP ceramics decreased from 4.80 × 10^?6 K^?1 (x = 0) to 3.61 × 10^?6 K^?1 (x = 0.3) with increase in lithium substitution. The reason can be attributed to the presence of spodumene after thermal treatment, which has a relatively low thermal expansion coefficient compared with pollucite. Meanwhile, molten spodumene could serve as a buffer phase between pollucite crystals also conducive to the decline of CTE of this system.