Fabrication of ultralight heat-insulating hollow-fiber mullite fiberboards

Mullite fiberboards have been extensively used in heat-insulating refractory materials. To further improve the thermal insulation properties and reduce the density, we fabricated mullite fiberboards by vacuum filtration using hollow mullite fibers based on a ceiba fiber template. The effects of sintering temperature and type and content of high-temperature adhesives on the density, compressive strength, thermal conductivity, microstructure, and volume stability of mullite fiberboards were investigated. The results showed that the obtained mullite fiberboards have ultralow density of 0.1–0.2 g/cm³, low thermal conductivity of 0.0988–0.1230 W/(m·K) (at 500 °C), and compressive strength of 0.08–0.12 MPa, and they exhibit good volume stability at 1300 °C.     

Porous ceramics based on high-thermal-stability Al2O3–ZrO2 nanofibers for thermal insulation and sound absorption applications

Al₂O₃ fibers are promising candidates for porous ceramics, but the sudden growth of grains in the fibers above 1200 °C will limit their applications for high temperature. Herein, we reported the successful fabrication of the Al₂O₃–ZrO₂ nanofibers by electrospinning and the nanofiber-based porous ceramics by a combination of gel-casting, freeze-drying and high-temperature sintering. Results show that the addition of Zr could greatly improve the thermal stability (up to 1400 °C) of the Al₂O₃-based nanofibers, owing to the inhibition of the sudden growth of the grains in the fibers at high temperature. The Al₂O₃–ZrO₂ nanofiber-based porous ceramics after sintering at 1100–1400 °C possessed a multi-level pore structure and exhibited high thermal stability, ultra-high porosity (97.79–98.04%), ultra-low density (0.075–0.091 g/cm³) and thermal conductivity (0.0474–0.0554 W/mK), and excellent sound absorption performance with the average sound absorption coefficient of 0.598–0.770. These porous ceramics are expected to be employed in the fields of high-temperature thermal insulation and sound absorption.     

Heat-insulating properties of hollow Al2O3–ZrO2(CeO2)fibers fabricated using pampas grass as the template

Al₂O₃–ZrO₂(CeO₂) ceramic fibers have good heat insulation and high-temperature resistance. Pampas grass is a large perennial grass, or a natural fiber, with a hollow structure that can improve the heat insulation of the fiber by changing its heat transfer mode. This study introduces a method for the preparation of biomorphic Al₂O₃–ZrO₂(CeO₂) fibers with a hollow structure and a double-layer-tube structure using the pampas grass as thetemplate. Hollow ceramic fibers with good thermal insulation properties were prepared by soaking the pampas grass in ZrOCl₂, Ce(NO₃)₃, and AlCl₃ solutions before sintering them at high temperatures. When the zirconia doping ratio was 11 mol%, the biomorphic fiber with a double-layer-tube structure was prepared. The biomorphic fibers inherited the hollow structure of the pampas grass and retained the template fiber'scharacteristics of excellent continuity and a high degree of hollowness, whichdecreased the thermal conductivity of the fibers.     

Preparation and fine thermal insulation performance of Gd2Zr2O7/ZrO2 composite fibers

Gd₂Zr₂O₇/ZrO₂ (GZC) composite fibers were prepared by electro-spinning method. The XRD, XPS and Raman results showed that there were three crystalline phases, tetragonal phase ZrO₂, cubic phase ZrO₂ and defect fluorite phase Gd₂Zr₂O₇ in GZC composite fibers. GZC fibers remained an intact fiber texture up to 1400 °C according to SEM photographs. The thermal conductivity of GZC fibers was between 0.173 W/(m·K) at 400 °C and 0.309 W/(m·K) at 800 °C, which was lower than that of 7YSZ under the same experimental conditions. The fiber sheet with density about 3.5 g/cm³ has thermal shrinkage less than 3% at 1400 °C. Hence, GZC fibers could be used as refractories for heat protection.     

Phase stability (at 1000°C) of hollow t-ZrO2 fibers costabilized by lanthana and yttria

Yttrium-stabilized ZrO₂ (YSZ) hollow fibers derived from a ceiba template present a 25%–53% reduction in thermal conductivity compared with traditional YSZ solid fibers. However, after prolonged preservation at 1000°C, tetragonal ZrO₂ (t-ZrO₂) can easily transform to monoclinic ZrO₂ (m-ZrO₂), which destroys the hollow structure of the YSZ fibers and results in loss of the structural advantages for heat insulation. To overcome this, in this study, biomorphic lanthana and yttrium costabilized zirconia (LaYSZ) fibers with a hollow structure are fabricated by doping appropriate amounts of lanthanum in the raw materials of YSZ fibers. X-ray diffraction, scanning electron microscopy, and thermal conductivity measurements are utilized to confirm the phase-stability superiority of LaYSZ fibers to that of YSZ fibers under harsh conditions. After preservation at 1000°C for 150 hours, the m-ZrO₂ content in the LaYSZ hollow fibers increases from 0 to 3.4 mol%, whereas that in the YSZ fibers increases from 0 to 10.25 mol%. Furthermore, owing to their better phase stability at 1000°C, the morphologies and heat-insulating properties of LaYSZ fibers are more improved in several aspects compared with YSZ fibers.     

Dielectric,mechanical and thermal properties of ZrO2–TiO2 materials obtained by microwave sintering at low temperature

The sinterability of 3Y-TZP/TiO₂ materials using micrometre-sized ZrO₂ and nanometre-sized TiO₂ (16 wt%) by one-step fast microwave sintering at low temperature (1200–1300 °C) was investigated. Firstly, in situ detailed analysis of the dielectric properties of the material with temperature was carried out in order to measure the capacity of the material to transform microwave energy into heat. Another related parameter associated to microwave sintering is the penetration depth of the microwave radiation into the material, which showed great homogeneity from 400 °C. Secondly, the effect of sintering conditions on microstructure, density, hardness and coefficient of thermal expansion was evaluated. The X-ray diffraction study and microstructural characterization demonstrate that it is possible to obtain fully dense pieces (>99%) by microwave sintering, a condition yielding to a coarse-grained (~1–2 μm), quite hard (~13.7 GPa) 3Y-TZP/TiO₂ material. However, the most important feature is the significant reduction of the thermal expansion coefficient (8·10⁻⁶ K⁻¹) as compared to that of 3Y-TZP. In addition, the results from conventional sintering at 1400–1500 °C with 2 and 6 h of dwell time are examined and compared. The materials obtained at 1500 °C showed high density with grain size and hardness similar to those obtained by microwave but with a dramatic difference in the power consumption of the sintering cycle, since the materials obtained by microwave used a maximum absorbed power of 120 W and a heating cycle of only 40 min.     

Polymer derived ZrO2 reinforced SiC-ZrB2 polycrystalline fiber

Most polycrystalline SiC-based fibers were prepared at a sintering temperature higher than 1600 °C. In this work, a ZrO₂ reinforced SiC-ZrB₂ polycrystalline fiber was prepared at 1400 °C via the polymer-derived ceramic method from a new Zr- and B-containing polycarbosilane. The morphology and microstructure of the polycrystalline nanocomposite fiber were studied using XRD, XPS, EDS, SEM, and TEM. The results showed that t-ZrO₂ was formed at relatively lower temperature (<1000 °C). The most interesting result is that the polycrystalline nanocomposite SiC-ZrB₂ was generated after heat-treatment at 1400 °C, producing an excellent ZrO₂ reinforced SiC-ZrB₂ polycrystalline fiber. The present study provides a novel strategy for the fabrication of SiC-based polycrystalline fiber at a relatively low temperature.     

Effect of lattice structure evolution and stacking fault energy on the properties of Cu–ZrO2/GNP nanocomposites

A hybrid nanocomposite comprising nanosized ZrO₂ and graphene nanoplatelet (GNP)-reinforced Cu matrix was synthesised via powder metallurgy. The influence of sintering temperature and GNP content on the electrical and mechanical behaviour of the Cu–ZrO₂/GNP nanocomposite was investigated. The ZrO₂ concentration was fixed at 10% for all the composites. Upon increasing the GNP concentration up to 0.5%, a significant improvement was observed in the compressive strength, microhardness, and electrical conductivity of the composite. Furthermore, the properties were significantly improved by increasing the sintering temperature from 900 to 1000 °C. The compressive strength, hardness, and electrical conductivity of Cu–10%ZrO₂/0.5%GNP were higher than those of the Cu–ZrO₂ nanocomposite by 60, 21, and 23.8%, respectively. This improvement in the mechanical properties is because of the decrease in the crystallite size and dislocation spacing, which increases the dislocation density, thereby increasing the impedance towards dislocation movement. The lower stacking fault energy of the hybrid nanocomposites enables easier electron transfer within and between the Cu grains, resulting in an improved electrical conductivity. The enhancement in strength and electrical conductivity were aided by the GNPs and ZrO₂ nanoparticles that were dispersed widely in the Cu matrix.     

Low-temperature Pr3Si2C2-assisted liquid-phase sintering of SiC with improved thermal conductivity

A novel Pr₃Si₂C₂ additive was uniformly coated on SiC particles using a molten-salt method to fabricate a high-density SiC ceramics via liquid-phase spark plasma sintering at a relatively low temperature (1400°C). According to the calculated Pr–Si–C-phase diagram, the liquid phase was formed at ∼1217°C, which effectively improved the sintering rate of SiC by the solution–reprecipitation process. When the sintering temperature increased from 1400 to 1600°C, the thermal conductivity of SiC increased from 84 to 126 W/(m K), as a consequence of the grain growth. However, an increasing amount of the sintering additive increased the interfacial thermal resistance, resulting in a decrease of thermal conductivity of the materials. The highest thermal conductivity of 141 W/(m K) was obtained for the material having the largest SiC grains and an optimized amount of the additive at the grain boundaries and triple junctions. The proposed Pr₃Si₂C₂-assisted liquid-phase sintering of SiC can be potentially used for the fabrication of SiC-based ceramic composites, where a low sintering temperature would inhibit the grain growth of SiC fibers.     

Investigation of the electrical conductivity of sintered monoclinic zirconia (ZrO2)

High-density monoclinic ZrO₂ was manufactured through sintering at ~1200 °C by using nanosized powders. Then, the electrical conductivity was measured at a range of high temperatures (700–900 °C) by electrical impedance spectroscopy (EIS). For the as-sintered monoclinic ZrO₂, the measured electrical conductivity was 3.2×10⁻⁵ s/cm (for 80% TD) and 4.4×10⁻⁵ s/cm (for 89% TD) at 900 °C. After aging at 900 °C for 100 h, the electrical conductivity of the monoclinic ZrO₂ of 80%-TD decreased by more than 50%. However, after reheating at 1200 °C for 1 h, approximately 80% of the conductivity was recovered compared to the value of the as-sintered monoclinic ZrO₂. The pure monoclinic crystal structure was retained despite the aging and reheating treatment. Based on microstructural observations of the aged and reheated monoclinic ZrO₂, the changes in electrical conductivity after aging and reheating were explained by the formation and recovery of micro-cracks, respectively.     

Preparation of porous Y2SiO5 ceramics with relatively high compressive strength and ultra-low thermal conductivity by a TBA-based gel-casting method

Porous Y₂SiO₅ ceramics with relative high compressive strength (as high as 24.45 MPa) and ultra-low thermal conductivity (～0.08 W/m K) were successfully fabricated by a tert-butyl alcohol based gel-casting method. The formation mechanism of the 3D interconnected pores and the properties of the green body are discussed. The porosity, pore size, compressive strength and thermal conductivity could be controlled by varying the initial solid loading and the sintering temperature. When regulating the initial solid loading (from 20 to 50 wt%) and sintering temperature (from 1200 to 1500 °C), the porosity can be controlled between 47.74% and 73.93%, and the compressive strength and the thermal conductivity of porous Y₂SiO₅ ceramics varied from 3.34 to 24.45 MPa and from 0.08 to 0.55 W/m K, respectively. It should be noted that the porous Y₂SiO₅ ceramics with 30 wt% solid loading and sintering at 1400 °C had an open porosity of 61.80%, a pore size of 2.24 μm, a low room-temperature thermal conductivity of 0.17 W/m K and a relatively high compressive strength of 13.91 MPa, which make this porous Y₂SiO₅ ceramics suitable for applications in high-temperature thermal insulators.     

Low‐Temperature Sintering of β‐Spodumene Ceramics Using Li2O–GeO2 as a Sintering Additive

Low‐temperature sintering of β‐spodumene ceramics with low coefficient of thermal expansion (CTE) was attained using Li₂O–GeO₂ sintering additive. Single‐phase β‐spodumene ceramics could be synthesized by heat treatment at 1000°C using highly pure and fine amorphous silica, α‐alumina, and lithium carbonate powders mixture via the solid‐state reaction route. The mixture was calcined at 950°C, finely pulverized, compacted, and finally sintered with or without the sintering additive at 800°C–1400°C for 2 h. The relative density reached 98% for the sample sintered with 3 mass% Li₂O–GeO₂ additive at 1000°C. Its Young's modulus was 167 GPa and flexural strength was 115 MPa. Its CTE (from R.T. to 800°C) was 0.7 × 10^?6 K^?1 and dielectric constant was 6.8 with loss tangent of 0.9% at 5 MHz. These properties were excellent or comparative compared with those previously reported for the samples sintered at around 1300°C–1400°C via melt‐quenching routes. As a result, β‐spodumene ceramics with single phase and sufficient properties were obtained at about 300°C lower sintering temperature by adding Li₂O–GeO₂ sintering additive via the conventional solid‐state reaction route. These results suggest that β‐spodumene ceramics sintered with Li₂O–GeO₂ sintering additive has a potential use as LTCC for multichip modules.     

Controllable preparation of porous ZrB2–SiC ceramics via using KCl space holders

In this work, a novel and facile technique based on using KCl as space holders, along with partial sintering (at 1900 °C for 30 min), was explored to prepare porous ZrB₂–SiC ceramics with controllable pore structure, tunable compressive strength and thermal conductivity. The as-prepared porous ZrB₂–SiC samples possess high porosity of 45–67%, low average pore size of 3–7 μm, high compressive strength of 32–106 MPa, and low room temperature thermal conductivity of 13–34 W m⁻¹ K⁻¹. The porosity, pore structure, compressive strength and thermal conductivity of porous ZrB₂–SiC ceramics can be tuned simply by changing KCl content and its particle size. The effect of porosity and pore structure on the thermal conductivity of as-prepared porous ZrB₂–SiC ceramics was examined and found to be consistent with the classical model for porous materials. The poring mechanism of porous ZrB₂–SiC samples via adding pore-forming agent combined with partial sintering was also preliminary illustrated.     

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.     

Flash sintering of Al2O3–ZrO2 ceramics under alternating current electric field

In this study, the influence of alternating current (AC) electric field on flash sintering and microstructural evolution of alumina–zirconia (Al₂O₃–ZrO₂) composite was systematically investigated at furnace temperature of 800 °C. Compared with direct current (DC) electric field, AC electric field not only promoted densification and grain growth of Al₂O₃–ZrO₂ composite, but also improved the uniformity of microstructure of ceramics. Grain size of AC flash-sintered samples was found to be inversely related to electric field, and positive correlation was observed with current density limit. Dense Al₂O₃–ZrO₂ composite ceramic was fabricated via AC flash sintering under 60 mA mm^?2 at low furnace temperature within 120 s, and as-sintered samples exhibited relatively good mechanical properties. The mechanism involving synergistic effect of Joule heating and defects generation under the influence of electric field was proposed to explain rapid densification during AC flash sintering. These results indicate the feasibility of preparation of dense composite ceramic with homogeneous microstructure via AC flash sintering.     

Amorphous silicon and silicates-stabilized ZrO2 hollow fiber with low thermal conductivity and high phase stability derived from a cogon template

ZrO₂ fibers are used as refractory materials owing to their excellent thermal resistance and thermal stability. Natural cogon fiber is a type of hollow Gramineae fiber, and usually contains a small amount of amorphous silicon and silicates, such as SiO₂, MgSiO₃, CaSiO₄, and Al₂SiO₅, which can effectively avoid the phase transition of ZrO₂. In this study, hollow ZrO₂ fibers with remarkable thermal insulation and phase stability were synthesized using a cogon fiber template. The results showed that the final ZrO₂ fibers successfully inherited the hollow structure and amorphous substance from the cogon template. The hollow structure of the biomorphic ZrO₂ fiber helped prevent heat flow more efficiently compared to solid fibers and reduced the thermal conductivity to a significant extent. In addition, the amorphous silicon and silicates played an important role in the phase stability of tetragonal ZrO₂; the transformation from the tetragonal to monoclinic phase was avoided at room temperature and in humid environment.     

Li2ZrO3 based Li-ion conductors doped with halide ions & sintered in oxygen-deficient atmosphere

All-solid-state batteries have recently attracted much attention for their high energy density and safety. Li₂ZrO₃-based Li-ion conductors with high electrochemical stability have potential applications for electrolytes in all-solid-state batteries. In this work, comparative investigations of Li₂ZrO₃ and halogen doped Li₂ZrO₃ ceramics were conducted by sintering at 700 °C in air or in oxygen-deficient atmosphere which was induced by a simple setup covering with corundum crucible. The analysis of phase composition reveals that the undoped Li₂ZrO₃ ceramic sintered in air contains pure monoclinic phase, while halogen-doped Li₂ZrO₃ sintered in air and all ceramics sintered in oxygen-deficient atmosphere are simultaneously composed of monoclinic and tetragonal phases. Li₂ZrO₃ ceramic with tetragonal phases has higher conductivity (0.28 mS cm⁻¹ for undoped Li₂ZrO₃) than the pure monoclinic Li₂ZrO₃ (0.07 mS cm⁻¹), and halogen doping can further enhance the conductivity of Li₂ZrO₃ ceramics higher than 0.5 mS cm⁻¹ at room temperature.     

Fast-low temperature microwave sintering of ZrSiO4–ZrO2 composites

Today, many industrial applications require components that work under extreme conditions, especially at very high temperatures (>1200 °C) for a long time. An excellent combination of properties such as low thermal conductivity, low coefficient of thermal expansion and high chemical resistance are required for such applications. Advanced ceramic materials based on zircon-zirconia composites (ZrSiO₄–ZrO₂) possess these properties, thus making them attractive for, i.e., high-level radioactive waste immobilisation. The main drawback of these materials are the high temperatures and long residence times required to sinter them and obtain high densities, which entails high energy consumption and costs. Therefore, non-conventional microwave sintering is a very powerful and efficient technique capable of reducing sintering temperatures and holding times. The objective of this study is to evaluate the microwave sinterability of zircon-zirconia powders obtained by colloidal methods (80–20 vol% and 20–80 vol% ZrSiO₄–ZrO₂). A stability study of the phases present was carried out by X-ray diffraction and the mechanical and microstructural properties were evaluated in order to obtain the best materials with outstanding final properties.     

Pressureless sintering and tribological properties of WC–ZrO2 composites

In a recent work [Basu, B., Lee, J. H. and Kim, D. Y., Development of WC-ZrO₂ nanocomposites by spark plasma sintering. J. Am. Ceram. Soc. 2004 87(2), 317–319], the processing of ultrahard WC–ZrO₂ nanocomposites using spark plasma sintering is reported. In the present work, we investigate the processing and properties of WC–6 wt.% ZrO₂ composites, densified by pressureless sintering route. The densification of the WC–ZrO₂ composites was performed in the temperature range of 1500–1700 °C with varying time (1–3 h) in vacuum. The experimental results indicate that significantly high hardness of 22–23 GPa and moderate fracture toughness of ∼5 MPa m^1/2 can be obtained with 2 mol% Y–stabilized ZrO₂ sinter-additive, sintered at 1600 °C for 3 h. Furthermore, the friction and wear behavior of optimized WC–ZrO₂ composite is investigated on a fretting mode I wear tester. The tribological results reveal that a moderate coefficient of friction in the range from 0.15 to 0.5 can be achieved with the optimised composite. A transition in friction and wear with load is noted. The dominant mechanisms of material removal are tribochemical wear and spalling of tribolayer.     

Structure and properties of Al2O3-bonded porous fibrous YSZ ceramics fabricated by aqueous gel-casting

To meet requirements for high porosity and high strength, novel aqueous gel-casting process has been successfully developed to fabricate Al₂O₃-bonded porous fibrous YSZ ceramics with ρ-Al₂O₃ and YSZ fibers as raw materials. Microstructure, phase composition, apparent porosity, bulk density, thermal conductivity, and compressive strength of fabricated porous ceramics were investigated, and effects of fiber content on properties were discussed. According to results, bird nest 3D mesh with interlaced YSZ fibers and Al₂O₃ binder was formed, ensuring the ability to obtain high performance, lightweight ceramics. An increase in the number of YSZ fibers led to more complex interlaced arrangement of fibers and denser network structure of porous ceramics at retaining their stability. Furthermore, their apparent porosity and bulk density increased, whereas thermal conductivity and compressive strength decreased with increasing the fiber content. In particular, comparatively high porosity (71.1–72.7%), low thermal conductivity (0.209–0.503 W/mK), and relatively high compressive strength (3.45–4.24 MPa) were obtained for as-prepared porous ceramics, making them promising for applications in filters, thermal insulation materials, and separation membranes.