Uniformly Porous Composites with 3-D Network Structure (UPC-3D) for High-Temperature Filter Applications

Uniformly porous composites with 3-D network structure (UPC-3D) have been recently developed via a pyrolytic reactive sintering process, which takes advantage of the evolved CO₂ gas from a decomposing carbonate source (e.g., dolomite, CaMg(CO₃)₂) and does not require any additional pore-forming agent nor long-time burning-out process. Through liquid formation via LiF doping, strong necks are formed between constituent particles before completion of the pyrolysis of carbonate, resulting in the formation of a strong 3-D network structure. The pore size distribution is very narrow (with typical pore size: ∼1 μm), and the porosity was controllable (∼30–60%) by changing the sintering temperature. This article presents the development details of UPC-3D, and reports the recent findings in CaZrO₃/MgAl₂O₄ system, which will be one of the more promising systems for practical applications.     

Reactive Synthesis of a Porous Calcium Zirconate/Spinel Composite with Idiomorphic Spinel Grains

Porous CaZrO₃/MgAl₂O₄ composites were synthesized in air by pressureless reactive sintering of an equimolar mixture of dolomite (CaMg(CO₃)₂), monoclinic zirconia ( m -ZrO₂), and α-alumina powders, with a 0.5 wt% lithium fluoride additive. The reaction behavior of the mixed powders (with/without LiF additive) was studied using high-temperature X-ray diffraction. A bulk porous composite resulted from sintering at 1300°C for 2 h (in a nearly closed container, so as to increase the LiF-doping effect), which consisted of fine grains (CaZrO₃ and MgAl₂O₄, ∼0.5–1 μm) and well-grown idiomorphic ones (MgAl₂O₄ octahedra ∼ 2–4 μm). The idiomorphic spinel grains were located around the inner walls of relatively large pores. The composite showed appreciably high bending strength (σ_f= 110 ± 8 MPa for a porosity of 31%). The porous CaZrO₃/MgAl₂O₄ composites can be applied as high-temperature filters and lightweight structural components.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

Manufacturing Nano-Diphasic Materials from Natural Dolomite: In Situ Observation of Nanophase Formation Behavior

Decomposition of the mixed carbonate mineral dolomite, CaMg(CO₃)₂, and formation of CaO/MgO nanopolycrystals were observed in situ using a field emission scanning electron microscope with a hot stage. The pyrolysis behavior of dolomite was studied with an in situ high–temperature X–ray powder diffraction technique. Nano–sized particles formed during pyrolysis then sintered into CaO/MgO nanopolycrystals. Dense bulk–form CaO/MgO and CaZrO₃/MgO nanocomposites were successfully fabricated using reactive hot–pressing from dolomite and dolomite/zirconia mixed powders, respectively.     

Dense Al2O3/ZrO2 Particulate Composites by Free Sintering of Coated Powders

Composite powders, prepared by coating coarse ZrO₂ particles with fine Al₂O₃ powder using a chemical precipitation technique, were compacted and sintered freely at a constant heating rate of 4°C/min to ∼1600°C. Composites containing up to ∼30 vol% inclusions were sintered to nearly full density under the same conditions used for the unreinforced matrix. Furthermore, the sintering kinetics were not influenced significantly by the inclusion volume fraction. The sinterability of the composites formed from the coated powders was significantly better than that for similar composites formed from mechanically mixed powders. The present data provide a further demonstration that the use of coated powders may have widespread applicability for the fabrication, by free sintering, of dense ceramic particulate composites.     

Low-Temperature Fabrication of Transparent Yttrium Aluminum Garnet (YAG) Ceramics without Additives

A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.9H₂O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ∼1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of ≤1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C in 1 h without additives.     

Preparation, Sintering, and Properties of Translucent Er2O3-Doped Tetragonal Zirconia

Tetragonal zirconia doped with 3 mol% Er₂O₃ was prepared by the gel-precipitation wet-chemical method. The compaction response of ultrafine (∼8.5 nm), calcined, deagglomerated powders was studied. Initial sintering was studied using both isothermal and nonisothermal techniques, and an activation energy of 270 ± 40 kJ/mol was obtained; therefore, grain-boundary diffusion was probably the predominant mechanism in the sintering stage. The microstructural development in the high-temperature-aged, sintered samples and its effect on density and mechanical properties were also studied. A theoretically dense body of tetragonal zirconia solid solution of 3 mol% Er₂O₃–ZrO₂, obtained from sintering below 1400°C, was translucent.     

Preparation and Characterization of Fine-Grained 3Y-TZP/BaFe12O19 In Situ Composites

Composites of BaFe₁₂O₁₉ particles dispersed throughout a 3-mol%-yttria-doped zirconia (3Y-TZP) matrix have been prepared using the pressureless reactive sintering of 3Y-TZP, BaCO₃, and γ-Fe₂O₃ powders. The reaction behavior of the mixed powder was studied with an in situ , high-temperature powder X-ray diffraction technique. The composite that was sintered at 1300°C consisted of submicrometer-sized 3Y-TZP grains and BaFe₁₂O₁₉ particles; the size of the 3Y-TZP grains was ∼100-300 nm, and the size of the BaFe₁₂O₁₉ particles was ∼200-500 nm. Based on the grain size, most of the BaFe₁₂O₁₉ grains presumably had a single-magnetic-domain structure. The 3Y-TZP/20-wt%-BaFe₁₂O₁₉ composite showed high magnetization and coercivity values, despite the low concentration of ferromagnetic phase. Preliminary mechanical tests revealed that the composite possessed moderately good mechanical properties.     

The Study on Carbon Nanofiber (CNF)-Dispersed B4C Composites

Carbon nanofiber (CNF)-dispersed B₄C composites have been synthesized and consolidated directly from mixtures of elemental raw powders by pulsed electric current pressure sintering (1800°C/10 min/30 MPa). A 15 vol% CNF/B₄C composite with ∼99% of dense homogeneous microstructures (∼0.40 μm grains) revealed excellent mechanical properties at room temperature and high temperatures: a high bending strength (σ_b) of ∼710 MPa, a Vickers hardness ( H _v) of ∼36 GPa, a fracture toughness ( K _{I C} ) of ∼7.9 MPa m^1/2, and high-temperature σ_b of 590 MPa at 1600°C in N₂. Interfaces between the CNF and the B₄C matrix were investigated using high-resolution transmission electron microscopy, EDS, and electron energy-loss spectroscopy.     

In Situ Synthesis of Ultrafine ZrB2–SiC Composite Powders and the Pressureless Sintering Behaviors

Ultrafine ZrB₂–SiC composite powders have been synthesized in situ using carbothermal reduction reactions via the sol–gel method at 1500°C for 1 h. The powders synthesized had a relatively smaller average crystallite size (<200 nm), a larger specific surface area (∼20 m²/g), and a lower oxygen content (∼1.0 wt %). Composites of ZrB₂+20 wt% SiC were pressureless sintered to ∼96.6% theoretical density at 2250°C for 2 h under an argon atmosphere using B₄C and Mo as sintering aids. Vickers hardness and flexural strength of the sintered ceramic composites were 13.9±0.3 GPa and 294±14 MPa, respectively. The microstructure of the composites revealed that elongated SiC grain dispersed uniformly in the ZrB₂ matrix. Oxidation from 1100° to 1600°C for 30 min showed no decrease in strength below 1400°C but considerable decrease in strength with a rapid weight increment was observed above 1500°C. The formation of a protective borosilicate glassy coating appeared at 1400°C and was gradually destroyed in the form of bubble at higher temperatures.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Fabrication and Microstructure Characterization of Ti3SiC2 Synthesized from Ti/Si/2TiC Powders Using the Pulse Discharge Sintering (PDS) Technique

Ti/Si/2TiC powders were prepared using a mixture method (M) and a mechanical alloying (MA) method to fabricate Ti₃SiC₂ at 1200°–1400°C using a pulse discharge sintering (PDS) technique. The results showed that the Ti₃SiC₂ samples with <5 wt% TiC could be rapidly synthesized from the M powders; however, the TiC content was always >18 wt% in the MA samples. Further sintering of the M powder showed that the purity of Ti₃SiC₂ could be improved to >97 wt% at 1250°–1300°C, which is ∼200°–300°C lower than that of sintered Ti/Si/C and Ti/SiC/C powders using the hot isostatic pressing (HIPing) technique. The microstructure of Ti₃SiC₂ also could be controlled using three types of powders, i.e., fine, coarse, or duplex-grained, within the sintering temperature range. In comparison with Ti/Si/C and Ti/SiC/C mixture powders, it has been suggested that high-purity Ti₃SiC₂ could be rapidly synthesized by sintering the Ti/Si/TiC powder mixture at relatively lower temperature using the PDS technique.     

Effect of CO32− Incorporation on the Mechanical Properties of Wet Chemically Synthesized β-Tricalcium Phosphate (TCP) Ceramics

Calcium-deficient hydroxyapatite (CDHA) powders were synthesized by a wet chemical precipitation method using Ca(OH)₂ and H₃PO₄ solutions. Single-phase β-tricalcium phosphate (β-TCP) powder with a molar (Ca+Mg)/P ratio of 1.5 was obtained after calcination of CDHA synthesized under vacuum. During synthesis in air, CO₂ can be adsorbed and HBO₄²⁻ is partly substituted by CO₃²⁻, resulting in a lower phosphorous content and consequently an increase of the molar (Ca+Mg)/P ratio to 1.53. A two-phase β-TCP powder containing 20 wt% hydroxyapatite (HA) was obtained after calcination. Samples prepared from β-TCP powders synthesized under vacuum achieved a compressive strength of 301±23 MPa at 99.6% fractional density, while TCP/HA samples prepared in air achieved a maximum compressive strength of 132±29 MPa at 91.7% fractional density. This decrease in strength can be correlated to the porosity retaining due to CO₂ release during sintering and residual tensile stresses in the TCP matrix caused by the thermal expansion mismatch of β-TCP and HA.     

Pressureless Sintering of High-Density ZrB2–SiC Ceramic Composites

Ultra-high-temperature ceramic composites of ZrB₂ 20 wt%SiC were pressureless sintered under an argon atmosphere. The starting ZrB₂ powder was synthesized via the sol–gel method with a small crystallite size and a large specific surface area. Dry-pressed compacts using 4 wt% Mo as a sintering aid can be pressureless sintered to ∼97.7% theoretical density at 2250°C for 2 h. Vickers hardness and fracture toughness of the sintered ceramic composites were 14.82±0.25 GPa and 5.39±0.13 MPa·m^1/2, respectively. In addition to the good sinterability of the ZrB₂ powders, X-ray diffraction and scanning electron microscopy results showed that Mo formed a solid solution with ZrB₂, which was believed to be beneficial for the densification process.     

Fabrication of Textured Bi3NbTiO9 Ceramics

Highly textured Bi₃NbTiO₉ ceramics are fabricated by normal sintering from molten salt-synthesized plate-like crystallites. Fine Bi₃NbTiO₉ plate-like crystallites (∼1 μm) not only facilitate the densification, but also enhance texture in Bi₃NbTiO₉ ceramics. Weak-agglomerated platelets exhibit higher sinterability and can be densified at a temperature as low as 1000°C, which is about 100°C lower than that of equiaxed powders prepared by directly calcining Bi₃NbTiO₉ precursor. Meanwhile, the orientation degree of textured Bi₃NbTiO₉ ceramics increases with sintering temperature. Highly oriented Bi₃NbTiO₉ (orientation degree of ∼0.91) ceramic with a relative density of ∼92% is obtained at 1150°C. Because of the oriented grain microstructure, textured Bi₃NbTiO₉ ceramic exhibits anisotropic electrical properties.     

Development of WC–ZrO2 Nanocomposites by Spark Plasma Sintering

In the present work, we report the processing of ultrahard tungsten carbide (WC) nanocomposites with 6 wt% zirconia additions. The densification is conducted by the spark plasma sintering (SPS) technique in a vacuum. Fully dense materials are obtained after SPS at 1300°C for 5 min. The sinterability and mechanical properties of the WC–6 wt% ZrO₂ materials are compared with the conventional WC–6 wt% Co materials. Because of the high heating rate, lower sintering temperature, and short holding time involved in SPS, extremely fine zirconia particles (∼100 nm) and submicrometer WC grains are retained in the WC–ZrO₂ nanostructured composites. Independent of the processing route (SPS or pressureless sintering in a vacuum), superior hardness (21–24 GPa) is obtained with the newly developed WC–ZrO₂ materials compared with that of the WC–Co materials (15–17 GPa). This extremely high hardness of the novel WC–ZrO₂ composites is expected to lead to significantly higher abrasive-wear resistance.     

Preparation of Ca3Co4O9 and Improvement of its Thermoelectric Properties by Spark Plasma Sintering

The precursor powders of Ca₃Co₄O₉ were synthesized by a sol–gel method. The results of X-ray diffraction and thermogravimetric and differential thermal analyses patterns indicate that pure Ca₃Co₄O₉ powders could be obtained by calcining the precursor at 800°C for 2 h. High dense Ca₃Co₄O₉ ceramic samples (∼99% of theoretical density) were prepared by the spark plasma sintering (SPS) method. Compared with the conventional sintering (CS), the SPS samples exhibit much higher electrical conductivity and power factor which are respectively about 118 S/cm and 3.51 × 10⁻⁴ W·(m·K²)⁻¹. The SPS method is greatly effective for improving the thermoelectric properties of Ca₃Co₄O₉ oxide ceramics.     

Processing and Dielectric Properties of Sillenite Compounds Bi12MO20−δ (M = Si, Ge, Ti, Pb, Mn, B1/2P1/2)

Six sillenite compounds Bi₁₂MO_20-δ (M = Si, Ge, Ti, Pb, Mn, B_1/2P_1/2) were synthesized, and the resulting single-phase powders were then sintered to obtain ∼97% dense ceramics. An analysis of their microwave dielectric properties, performed at ∼5.5 GHz, revealed a permittivity of ∼40 for all six compounds. The temperature coefficient of resonant frequency was the lowest for the Pb analogue (−84 ppm/K) and was found to increase with increasing ionic radius of the B-site ion to a value of −20 ppm/K for the Bi₁₂SiO₂₀ and Bi₁₂(B_1/2P_1/2)O₂₀ compounds. The Q × f value is a maximum for Bi₁₂SiO₂₀ and Bi₁₂GeO₂₀ with 8100 and 7800 GHz, respectively. The dielectric properties of the sillenites have been correlated with the structure of the oxygen network of the sillenite crystal lattice. As a result of its low sintering temperature (850°C), chemical compatibility with silver, low dielectric losses, and temperature-stable permittivity, the Bi₁₂SiO₂₀ compound is a suitable material for applications in low-temperature cofiring ceramic (LTCC) technology.     

Reaction Processing of Al2O3 Composites Containing Iron and Iron Aluminides

Different Fe-Al₂O₃ and FeAl-Al₂O₃ composites with metallic contents up to 30 vol% have been fabricated via reaction processing of Al₂O₃, Fe, and Al mixtures. Low Al contents (<∼10 vol%) within the starting mixture lead to composites consisting of Fe embedded in an Al₂O₃ matrix, whereas aluminide-containing Al₂O₃ composites result from powder mixtures with higher Al contents. In both cases, densification up to 98% TD can be achieved by pressureless sintering in inert atmosphere at moderate temperatures (1450°-1500°C). The proposed reaction sintering mechanism includes the reduction of native oxide layers on the surface of the Fe particles by Al and, in the case of mixtures with high Al contents, aluminide formation followed by sintering of the composites. Density and bending strengths of the reaction-sintered composites depend strongly on the Al content of the starting mixture. In the case of samples containing elemental Fe, crack path observations indicate the potential for an increase of fracture toughness, even at room temperature, by crack bridging of the ductile Fe inclusions.     

Sintering Kinetics of a MgAl2O4 Spinel Doped with LiF

The sintering kinetics of a pure magnesium aluminate spinel, MgAl₂O₄, and that doped with LiF were determined through the use of the master sintering curve technique developed by Su and Johnson. ²⁰ Powders with 0%, 0.5%, and 1.0% by mass LiF were densified in a vacuum hot press under a range of unaxial pressures. After the sintering mechanisms in each temperature and pressure regime were determined, an optimized vacuum hot-pressing schedule was formulated for spinel powders doped with 1.0% by mass of LiF. In addition to forming a transient liquid phase, the presence of LiF leads to the formation of oxygen vacancies that promote late-stage sintering in MgAl₂O₄.