Synthesis of Hexacelsian Barium Aluminosilicate by a Solid-State Process

Synthesis of hexacelsian barium aluminosilicate (BaAl₂Si₂O₈ or BAS) from BaCO₃, Al₂O₃, and amorphous SiO₂ was examined. BaCO₃ can react with SiO₂ and Al₂O₃ to form barium silicates (Ba₂SiO₄ or B₂S, BaSiO₃ or BS, and BaSi₂O₅ or BS₂) and barium aluminate (BaAl₂O₄ or BA). It is shown that there are two competitive reactions leading to the formation of hexacelsian BAS. One involves a reaction between BS₂ and Al₂O₃ and the other involves a reaction between BA and SiO₂. In experiments with the model BS₂–Al₂O₃ and BA–SiO₂ systems it is shown that the reaction between BS₂ and Al₂O₃ is much faster than the reaction between BA and SiO₂. However, in the BAS system, Al₂O₃ suppresses the reactions which form BS₂ and instead reacts with B₂S and BS to form BA. The kinetics of hexacelsian BAS formation are greatly enhanced when BS₂ is made separately and fired with Al₂O₃ to yield BAS.     

Low-Temperature Synthesis of BaAl2O4/Aluminum-Bearing Composites by the Oxidation of Solid Metal-Bearing Precursors

BaAl₂O₄/aluminum-bearing composites have been synthesized via the low-temperature oxidation of Ba-Al precursors. Ba-Al powder mixtures that were prepared via high-energy vibratory milling were uniaxially pressed into bar-shaped specimens that were then exposed to a series of heat treatments in pure, flowing oxygen at temperatures up to 640°C. Oxidation at a temperature of 300°C resulted in the formation of barium peroxide (BaO₂). Additional heat treatment at a temperature of 550°C resulted in the consumption of BaO₂ and some aluminum to yield BaAl₂O₄ and Al₄Ba. The oxidation of Al₄Ba at a temperature of 640°C yielded additional BaAl₂O₄. Microstructural analyses revealed that a well-dispersed, co-continuous mixture of Al₂O₃-excess BaAl₂O₄ and 99.5% pure aluminum was produced.     

High-Toughness Ce-TZP/Al2O3 Ceramics with Improved Hardness and Strength

Simulataneous additions of SrO and Al₂O₃ to ZrO₂ (12 mol% CeO₂) lead to the in situ formation of strontium aluminate (SrO · 6Al₂O₃) platelets (∼0.5 μm in width and 5 to 10 μm in length) within the Ce-TZP matrix. These platelet-containing Ce-TZP ceramics have the strength (500 to 700 MPa) and hardness (13 to 14 GPa) of Ce-TZP/Al₂O₃ while maintaining the high toughness (14 to 15 MPa ± m^1/2) of Ce-TZP. Optimum room-temperature properties are obtained at SrO/Al₂O₃ molar ratios between 0.025 and 0.1 for ZrO₂ (12 mol% CeO₂) with starting Al₂O₃ contents ranging between 15 and 60 vol%. The role of various toughening mechanisms is discussed for these composite ceramics.     

Mullite Joining by the Oxidation of Malleable, Alkaline-Earth-Metal-Bearing Bonding Agents

Metallic Ba-Al-Si bonding agents have been used to produce all-ceramic, BaO-Al₂O₃-SiO₂ bonds between plates of mullite (Al₆Si₂O₁₃). Ba-Al-Si tapes (200 (μm thick) were fabricated by compaction and rolling of mechanically alloyed powder. The Ba-Al-Si tapes were placed between mullite plates and then oxidized by heating to a peak temperature of 1230°C in air. The oxidized tapes strongly adhered to the mullite plates at 25° and 1000°C, as indicated by the fracture morphologies obtained from compressive shear tests. Electron microscopy (EPMA, TEM) revealed that the bulk of the oxidized Ba-Al-Si tapes (away from the interfaces with mullite) consisted largely of the compound BaAl₂Si₂O₈, along with some BaSiO₃ and an amorphous, barium-rich aluminosilicate. The interface between the oxidized bonding agent and bulk mullite consisted of a mixture of BaAl₂Si₂O₈, Al₆Si₂O₁₃, A1₂O₃, BaAl₂O₄, and an amorphous, barium-bearing aluminosilicate.     

Processing and Characterization of BaTi4O9

BaTi₄O₉ powder prepared by calcining BaCO₃ and TiO₂ powders was sintered to over 97% of theoretical density. Less than 5% Ba₂Ti₉O₂₀ occurred as a second phase in "pure" BaTi₄O₉, and Al₂O₃ impurities from processing formed isolated hollandite (∼BaAl₂Ti₆O₁₆) grains, which were identified by fringes in bright-field TEM images. For pure BaTi₄O₉ at 1 MHz, a dielectric loss (tan δ) of 5 × 10⁻⁴ and dielectric constant of 39 were recorded. Hollandite impurities were found to increase tan δ by 2 orders of magnitude, whereas firing in oxygen decreased tan δ by an order of magnitude.     

Size Control of α-Al2O3 Platelets Synthesized in Molten Na2SO4 Flux

α-Al₂O₃ platelet powders were synthesized in molten Na₂SO₄ flux. The size of α-Al₂O₃ platelets was significantly reduced when partially decomposed rather than pure Al₂(SO₄)₃ was used as the source of Al₂O₃; a further reduction in the platelet size was realized through additional seeding with nanosized α-Al₂O₃ seeds. The addition of microsized α-Al₂O₃ platelet seeds significantly influenced the platelet morphology of the final powder, as well. The platelet size of the final powder was in direct proportion to the size of the platelet seeds, and was in reverse proportion to the cube root of the platelet seed content.     

Homogeneous ZrO2–Al2O3 Composite Prepared by Nano-ZrO2 Particle Multilayer-Coated Al2O3 Particles

ZrO₂–Al₂O₃ nanocomposite particles were synthesized by coating nano-ZrO₂ particles on the surface of Al₂O₃ particles via the layer-by-layer (LBL) method. Polyacrylic acid (PAA) adsorption successfully modified the Al₂O₃ surface charge. Multilayer coating was successfully implemented, which was characterized by ξ potential, particle size. X-ray diffraction patterns showed that the content of ZrO₂ in the final powders could be well controlled by the LBL method. The powders coated with three layers of nano-ZrO₂ particles, which contained about 12 wt% ZrO₂, were compacted by dry press and cold isostatically pressed methods. After sintering the compact at 1450°C for 2 h under atmosphere, a sintered body with a low pore microstructure was obtained. Scanning electron microscopy micrographs of the sintered body indicated that ZrO₂ was well dispersed in the Al₂O₃ matrix.     

Microstructural Characterization of Cofired Tungsten-Metallized High-Alumina Electronic Substrates

Microstructural characterization of a high-Al₂O₃ substrate containing cofired thick-film tungsten metallization, with particular emphasis on the metal/ceramic interface, was conducted. The substrate contained tabular Al₂O₃ grains surrounded by a continuous calcium magnesium aluminum silicate glass containing particles of monoclinic ZrO₂ and reduced rutile (TiO_{2- x} ). The metal/ceramic adhesion was caused by mechanical interlocking between the W and Al₂O₃ grains by the glass phase which penetrated the porous W layers during sintering; there was no interfacial reaction or diffusion zone. The mechanical properties of the W metallization did not limit interfacial strength. Heat treatments of the substrate at 1400 K in air and under vacuum resulted in the devitrification of the intergranular glass. The most abundant devitrification product was anorthite (CaAl₂Si₂O₈), accompanied by magnesium aluminate titanate, magnesium aluminate spinel, α-cristobalite (SiO₂), and α-cordierite (Mg₂Al₄Si₅O₁₈). In addition, small rutile particles precipitated within the Al₂O₃ grains.     

Analytical Electron Microscope Analysis of the Formation of BaO · 6Al2O3

The formation process of barium hexaaluminate (BaO 6Al₂O₃) from BaCO₃/γ-Al₂O₃ powders or hydrolyzed alkoxides was studied by analytical electron microscopy. Barium hexaaluminate is produced by a two-step solid-state reaction from BaCO₃ and Al₂O₃ via formation of BaO·Al₂O₃. Marked grain growth and inclusion of nonequilibrium phase were inevitable in this powder mixture process. However, in an alkoxide-derived precursor, homogeneous mixing of components is attained and hence the formation of BaO·6Al₂O₃ proceeds readily. Powders obtained by this latter route consisted of fine planar particles with a uniform size and retained a large surface area (20.2 m²/g) even after heating at 1300°C. Electron diffraction results implied that suppression of crystal growth along the c axis is the reason for the large surface area of BaO·6Al₂O₃.     

Synthesis, Sintering, Microstructure, and Mechanical Properties of Ceramics Made by Exothermic Reactions

Al₂O₃-TiC, Al₂O₃-TiC_0.5N_0.5, Al₂O₃-WC, Al₂O₃-SiC, and Al₂O₃-HfB₂ powders were synthesized by the aluminothermic reduction of oxides in the presence of carbon or boron. The reacted powders were milled to reduce the size of agglomerates and subsequently densified without applied pressure to near-theoretical density. Microstructures and mechanical properties of composites made from exothermically reacted powders were compared with similar ceramics made from commercially available powders. In situ sintering was possible in the Al₂O₃-TiC system using a closed graphite crucible to contain reaction gases. The synthesis of β -SiC at temperatures above 1400°C via the direct reaction of the elements (SHS) was compared with SiC made by the magnesiothermic reduction of SiO₂ in the presence of C after removing the MgO by leaching.     

Synthesis of AlN Powder by Carbothermal Reduction-Nitridation of Various Al2O3 Powders with CaF2

We investigated the effect of characteristics of raw Al₂O₃ powder on the synthesis of AlN by the carbothermal reduction-nitridation method, in which CaF₂ was added as a catalytic material. Four types of Al₂O₃ powders were selected. An Al₂O₃/C molar ratio of 0.29 was fixed, and the amount of CaF₂ was varied from 3 to 30 wt%. The carbothermal reduction-nitridation was conducted from 1350° to 1450°C in N₂ flow. The nitridation rate tended to increase with decreasing particle size of raw Al₂O₃ and was found to depend on the Al₂O₃ synthesizing method. The particle sizes of the synthesized AlN increased somewhat with increasing reaction temperatures. However, even though different particle sizes of Al₂O₃ powders were used, AlN powders synthesized under the same conditions exhibited almost the same particle size, round shape, and narrow size distribution. From XRD analysis, CaO·6Al₂O₃ and CaO·2Al₂O₃ were identified as intermediate compounds during these reactions. The above phenomena suggest that the synthesis mechanism of AlN powder by carbothermal reduction-nitridation of Al₂O₃ with CaF₂ addition was the nitridation of the intermediate compounds through the liquid phase of the system CaF₂-CaO·6Al₂O₃-CaO·2Al₂O₃.     

Synthesis and Properties of CaAl2O4-Coated AI2O3 Microcomposite Powders

Novel microcomposite powders, consisting of inert cores (αAL-Al₂O₃) surrounded by reactive cement-based coatings (CaAl₂O₄), were synthesized by a modified Pechini process. The evolution of the crystalline CaAl₂O₄ phase during calcination was studied using multiple analytical techniques, including DRIFTS,¹³C and ²⁷AlMAS FT-NMR, and XRD, for both pure CaAl₂O₄ and CaAl₂O₄-coated Al₂O₃ precursor powders. In both powders, decomposition proceeded via hydrocarbon chain scission and removal of ester groups at low temperatures ( T < 450°C), followed by the formation of inorganic carbonates at higher temperatures ( T > 450°C). These decomposition processes were accelerated by the underlying Al₂O₃ cores. Transmission electron microscopy (TEM) of the fully calcined powders showed that the inert αAL-Al₂O₃ particles were surrounded by relatively uniform CaAl₂O₄ coatings ranging in thickness from approximately 10 to 100 nm.     

Synthesis of (Mg,Si)Al2O4 Spinel from Aluminum Dross

The spinel (Mg,Si)Al₂O₄ was synthesized from aluminum dross using an induction synthesis method. X-ray diffraction analyses on products formed at different temperatures provided an understanding of the formation mechanism of the spinel. After removal of soluble components, the induction heating of the dross resulted first in the oxidation of some of the AlN component and the subsequent formation of the spinel by the following reaction: x SiO₂+ (1− x )MgO + [1−( x /3)]Al₂O₃+ (2 x /3)AlN = (Mg_{1− x} ,Si _x )Al₂O₄+ ( x /3)N₂( g ).     

Optical Properties of Zinc Aluminate, Zinc Gallate, and Zinc Aluminogallate Spinels

The optical spectra of zinc aluminate (ZnAl₂O₄), zinc gallate (ZnGa₂O₄), and zinc aluminogallate (ZnAlGaO₄) spinel powders were studied at wavelengths in the range of 250-900 nm using reflectance spectroscopy. The ZnAl₂O₄ and ZnGa₂O₄ powders were synthesized by using conventional ceramic processing techniques and had systematic variations in the molar ratio of ZnO to M₂O₃ (M = Al or Ga). The cubic spinel crystal structure of each composition was confirmed via powder X-ray diffractometry. The ZnAl₂O₄ powders showed optical properties in the ultraviolet wavelength region and had combined characteristics that were similar to that of ZnO (wurtzite structure) and Al₂O₃ (corundum structure), which result from the similar local environments of the zinc and aluminum cations within the cubic spinel crystal structure. A mechanically induced optical absorption (optomechanical effect) in the ultraviolet wavelength region was also observed in ZnAl₂O₄. The ZnGa₂O₄ powder followed a similar behavior, with the exception that the optomechanical effect did not occur in the gallate. The ZnAlGaO₄ showed optical spectra that were intermediate to that of the endpoint compositions.     

Chemical Durability of Lead-Oxide-Based, Thick-Film Binder Glasses

Lead-oxide-bearing glasses are incompatible with aluminum nitride metallizations, and federal legislation is recommending their replacement in thick-film electronics and labeling. To evaluate alternatives, the benchmark chemical durabilities of the lead borosilicate and lead aluminate thick-film binder glasses have been determined. Aluminum oxide (Al₂O₃) benefits water durability moderately, acid durability substantially, and basic durability indistinguishably. The rate of attack in water is approximately two orders of magnitude greater than for soda–lime glass. The thermal contractions of the glasses are compatible. An apparent, spontaneous phase separation in the Al₂O₃-free glass is suppressed by Al₂O₃ that is included as a batch component.     

Synthesis and Preparation of Lanthanum Aluminate Target for Radio-Frequency Magnetron Sputtering

A polycrystalline LaAlO₃ target for the radio-frequency (rf) magnetron sputterings of LaAlO₃ thin films has been prepared. These films serve as a buffer layer for high- T _c YBa₂Cu₃O_{7– x} superconducting thin films on Si. Synthesis of lanthanum aluminate powder from a mixture of La₂O₃ and Al₂O₃ powders was performed by calcining from 1000° to 1600°C in air. Characterization of the calcined powders by X-ray diffraction indicates that full development of LaAlO₃ phase was evident in the sample calcined for 3 h at 1600°C in air. A polycrystalline LaAlO₃ target was prepared by heat treatment at 1500°C for 2 h in air after pressureless sintering at 1750°C for 3 h in Ar. Thin films of the LaAlO₃ on Si (100) were obtained by rf magnetron sputtering using the target and oxygen-annealing the as-deposited films.     

Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

Calcium Phosphate Cements Prepared by Acid–Base Reaction

We investigated the characteristics of calcium phosphate cements (CPC) prepared by an exothermic acid–base reaction between NH₄H₂PO₄-based fertilizer (Poly-N) and calcium aluminate compounds (CAC), such as 3CaO · Al₂O₃ (C₃A), CaO · Al₂O₃ (CA), and CaO · 2Al₂O₃ (CA₂), in a series of integrated studies of reaction kinetics, interfacial reactions, in-situ phase transformations, and microstructure development. Two groups were compared: untreated and hydrothermally treated CPC specimens. The extent of reactivity of CAC with Poly-N at 25°C was in the following order: CA > C₃A ≫ CA₂. The formation of a NH₄CaPO₄· x H₂O salt during this reaction was responsible for the development of strength in the CPC specimens. The in-situ phase transformation of amorphous NH₄CaPO₄· x H₂O into crystalline Ca₅(PO₄)₃(OH) and the conversion of hydrous Al₂O₃ gel →γ-AIOOH occur in cement bodies during exposure in an autoclave to temperatures up to 300°C. This phase transformation significantly improved mechanical strength.     

Conversion from β-Ce2O3·11Al2O3 to α-Al2O3 in Tetragonal ZrO2 Matrix

Composites of β-Ce₂O₃·11Al₂O₃ and tetragonal ZrO₂ were fabricated by a reductive atmosphere sintering of mixed powders of CeO₂, ZrO₂ (2 mol% Y₂O₃), and Al₂O₃. The composites had microstructures composed of elongated grains of β-Ce₂O₃·11Al₂O₃ in a Y-TZP matrix. The β-Ce₂O₃·11Al₂O₃ decomposed to α-Al₂O₃ and CeO₂ by annealing at 1500°C for 1 h in oxygen. The elongated single grain of β-Ce₂O₃·11Al₂O₃ divided into several grains of α-Al₂O₃ and ZrO₂ doped with Y₂O₃ and CeO₂. High-temperature bending strength of the oxygen-annealed α-Al₂O₃ composite was comparable to the β-Ce₂O₃·11Al₂O₃ composite before annealing.     

Synthesis of α-Alumina Hexagonal Platelets Using a Mixture of Boehmite and Potassium Sulfate

Single-crystal α-alumina (Al₂O₃) hexagonal platelets with a diameter of about 200 nm and 25 nm in thickness were synthesized by heating a mixture of boehmite and potassium sulfate at 1000°C for 2 h and washing with water. The potassium sulfate addition effects on the Al₂O₃ phase and morphology were investigated using differential thermal analysis (DTA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). It was found that potassium sulfate addition helps in the formation of single-crystal α-Al₂O₃ hexagonal platelets and promotes phase transformation from intermediate γ-Al₂O₃ to α-Al₂O₃.