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.     

Low-Temperature Synthesis of Nanocrystalline Yttrium Aluminum Garnet Powder Using Triethanolamine

Nanocrystalline yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized by pyrolysis of complex compounds of aluminum and yttrium with triethanolamine [(HOCH₂CH₂)₃N, (TEA)]. Loose and porous precursor was obtained on complete dehydration of the metal ion–triethanolamine complexes. Pure YAG powder was obtained by calcination of the precursor at 950°C. The precursor was characterized by simultaneous thermogravimetry, differential scanning calorimetry, and mass spectra analyses (TG–DSC–MS). The heat-treated powders were characterized by X-ray diffractometry (XRD), specific surface area measurements, and transmission electron microscopy (TEM). The average crystallite size as determined from X-ray line broadening and transmission electron microscopy studies was ∼40 nm. The effects of the calcination temperature and the ratio of triethanolamine to mixed metal ions were also studied.     

Fabrication of Translucent Magnesium Aluminum Spinel Ceramics

A precursor for magnesium aluminum spinel powder, composed of crystalline ammonium dawsonite hydrate (NH₄Al(OH)₂CO₃·H₂O) and hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O) phases, was synthesized via precipitation, using ammonium bicarbonate as the precipitant. The precursor was characterized by differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. Reactive spinel powder, which could be densified to translucency under vacuum at 1750°C in 2 h without additives, was obtained by calcining the precursor at 1100°C for 2 h.     

Synthesis and Processing of Barium Hexaaluminogallates

Barium hexaaluminogallate was synthesized by the mixed-oxide method and the coprecipitation method. Barium hexaaluminate and barium hexagallate were found to form barium hexaaluminogallate, Ba(Al,Ga)₁₂O₁₉, at 1400°C. BaCo(Al,Ga)₁₁O₁₉ of magnetoplumbite structure was formed from a mixture of metal oxides at 1200°C for 6 h. Co was successfully introduced to barium hexaaluminogallate, although the synthesis of BaCoAl₁₁O₁₉ is quite difficult via solid-state reaction of oxide powders. Anisotropic BaCo(Al,Ga)₁₁O₁₉ particles crystallized at 1100°C for 2 h through the coprecipitation method using metal nitrates and ammonium carbonate. BaCo(Al,Ga)₁₁O₁₉ supported on a cordierite honeycomb had the ability to reduce NO _x with methane over 500°C in the presence of excess oxygen.     

Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders

Praseodymium-doped ceria (CeO₂) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH₄)_0.16Ce_{1− x} Pr _x (CO₃)_1.58·H₂O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO₂ lattice, because of the oxidation of Pr³⁺ to smaller Pr⁴⁺, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.     

A Novel Way to Synthesize Yttrium Aluminum Garnet from Metal–Inorganic Precursors

Yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized by sol–gel processing from the stoichiometric amounts of aluminum pellets, Y(NO₃)₃·6H₂O, and Al(NO₃)₃·9H₂O or AlCl₃·6H₂O, with suitable kinds of acid (citric acid, acetic acid, etc.) as catalysts. Polycrystalline YAG powder was obtained by drying the YAG precursor followed by calcination at temperatures above 900°C. Thermogravimetry/differential thermal analysis and Fourier transform infrared specotrscopic analyses in air showed an exothermic peak at ∼900°C, attributed to the formation of a polycrystalline YAG phase and weight loss of 60% at 1000°C, caused by the decomposition of hydroxyl and NO₃⁻, etc. X-ray diffraction analysis showed that YAG can be formed at 900°C, and no other intermediate was observed. In particular, the YAG sol can be used for dry-spinning fibers with the aid of some organic polymer.     

Synthesis of Bulk, Dense, Nanocrystalline Yttrium Aluminum Garnet from Amorphous Powders

Amorphous powders of Al₂O₃—37.5 mol% Y₂O₃ (yttrium aluminum garnet (YAG)) were prepared by coprecipitation, decomposed at 800°C, and hot-pressed uniaxally at low temperature (600°C) and a moderate pressure (750 MPa). Optimum conditions yielded microstructures with only 2% porosity and partial crystallization of YAG. Further processing using high quasi-hydrostatic pressure (1 GPa) at 1000°C enabled the production of fully crystallized YAG with >96% relative density and a nanocrystalline grain size of ∼70 nm.     

Reactive Ceria Nanopowders via Carbonate Precipitation

Nanocrystalline CeO₂ powders have been successfully synthesized via a carbonate precipitation method, using ammonium carbonate (AC) as the precipitant and cerium nitrate hexahydrate as the cerium source. The AC/Ce³⁺ molar ratio ( R ) affects significantly precursor properties, and spherical nanoparticles can be produced only in a narrow range of 2 < R ≤ 3. The precursor, having an approximate composition of Ce(OH)CO₃·2.5H₂O, decomposes to CeO₂ at temperatures ≥300°C. The CeO₂ powder calcined at 700°C exhibits high reactivity and can be densified to >99% of theoretical at 1000°C.     

Transparent Nd:YAG Ceramics Fabricated Using Nanosized γ-Alumina and Yttria Powders

Neodymium (Nd):Y₃Al₅O₁₂ (YAG) ceramics of excellent transparency have been fabricated by solid-reactive sintering, using nanosized γ-Al₂O₃ and Y₂O₃ powders as the starting materials. Reaction sequences and sintering behaviors of the powder mixture were characterized by X-ray diffractometry and dilatometry. One feature of the solid reactions involving γ-Al₂O₃ is the occurrence of hexagonal YAlO₃, which is unstable and transforms to perovskite YAlO₃ (YAP) upon further heating. Because of the high reactivities of the starting nanopowders, a complete conversion of the powder mixture to YAG has been achieved by heating at 1300°C for 4 h, via Y₄Al₂O₉, hexagonal YAlO₃, and YAP phases. In-line transmittances of the 1.5 at.% Nd:YAG ceramics (doped with 0.5 wt% of tetraethyl orthosilicate) at 700 nm are 81.0% and 65.7% after vacuum sintering at 1700° and 1600°C for 5 h, respectively.     

A Simple Way to Synthesize Yttrium Aluminum Garnet by Dissolving Yttria Powder in Alumina Sol

A precursor for Y₃Al₅O₁₂ was synthesized as a YAG sol by simply dissolving Y₂O₃ powder in an alumina sol. Phase-pure Y₃Al₅O₁₂ powder was obtained by precipitating the YAG sol with an aqueous dilute ammonia solution followed by calcination at 1100°C. TG/DTA analysis showed an exotherm at 938°C attributed to formation of YAG phase and weight loss of 44% at 1000°C. XRD and FT-IR analysis showed that phase-pure YAG can be formed through noncrystalline and metastable hexagonal YAlO₃ without forming either yttrium or aluminum formate intermediate.     

Solid-State Reactive Sintering of Transparent Polycrystalline Nd:YAG Ceramics

Transparent polycrystalline Nd:YAG ceramics were fabricated by solid-state reactive sintering a mixture of commercial Al₂O₃, Y₂O₃, and Nd₂O₃ powders. The powders were mixed in methanol and doped with 0.5 wt% tetraethoxysilane (TEOS), dried, and pressed. Pressed samples were sintered from 1700° to 1850°C in vacuum without calcination. Transparent fully dense samples with average grain sizes of ∼50 μm were obtained at 1800°C for all Nd₂O₃ levels studied (0, 1, 3, and 5 at.%). The sintering temperature was little affected by Nd concentration, but SiO₂ doping lowered the sintering temperature by ∼100°C. Abnormal grain growth was frequently observed in samples sintered at 1850°C. The Nd concentration was determined by energy-dispersive spectroscopy to be uniform throughout the samples. The in-line transmittance was >80% in the 350–900 nm range regardless of the Nd concentration. The best 1 at.% Nd:YAG ceramics (2 mm thick) achieved 84% transmittance, which is equivalent to 0.9 at.% Nd:YAG single crystals grown by the Czochralski method.     

Sol–Gel Combustion Synthesis of Nanocrystalline YAG Powder from Metal-Organic Precursors

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) nanocrystalline powders were synthesized by a novel sol–gel combustion process. Yttrium acetate and aluminum sec-butoxide were used as the precursors and triethanolamine was used as the chelating agent and fuel. Thermal and crystallization behaviors of the YAG precursor powders were investigated by thermal gravimetric differential thermal analysis (DTA), Fourier transform infrared spectrum, and X-ray diffraction. The combustion-synthesized powders are amorphous and transform to a pure YAG crystalline phase at 900°C. The crystallization activation energy of amorphous YAG precursor was investigated by variable heating rate DTA. The calculated activation energy is 58.9 KJ/mol. The average crystalline size of heat-treated YAG powders at 900°C is ∼20 nm.     

Double Precursor Solution Coating Approach for Low-Temperature Sintering of [Pb(Mg1/3Nb2/3)O3]0.63[PbTiO3]0.37 Solids

Lead-based piezoelectric ceramics typically require sintering temperatures higher than 1000°C at which significant lead loss can occur. Here, we report a double precursor solution coating (PSC) method for fabricating low-temperature sinterable polycrystalline [Pb(Mg_1/3Nb_2/3)O₃]_0.63-[PbTiO₃]_0.37 (PMN–PT) ceramics. In this method, submicrometer crystalline PMN powder was first obtained by dispersing Mg(OH)₂-coated Nb₂O₅ particles in a lead acetate/ethylene glycol solution (first PSC), followed by calcination at 800°C. The crystalline PMN powder was subsequently suspended in a PT precursor solution containing lead acetate and titanium isopropoxide in ethylene glycol to form the PMN–PT precursor powder (second PSC) that could be sintered at a temperature as low as 900°C. The resultant d ₃₃ for samples sintered at 900°, 1000°, and 1100°C for 2 h were 600, 620, and 700 pm/V, respectively, comparable with the known value. We attributed the low sintering temperature to the reactive sintering nature of the present PMN–PT precursor powder. The reaction between the nanosize PT and the submicrometer-size PMN occurred roughly in the same temperature range as the densification, 850°–900°C, thereby significantly accelerating the sintering process. The present PSC technique is very general and should be readily applicable to other multicomponent systems.     

Synthesis of Nd:YVO4 Thin Films by a Sol-Gel Method

Nd: YVO₄ powders and thin films were successfuly synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of Y(OEt)₃, VO(O i Pr)₃, and Nd(OEt)₃ in 2-methoxyethanol. The precursor was a mixture of vanadium and yttrium double alkoxide. Precursor films were prepared by dip coating and crystallization to single-phase YVO₄ at 500°C. Nd:YVO₄ films were crystallized with (200) preferred orientation on glass substrates, which showed the characteristic optical absorption of neodymium.     

10-mol%-Gd2O3-Doped CeO2 Solid Solutions via Carbonate Coprecipitation: A Comparative Study

10mol% Gd₂O₃-doped CeO₂ solid solutions (20GDC) have been synthesized via carbonate coprecipitation using ammonium bicarbonate (AHC) and urea as the precipitants. The precursors and the resultant oxide powders were characterized via chemical analysis, X-ray diffractometry (XRD), Brunauer–Emmett–Teller (BET) analysis, and high-resolution scanning electron microscopy (HRSEM). Sinterabilities of the 20GDC oxides in air were studied by constant-rate-of-heating (CRH) sintering and the conventional ramp-and-holding sintering methods. The precursor processed by both methods is hydroxyl carbonate but shows quite different particle morphologies in the two cases. Highly sinterable 20GDC oxides that can be densified to >99% of the theoretical at 1050°C within 4 h have been obtained via the AHC method.     

Transparent Cr4+-Doped YAG Ceramics for Tunable Lasers

Transparent Cr⁴⁺:YAG (Y₃Al_SO₁₂) ceramics doped with Ca and Mg as counterions and SiO₂ as a sintering aid were fabricated by a solid-state reaction method using high-purity powders of Al₂O₃, Y₂O₃, and Cr₂O₃. The mixed powder compacts were sintered at 1750°C for 10 h in oxygen, or 1750°C for 10 h under vacuum, and then annealed at 1400°C for 10 h in oxygen. Cr-doped YAG ceramics sintered in oxygen had a brown color and characteristic absorption by Cr⁴⁺ ions, whereas these YAG ceramics sintered under different conditions (vacuum + oxygen) had a green color and absorption at ∼590 and 430 nm by Cr³⁺ ions. The absorption behavior of YAG ceramics sintered in oxygen was almost equivalent to that of Cr⁴⁺:YAG single crystals fabricated by the Czochralski method.     

Agglomeration Control of Nd:YAG Nanoparticles Via Freeze Drying for Transparent Nd:YAG Ceramics

Neodymium-doped yttrium aluminum garnet (Nd:YAG) nanopowders were synthesized by the carbonate coprecipitation method. The effects of freeze drying and conventional oven drying of the precursor on the agglomeration of the Nd:YAG nanopowders were compared. The optical properties of the Nd:YAG nanopowders and the corresponding sintered Nd:YAG transparent ceramics were also investigated. The Nd:YAG nanopowders synthesized from freeze-dried precursor showed better dispersion and narrower particle size distribution compared with the powders synthesized from conventional oven drying. As a result, the Nd:YAG nanopowders synthesized from freeze-dried precursor have good sinterability, and Nd:YAG transparent ceramics were fabricated by vacuum sintering at 1750°C for 5 h.     

Aluminum Nitride Prepared by Nitridation of Aluminum Oxide Precursors

Aluminum nitride (AlN) powders were prepared from the oxide precursors aluminum nitrate, aluminum hydroxide, aluminum 2-ethyl-hexanoate, and aluminum isopropoxide (i.e., Al(NO₃)₃, Al(OH)₃, Al(OH)(O₂CCH(C₂H₅)(C₄H₉))₂, and Al(OCH(CH₃)₂)₃). Pyrolyses were performed in flowing dry NH₃ and N₂ at 1000°–1500°C. For comparison, the nitride precursors aluminum dimethylamide (Al(N(CH₃)₂)₃) and aluminum trimethylamino alane (AlH₃·N(CH₃)₃) were exposed to the same nitridation conditions. Products were investigated using XRD, TEM, EDX, SEM, and elemental analysis. The results showed that nitridation was primarily controlled by the water:ammonia ratio in the atmosphere. Single-phase AlN powders were obtained from all oxide precursors. Complete nitridation was not obtained using pure N₂, even for the non-oxide precursors.     

Synthesis of Anatase-Type TiO2 Nanocrystallites Via a Redox Route

Anatase nanocrystallites showing high surface area (∼62 m²/g) and good photocatalytic property have been obtained by pyrolyzing at 600°C for 4 h an ammonium titanyl double sulfate precursor (α-(NH₄)₂TiO(SO₄)₂) synthesized via a redox approach, that is, by oxidizing an aqueous solution of titanium trichloride (TiCl₃) with ammonium peroxodisulfate ((NH₄)₂S₂O₈), followed by reacting with ammonium sulfate ((NH₄)₂SO₄).     

Synthesis of Ultrafine β"-Alumina Powders via Flame Spray Pyrolysis of Polymeric Precursors

Flame spray pyrolysis of a polymeric precursor is used to prepare ultrafine powders that, when sintered, convert to essentially pure phase lithium-doped sodium β"-alumina. The precursor Na_1.67 Al_10.67 Li_0.33 [N(CH₂CH₂O)₃]_10.67-[OCH₂CH₂O]·x(HOCH₂CH₂OH) has been synthesized from stoichiometric amounts of metal hydroxides and tri-ethanolamine (N(CH₂CH₂OH)₃, TEA) in excess ethylene glycol. The precursor is dissolved in ethanol, and an atom-ized spray of the solution is combusted in a specially con-structed flame spray apparatus. Combustion occurs at ∼2000°C, followed by immediate quenching. This proce-dure provides for a measure of kinetic control over the process. The resulting nanopowder particles are 50–150 nm in diameter and exhibit powder X-ray diffractometry pat-terns similar to β"-alumina. Heating the nanopowder at 30°C/min to 1200°C with a 1 hisotherm converts it to pure β"-alumina. In preliminary sintering studies, green powder compacts (∼65% theoretical density) sintered at 1600°C for 12 min densify to 3.0 ± 0.1 g/cm 3 (∼92% theoretical density) with minimal loss of Na₂O. This procedure offers several processing and cost advantages over conventional β"-alumina syntheses.