Rapid Formation of Zinc Oxide Films by an Atmospheric-Pressure Chemical Vapor Deposition Method

Zinc oxide films were prepared by an atmospheric-pressure chemical vapor deposition method using (acetylacetonato)-zinc as a source material. Transparent and uniform ZnO films of considerable area (20 × 70 mm, ∼0.3 μm thick) could be obtained easily on a crown glass (CGW #200) with a high deposition rate. The deposition rate first remained constant with increasing substrate temperature ( T _s), then increased abruptly from 120 nm/min at T _s= 550°C to 220 nm/min at T _s= 600°C, and finally stopped increasing above T _s= 600°C. The maximum preferred orientation and best crystallinity of the films were obtained at T _s= 550°C.     

Sintering and Characterization of Nanophase Zinc Oxide

Nanocrystalline, single-phase undoped ZnO was sintered to 95%–98% of theoretical density at 650°–700°C, using pressureless isothermal sintering. The density increased very rapidly at 500°–600°C, remained constant with sintering temperature until ∼900°C, and then decreased slightly. The estimated activation energy for densification at 600°–700°C (275 kJ/mol) was comparable to grain-growth activation energies previously reported for microcrystalline ZnO but much greater than the grain-growth activation energy measured in the present work. A bimodal microstructure, consisting of nanocrystalline grains within larger ensembles ("supergrains"), was observed, and both modes grew as the sintering temperature increased. The grain-growth activation energy for the nanocrystalline grains was extremely low, ∼20 kJ/mol. The activation energy for the growth of the supergrains depended strongly on temperature but was ∼54 kJ/mol at >500°C. The important mechanisms probably are rearrangement of the nanoparticle grains, with simultaneous surface and boundary diffusion, and vapor transport above 900°C.     

Microstructure of Varistors Prepared with Zinc Oxide Nanoparticles Coated with Bi2O3

Zinc oxide (ZnO) nanoparticles coated with 1–5 wt% Bi₂O₃ were prepared by precipitating a Bi(NO₃)₃ solution onto a ZnO precursor. Transmission electron microscopy showed that a homogeneous Bi₂O₃ layer coated the surface of the ZnO nanoparticles and that the ZnO particle size was ∼30–50 nm. Scanning electron microscopy showed that ZnO grains sintered at 1150°C were homogeneous in size and surrounded by a uniform Bi₂O₃ layer. When the ZnO grains were surrounded fully by Bi₂O₃ liquid phases, further increases in the ZnO grain size were not affected by the Bi₂O₃ content. This predesigned ZnO nanoparticle structure was shown to promote homogeneous ZnO grains with perfect crystal growth.     

Direct-Write Fabrication of Zinc Oxide Varistors

Zinc oxide (ZnO)-based pastes with tailored rheological properties have been developed for direct-write fabrication of thick-film varistor elements in highly integrated, multifunctional electroceramic devices. Such pastes exhibited pseudoplastic behavior with a low shear apparent viscosity of roughly 1 × 10⁴ Pa·s. Upon aging, the pastes attained printable, steady-state viscosities of approximately 3 × 10² Pa·s at 10 s⁻¹. Square and rectangular elements were patterned on dense alumina substrates and sintered at varying temperatures between 800° and 1250°C. Varistor elements fired at 900°C exhibited nonlinearity coefficients (α= 30) that were equivalent to high-density (>95%) varistors formed by cold isostatic pressing at 100 MPa (15 ksi) of a similar chemically derived powder heat-treated under analogous conditions.     

Photoluminescence Properties of Nanocrystalline ZnO Ceramics Prepared by Pressureless Sintering and Spark Plasma Sintering

Nanocrystalline ZnO ceramics with grain sizes of ∼100 nm were prepared by pressureless sintering at 800°C for 2 h and spark plasma sintering (SPS) at 550°C for 2 min, respectively. Excellent green emission properties were obtained in the ZnO ceramic prepared by the SPS process and in the pressureless-sintered ZnO ceramic prepared at 1000°C for 2 h, which are attributed to the vacuum ambience of the SPS process and the sublimeness of the interstitial Zn at >900°C in air, respectively.     

Controlled synthesis of defects-containing ZnO by the French process modified with pulsed injection and its luminescence properties

Zinc oxide (ZnO) nanoparticles containing oxygen vacancies were synthesized by the French process modified with pulsed injection of nitrogen. Zinc vapor was generated by evaporation of zinc foil and carried by a carrier gas to react with co-currently supplied air. During the reaction, nitrogen gas was injected in pulse, perpendicular to the flow direction of both zinc vapor and air. Low partial pressure of oxygen and turbulence caused by pulsed injection yielded uniform ZnO nanotetrapods that contained oxygen vacancies. The content of oxygen vacancies depended upon the characteristics of the pulse, i.e. flowing and non-flowing period of the gas, pulsing cycle time, and the supplied pressure of the injected gas. Strong correlation between the presence of oxygen vacancies and the intensity of green emission in the photoluminescence spectra of ZnO was also observed.     

Effect of Heat Treatments on the Wetting Behavior of Bismuth-Rich Intergranular Phases in ZnO:Bi:Co Varistors

The effect of annealing on the wetting behavior of Bi-rich intergranular phases in ZnO:Bi:Co varistors was studied. The intergranular phase exhibits temperature-dependent grain-boundary wetting, with an average equilibrium dihedral angle of 0° at 1140°C and over 55° at 610°C. The temperature-dependent wetting may be related to the temperature dependence of the ZnO concentration in the Bi₂O₃ liquid phase. The effect of the intergranular phase distribation on the electrical properties of ZnO varistors is discussed.     

Crystallite Growth of Beryllium Oxide Powders and Its Inhibition by Adsorbed Phosphate

Low-temperature sintering of beryllium oxide powders from 300° to 1000°C was studied by measuring crystallite growth and changes in specific surface. Sintering is enhanced by the presence of water vapor. Adsorbed phosphate almost completely inhibits sintering up to 850°C, even in water vapor at 10 torr.     

Sol-Gel Preparation of Transparent and Conductive Aluminum-Doped Zinc Oxide Films with Highly Preferential Crystal Orientation

Transparent aluminum-doped zinc oxide (ZnO) films were prepared via the sol-gel method on silica-glass substrates from 2-methoxyethanol solutions of zinc acetate and aluminum chloride that contained monoethanolamine. Dip coating was conducted at room temperature, with substrate withdrawal rates of 1.2-7.0 cm/min. After each deposition, the films were heat-treated in air at 200°-450°C for 10 min (pre-heat-treatment). After six to fourteen layers had been deposited, the films were then subjected to annealing in air at 500°-800°C for 1 h (the first post-heat-treatment), followed by annealing in nitrogen at 500°-700°C for 15 min to 4 h (the second post-heat-treatment). All the films obtained were transparent and showed only an extremely sharp ZnO (002) peak in the X-ray diffractometry (XRD) patterns. The effects of the aluminum content, the substrate withdrawal speed, and the heat-treatment conditions on the electrical resistivity of the films were studied. All these factors strongly affected the resistivity. The lowest resistivity value (6.5 10^-3 Omegacm) was achieved in a film that contained 0.5 at.% aluminum, prepared with a low substrate withdrawal speed (1.2 cm/min), and a pre-heat-treatment of individual layer at 400°C in air and a post-heat-treatment of the entire film at 600°C in air, followed by a post-heat-treatment at 600°C in nitrogen. These preparation parameters also affected the degree of crystal orientation, which was revealed by the intensity of the ZnO (002) XRD peak. Higher crystal orientation was effective in reducing the film resistivity, whereas the higher grain-packing density and possible aluminum segregation were thought to have positive and negative effects, respectively, in reducing the resistivity.     

Low-Temperature Synthesis of Zinc Oxide Nanoparticles

Crystalline zinc oxide nanoparticles have been prepared by mixing aqueous solutions of zinc nitrate and hexamethylenetetramine (HMT) at 60°C and 80°C. Transmission electron microscopy and X-ray diffraction show that the ZnO nanoparticles of diameters ranging from 15–33 nm and 25–43 nm long are formed. Aspect ratio is observed to range from 1.18 to 1.74 at 60°C and 1.22 to 1.70 at 80°C as the HMT to zinc nitrate concentration ratio increases from 10 to 150. Nanoparticle size decreases as the concentration of HMT increases. Much larger ZnO particles are formed with ammonium hydroxide as a hydrolysis agent without HMT. In summary, HMT is an ammonium-hydroxide source in the reaction, a surfactant for retaining nanosize, and not necessarily a template for ZnO nucleation.     

Corrosion Behavior of Single-Crystal Alumina in Argon, Air, and Water Vapor Atmospheres at 1700-2000°C

The results for the corrosion of alumina single crystals at 1700-2000°C in argon, argon/water vapor, air, and air/water vapor for 10 h are reported. There were no obvious weight and volume changes after corrosion. White spots were observed on the surfaces of the specimens after corrosion tests. The initial temperature for the appearance of these white spots was 1800°C for argon and air, 1900°C for argon/water vapor, and 2000°C for air/water vapor. These white spots were likely formed by internal impurities, which diffused outward to the surface and coalesced at high temperatures. There was no evidence of corrosion damage inside the specimens. The flexural strength of the specimens was clearly enhanced after the corrosion tests and showed no evident relation to the corrosion conditions. This increase in strength after corrosion was likely due to the healing of surface machining flaws. The surface flaw healing temperature for alumina crystals was higher than 1400°C.     

Two-Step Sintering of Nanocrystalline ZnO Compacts: Effect of Temperature on Densification and Grain Growth

Two-step sintering (TSS) was applied on nanocrystalline zinc oxide (ZnO) to control the accelerated grain growth occurring during the final stage of sintering. The grain size of a high-density (>98%) ZnO compact produced by the TSS was smaller than 1 μm, while the grain size of those formed by the conventional sintering method was ∼4 μm. The results showed that the temperature of both sintering steps plays a significant role in densification and grain growth of the nanocrystalline ZnO compacts. Several TSS regimes were analyzed. Based on the results obtained, the optimum regime consisted of heating at 800°C (step 1) and 750°C (step 2), resulting in the formation of a structure containing submicrometer grains (0.68 μm). Heating at 850°C (step 1) and then at 750°C (step 2) resulted in densification and grain growth similar to the conventional sintering process. Lower temperatures, e.g., 800°C (step 1) and 700°C (step 2), resulted in exhaustion of the densification at a relative density of 86%, above which the grains continued to grow. Thermogravimetric analysis results were used to propose a mechanism for sintering of the samples with transmission electron micrographs showing the junctions that pin the boundaries of growing grains and the triple-point drags that result in the grain-boundary curvature.     

Variation of the Oxidation Rate of Silicon Carbide with Water-Vapor Pressure

Chemically vapor deposited silicon carbide (CVD SiC) was oxidized at temperatures of 1000°-1400°C in H₂O/O₂ gas mixtures with compositions of 10-90 vol% water vapor at a total pressure of 1 atm. Additional experiments were conducted in H₂O/argon mixtures at a temperature of 1100°C. Experiments were designed to minimize impurity and volatility effects, so that only intrinsic water-vapor effects were observed. The oxidation kinetics increased as the water-vapor content increased. The parabolic oxidation rates in the range of 10-90 vol% water vapor (the balance being oxygen) were approximately one order of magnitude higher than the rates that were observed in dry oxygen for temperatures of 1200°-1400°C. The power-law dependence of the parabolic oxidation rate on the partial pressure of water vapor at all temperatures of the study indicated that the molecular species was not the sole rate-limiting oxidant. The determination of an activation energy for diffusion was complicated by variations in the oxidation mechanism and oxide-scale morphology with the partial pressure of water vapor and the temperature.     

Kinetics of Vapor-Phase Hydration of Magnesium Oxide

Magnesium oxide prepared by calcining the carbonate at 1000°C was sintered at 1800°C and ground under methanol. Well-sized 2 to 5 and 5 to 10 μ fractions were separated. After outgassing at 550°C in vacuum, the powders were hydrated at various temperatures to 98°C in a wide range of water vapor pressures, and the reaction was followed gravimetrically with a silica helix balance. At low temperatures and vapor pressures, where nucleation is exceedingly slow, the hydration of prenucleated samples was studied. The results are interpreted on the basis of a model in which an interface reaction controls the progress of the reaction. The reaction rate varies with the water vapor pressure p according to a function ( p / p * - 1), where p 0.3 p _sat. and appears to correspond to the equilibrium adsorption pressure. This suggests that the reaction depends on an adsorbed water vapor film which has free access to the reacting surface. The reaction rate constant varies with temperature according to an Arrhenius-type equation, with an activation energy 16,100 cal per mole.     

A New Water-Resistant Coating for AlN Powders

AlN powders were coated with a protective layer at temperatures between 1150-1450°C in a vacuum by the reaction with SiO vapor. The waterproof property of the coated AlN powders was studied. The composition of the protective coating varied with the coating temperature. Si-coated AlN powders at 1200°C showed excellent stability against hydrolysis. On the other hand, the coated layer was not observed when the AlN powders were treated at 1450°C. A silica layer was formed on the surface of the Si-coated AlN powders by the oxidation treatment at 1000°C. These powders showed superior stability against hydrolysis compared to that of the commercially available silica-coated AlN produced by the sol-gel method. Compared to other processes used to coat AlN powders, the vaporization process using SiO vapor is simple and effective.     

Phase Equilibria in the System ZnO—TiO2

The system ZnO-Ti0₂ has been investigated using quenching, hydrothermal, strip-furnace, and solid-state-reaction techniques. Two compounds were found: Zn₂Ti0₄, which melts congruently at 1549 C., and ZnTiO₂, which dissociates hydrothermally at 945° C. to give Zn₂Ti0₄and rutile. Two eutectics were found: one between ZnO and Zn₂Ti0₄ at 32 mole % TiO, and 1537°C. and the other between Zn₂TiO₄and TiO₂ at 58 mole % TiO₂ and 1418°C. Rutile was the only modification of TiO₂ observed. The reported melting points of ZnO and TiO₂ are 1975° and 1830°C. respectively; however, data exist which indicate the sublimation of ZnO at atmospheric pressure. Loss of ZnO by volatilization slightly decreased the accuracy of the liquidus relations. Reported solid solutions of TiO₂ in Zn₂Ti0₄ and ZnTiO₃ were not encountered, and explanations of this discrepancy are proposed.     

Sintering TiO2 in HCl Atmospheres

TiO₂ was sintered in HCl atmospheres to enhance the effects of vapor transport. Little or no densification is observed for temperatures between 1000° and 1300°C. Particle coarsening occurs at temperatures above 1200°C. The apparent activation energy for particle growth is 114 kJ/mol. It is concluded that the primary mass-transport mechanism is vapor transport while the particle growth rate is limited by grain-boundary mobility .     

Sintering of Zinc Oxide Doped with Antimony Oxide and Bismuth Oxide

The phase change, densification, and microstructure development of ZnO doped with both Bi₂O₃ and Sb₂O₃ are studied to better understand the sintering behavior of ZnO varistors. The densification behavior is related to the formation of pyrochlore and liquid phases; the densification is retarded by the former and promoted by the latter. The pyrochlore phase, whose composition is Bi_3/2ZnSb_3/2O₇, appears below 700°C. The formation temperature of the liquid phase depends on the Sb/Bi ratio: about 750°C for Sb/Bi < 1 by the eutectic melting in the system ZnO—Bi₂O₃, and about 1000°C for Sb/Bi > 1 by the reaction of the pyrochlore phase with ZnO. Hence, the densification rate is determined virtually by the Sb/Bi ratio and not by the total amount of additives. The microstructure depends on the sintering temperature. Sintering at 1000°C forms intragrain pyrochlore particles in ZnO grains as well as intergranular layers, but the intragrain particles disappear at 1200°C by the increased amount of liquid phase, which enhances the mobility of the solid second phase.     

Preparation and Electric Properties of Dense Nanocrystalline Zinc Oxide Ceramics

This communication reports on the preparation and electric properties of dense nanocrystalline ZnO ceramics. By spark plasma sintering, nanocrystalline (∼100 nm) ZnO ceramics with a high density of 98.5% were obtained at a very low temperature of 550°C. Electric property measurement revealed a novel conduction nonlinearity in the sample sintered at 500°C. This phenomenon is due to the nanometerization of ZnO crystal and the grain boundary layer with an amorphous interfacial layer.     

Characterization of Thermoelectric Metal Oxide Elements Prepared by the Pulse Electric-Current Sintering Method

Thermoelectric elements consisting of the layered polycrystalline materials of Al-doped ZnO and NaCo₂O₄ were prepared using the pulse electric-current sintering (PECS) method at 900°C for 3 min. Direct contact between the polycrystalline Al-doped ZnO and the NaCo₂O₄ was obtained in a single-step process for the stacked powders. The electrical conductivities of the polycrystalline materials prepared by PECS were higher than those of materials prepared by conventional sintering, despite their porous structure. The thermoelectric voltage of the 1-mol%-Al-doped ZnO and NaCo₂O₄ polycrystalline element (measuring ∼6 mm × 3 mm × 15 mm) was 83 mV at d T = 500 K, when the junction of the elements was at 800°C.