Synthesis of Oriented Ba2NaNb5O15 (BNN) Thin Films from an Alkoxy-derived Precursor

Ba₂NaNb₅O₁₅ (BNN) thin films with a tungsten bronze structure were fabricated using a precursor solution that was synthesized from barium, sodium, and niobium alkoxides. Highly (002)-oriented BNN films were prepared successfully on Pt(100)/MgO(100) substrates at 700°C, using a BNN underlayer. X-ray pole-figure measurement showed that the BNN films that crystallized on Pt(100)/MgO(100) substrates had two in-plane orientations. The remanent polarization and coercive field of the BNN film (thickness of 1.0 µm) that was crystallized at 700°C were 12.3 µC/cm² and 101 kV/cm, respectively, at –150°C (123 K). BNN films on fused silica substrates exhibited second harmonic generation upon irradiation with 1064 nm light.     

Molten Salt Synthesis of Acicular Ba2NaNb5O15 Seed Crystals

Well-developed acicular Ba₂NaNb₅O₁₅ (BNN) seed crystals have been successfully prepared by the reaction between BaCO₃, Nb₂O₅ and molten NaCl salt. The effects of the calcination temperature, time, and weight ratio of the oxide mixture to salt on the morphology of BNN particles were investigated. Uniform acicular seeds with high aspect ratio could be achieved at 1200°C for 4 h with the salt-to-oxide mixture of 2:1.     

Synthesis of ZrO2 and Y2O3-Doped ZrO2 Thin Films Using Self-Assembled Monolayers

Undoped or Y₂O₃-doped ZrO₂ thin films were deposited on self-assembled monolayers (SAMs) with either sulfonate or methyl terminal functionalities on single-crystal silicon substrates. The undoped films were formed by enhanced hydrolysis of zirconium sulfate (Zr(SO₄)·4H₄O) solutions in the presence of HCl at 70°C. Typically, these films were a mixture of two phases: nanocrystalline tetragonal- ( t -) ZrO₂ and an amorphous basic zirconium sulfate. However, films with little or no amorphous material could be produced. The mechanism of film formation and the growth kinetics have been explained through a coagulation model involving homogeneous nucleation, particle adhesion, and aggregation onto the substrate. Annealing of these films at 500°C led to complete crystallization to t -ZrO₂. Amorphous Y₂O₃-containing ZrO₂ films were prepared from a precursor solution containing zirconium sulfate, yttrium sulfate (Y₂(SO₄)₃8·H₂O), and urea (NH₂CONH₂) at pH 2.2–3.0 at 80°C. These films also were fully crystalline after annealing at 500°C.     

Effect of Seeding Sr2KNb5O15 on the Phase Formation and Microstructural Development in Reactive Sintering of Sr2NaNb5O15 Ceramics

The phase formation, densification behavior, and microstructure development of Sr₂NaNb₅O₁₅ (SNN) ceramics in both 10 wt% acicular Sr₂KNb₅O₁₅ (SKN) seed-containing and unseeded systems were investigated in this work. SNN ceramics were reactively sintered from SrNb₂O₆ and NaNbO₃ powders. The results show that the acicular SKN seeds not only accelerate SNN phase formation but also promote the densification at lower temperature. In reactive sintering, the acicular SKN seeds prepared by the molten salt synthesis method can give rise to the formation of a liquid phase and provide the structural framework for the grain growth of ceramics, leading to the formation of large anisotropic grains (>80 μm) in ceramics sintered at 1340°C. However, there are no such large anisotropic grains obtained in the SKN-free system. Observation of the large anisotropic grain growth is explained by the liquid-phase-assisted growth mechanism. For comparison, the microstructure evolution in the system with 10 wt% SKN seed, which was prepared by the conventional mixed-oxide method and without acicular morphology, was also investigated to further support the new growth mechanism.     

Microwave Dielectric Properties of Low-Fired Ba5Nb4O15

The effect of B₂O₃ on the sintering temperature and microwave dielectric properties of Ba₅Nb₄O₁₅ has been investigated using X-ray powder diffraction, scanning electron microscopy, and a network analyzer. Interactions between Ba₅Nb₄O₁₅ and B₂O₃ led to formation of second phases, BaNb₂O₆ and BaB₂O₄. The addition of B₂O₃ to Ba₅Nb₄O₁₅ resulted in lowering the sintering temperature from 1400° to 925°C. Low-fired Ba₅Nb₄O₁₅ could be interpreted by measuring changes in the quality factor ( Q × f ), the relative dielectric constant (ɛ_r), and the temperature coefficient of resonant frequency (τ_f) as a function of B₂O₃ additions. More importantly, the formation of BaNb₂O₆ provided temperature compensation. The microwave dielectric properties of low-fired Ba₅Nb₄O₁₅ had good dielectric properties: Q × f = 18700 GHz, ɛ_r= 39, and τ_f= 0 ppm/°C.     

Synthesis of Metal Phosphates Using SiP2O7

A new, alternative, route to the synthesis of metal phosphates, using SiP₂O₇ as a reagent, was studied. The reaction of SiP₂O₇ with metal oxides yields a solid mixture of silica and metal phosphates; its reaction with metal fluorides yields solid metal phosphates and gaseous mixtures of SiF₄ and POF₃ .     

Raman Study of Glasses of Ba2TiSi2O8 and Ba2TiGe2O8

Raman spectra are reported for fresnoite (Ba₂Ti(Si,Ge)₂O₈ glasses, and comparison is made between the Raman spectra of the corresponding crystalline powders and glasses of Ba₂TiSi₂O₈ and Ba₂TiGe₂O₈. The Ba₂TiGe₂O₈ glass spectra show correspondence with the Ba₂TiGe₂O₈ crystalline Raman spectra; the v _s(Ge–O–Ge) mode occurs at 518 cm⁻¹ in the glass and at 521 cm⁻¹ in the crystalline material. Five-fold coordinated titanium is the majority species present in the Ba₂TiGe₂O₈ glass as revealed by a strong band at 824 cm⁻¹ in the I glass spectrum. The Ba₂TiSi₂O₈ glass spectra are similar to the Ba₂TiSi₂O₈ crystalline spectrum; the strongest band is found at 836 cm⁻¹ in the I glass spectrum. Through comparison with the previous Raman data of other titania silicate glasses, we conclude that the Ba₂TiSi₂O₈ glass has a structure similar to the crystalline phase.     

Growth of Single-Crystal and Polycrystalline Thin Films of MgAl2O4 and MgFe2O4

Single-crystal and polycrystalline films of Mg-Al₂O₄ and MgFe₂O₄ were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al₂O₈ film. Above 540°C, the γ-Al₂O, and MgO reacted to form a single-crystal MgAl₂O₄ film with {001} MgAl₂O₄‖{001} MgO. Above 590°C, an additional layer of MgAl₂O₄, which is polycrystalline, formed between the γ-Al₂O₃ and the single-crystal spinel. Polycrystalline Mg-Al₂O₄ formed only when diffusion of Mg²⁺ ions proceeded into the polycrystalline γ-Al₂O₃ region. Corresponding results were obtained for Mg-Fe₂O₄. MgAl₂O₄ films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al₂O₃ crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate.     

TIOx/Al2O3 Multilayer Ceramic Thin Films

Titanium oxide/aluminum oxide films have been deposited using molecular beam epitaxy methods and characterized by reflection high-energy electron diffraction and transmission electron microscopy techniques. Growth on silicon substrates below 973 K resulted in primarily amorphous multilayers. At 1323 K, the deposition of titanium in an oxygen atmosphere on (0001) Al₂O₃ substrates resulted in films of Ti₂O₃. These films consisted of small domains, up to 60 nm, slightly misoriented from a [1120] ∥ [1120] orientation relationship. Two variants of Ti₂O₃ were observed due to multiple positioning during growth. Closing the titanium shutter during growth resulted in an oriented TiO₂ film.     

Sol-Gel Process for the Preparation of Ba2Ti9O20 and BaTi5O11

Barium titanate precursors with Ba/Ti ratio 2:9 and 1:5 were prepared by first hydrolyzing titanium alkoxide and then mixing the resulting titania sol with a barium alkoxide-methanol solution. After drying, the xerogels of the precursors of barium titanates were sintered at temperatures from 700°C (4 h) to 1200°C (110 h or longer). Characterization of the product was performed using X-ray diffraction and laser Raman spectroscopy. At 700°C, BaTi₅O₁₁ was formed from the 1:5 precursor and a two-phase mixture of BaTi₂O₅ and BaTi₅O₁₁ was formed from the 2:9 precursor. After prolonged heating at 1200°C, the latter mixture converted to a single-phase material, Ba₂Ti₉O₂₀.     

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.     

Evolution of the Formation of Inorganic Polymers in the CaO—SiO2—P2O5 System Using Metal Alkoxides

Inorganic polymers provide structural integrity to cements. The CaO—SiO₂—P₂O₅ system has not been studied extensively, but it is important to both cementitious and biological systems. The hydrolytic decomposition of homogeneous mixtures of tetraethyl orthosilicate and calcium isopropoxide with additions of anhydrous phosphoric acid in isopropyl alcohol was used to evaluate the formation of inorganic polymers. XRD, DTA, and TGA were used to characterize the decomposition products. The formation of inorganic polymers at room temperature and pressure was used as a criterion to establish cement-forming compositions within the CaO—SiO₂—P₂O₅ system.     

Ba2Ti9O20 Phase Equilibria

The heterogeneous phase distribution found in Ba₂Ti₉O₂₀ ceramic resonators results from a temperature-dependent phase boundary and slow reaction kinetics. When sintered at 1350°C or higher in oxygen the Ba₂Ti₉O₂₀ phase becomes slightly reduced and barium-rich. Thus a stoichiometric composition forms rutile and "Ba₂Ti₉O₂₀'phase. On slow cooling the excess barium diffuses to the oxygen-rich surface where it reacts to form an envelope of rutile-free material surrounding a core containing a small amount of rutile.     

Preparation of Ba2YCu3O7-δ Thin Films with Preferred Orientation through an Organometallic Route

Superconducting Ba₂YCu₃O_7-δ thin films were prepared through an organometallic route. Single-phase Ba₂YCu₃O_7-δ thin films with preferred orientation were successfully prepared on SrTiO₃ (100) single-crystal substrates at 800°C by a dip coating method using partially hydrolyzed Ba-Y-Cu organometallic solutions. Preferentially oriented Ba₂YCu₃-O_7-δ thin films were also prepared on MgO (100) substrates. By controlling the partial hydrolysis conditions, a coating solution for precursor thin films was kept accurately at the stoichiometric composition. The use of ozone gas during the pyrolysis of the precursor thin films was found to suppress the formation of BaCO₃. Ba₂YCu₃O_7-δ thin films with c -axis orientation perpendicular to a SrTiO₃ (100) substrate, which were heat-treated at 900°C for 15 min, exhibited a superconductivity transition with an onset of 90 K and an end of 75 K.     

Synthesis of LiAlO2 Powder by Hydrolysis of Metal Alkoxides

Crystalline fine powders of LiAlO₂ can be prepared by hydrolysis of metal alkoxides for application to the breeder blanket in nuclear fusion. The precipitates were fine crystalline particles of β-LiAlO₂of ∼0.1-μm size under controlled conditions. The transformation from β- to γ-phase was confirmed at temperatures between 700° and 750°C. The microstructure of a sintered body at 1000°C consisted of homogeneous fine particles and pores suitable for the blanket material. A fully dense sintered body could be prepared at 1400°C.     

Preparation of β-BaB2O4 Powders and Thin Films by Sol–Gel Method

β-BaB₂O₄ (β-BBO) powders and films were successfully synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of barium metal with boron triethoxide in ethanol by addition of diethanolamine. The drip-coated precursor films began to crystallize to β-BBO on Pt substrates at 500°C and converted to β-BBO films with preferred orientation to the c -axis at 700°C.     

Atmospheric Dependence on Dielectric Loss of 1/6Ba5Nb4O15· 5/6BaNb2O6 Ceramics

Samples of 1/6Ba₅Nb₄O₁₅·5/6BaNb₂O₆ along with the pure end members, Ba₅Nb₄O₁₅ and BaNb₂O₆, were sintered under low oxygen partial pressure. The degradation mechanisms of dielectric loss in this reducing atmosphere have been studied. We found that the degradation occurred primarily due to the formation of oxygen vacancies caused by the reduction of Nb⁵⁺. This was determined by measuring the electrical conductivity, and through X-ray photoelectron spectroscopy. More importantly, the dielectric loss of 1/6Ba₅Nb₄O₁₅·5/6BaNb₂O₆ samples with higher temperature stability was further decreased on sintering in a reducing atmosphere. This observation has been explained by considering the increased porosity and formation of a reduced second phase, Ba_0.65NbO₃.     

Critical Current Densities in Thin Ceramic Tapes of Superconducting Ba2YCu3O7

Tape-cast slurries of Ba₂YCu₃O₇ powders offer a convenient means of preparing sintered ceramic samples for critical current density (J_c) measurements where the transport cross section is small and the current electrode areas are large. Samples were sintered from 900° to 1000°C and characterized for bulk density, grain size, phase composition, T_c, and J_c. Bulk density and grain size both increase with sintering temperature while all samples were single-phase perovskite except for those sintered at 900°C. The onset temperature for superconductivity is constant at about 93 K while the transition sharpens to R=0 at about 92 K for the densest samples. J_c rises with sintering temperature to a maximum of ∼10³ A/cm². A linear relationship between J_c and bulk density is predicted from microstructural considerations.     

Orientation Relationships of Reactively Grown Ba6Ti17O40 and Ba2TiSi2O8 on BaTiO3 (001) Determined by X-ray Diffractometry

Fresnoite grows at 700° and 800°C, and Ba₆Ti₇O₄₀ grows at 1200°C with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured fresnoite is formed at higher temperatures. The SiO₂ films react with the BaTiO₃ crystals, forming the phases Ba₂TiSi₂O₈ (fresnoite) and Ba₆Ti₁₇O₄₀. At 700° and 800°C, both phases grow with definite orientations, which are determined by X-ray diffraction pole figure analysis. Partially textured polycrystalline phases are formed at higher temperatures.