Sol–Gel Route to Porous Lanthanum Cobaltite (LaCoO3) Thin Films

Sol–gel-derived LaCoO₃ thin films were deposited on yttria-stabilized zirconia (YSZ) substrates from a lanthanum isopropoxide–cobalt acetate (with 2-methoxyethanol) precursor solution. A chelating agent (2-ethylacetoacetate) and polyethylene glycol (PEG) were used to modify the above-mentioned precursor solution. The La-Co precursor solution was sufficiently viscous, and transparent LaCoO₃ gel films were prepared successfully using a spin-coating technique. Crystallization behavior and microstructure evolution were investigated using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). A single-phase perovskite thin film with the grain size of ∼50 nm was obtained by heat-treating the spin-coated gel film at a temperature of 600°C. SEM observations revealed that the microstructure of LaCoO₃ thin films that were prepared from the precursor solution with PEG was porous, and the LaCoO₃ thin film maintained its porous microstructure to a temperature of 800°C.     

Synthesis of KTiOPO4 (KTP) Thin Films Using Metallo-organics

Crack-free and highly transparent KTiOPO₄ (KTP) thin films were synthesized by the sol-gel method using a homogeneous precursor solution prepared from ("BuO)₂-P(O)(OH), Ti(OEt)₄, and KOEt in EtOH. Precipitated powders from the solution crystallized directly to KTP above 550°C. Polycrystalline KTP thin films were obtained at 600°C on various substrates. On NdAlO₃(100) substrates, KTP films with (101) and (240) preferred orientations were formed at 600°C. KTP films on glass substrates showed a refractive index of 1.75 and an absorption edge of 350 nm. KTP films exhibited the second harmonic generation of the 532 nm light on irradiaton with 1064 nm light.     

The System HfO2-Y2O3-Er2O3

A mathematical model of the liquidus surface based on a reduced polynomial method was proposed for the system HfO₂-Y₂O₃-Er₂O₃. The results of calculations according to this model agree fairly well with the experimental data. Phase equilibria in the system HfO₂-Y₂O₃-Er₂O₃ were studied on melted (as-cast) and annealed samples using X-ray diffraction (at room and high temperatures) and micro-structural and petrographic analyses. The crystallization paths in the system HfO₂-Y₂O₃-Er₂O₃ were established. The system HfO₂-Y₂O₃-Er₂O₃ is characterized by the formation of extended solid solutions based on the fluorite-type (F) form of HfO₂ and cubic (C) and hexagonal (H) forms of Y₂O₃ and Er₂O₃. The boundary curves of these solid solutions have the minima at 2370°C (15. 5 mol% HfO₂, 49. 5 mol% Y₂O₃) and 2360°C (10. 5 mol% HfO₂, 45. 5 mol% Y₂O₃). No compounds were found to exist in the system investigated.     

Effective Ionic Radius of Y3+ Determined from Lattice Parameters of Fluorite-Type HfO2 and ZrO2 Solid Solutions

Fluorite type HfO₂ and ZrO₂ solid solutions were prepared by doping with 8 to 14 mol% of Ho₂O₃ and Y₂O₃, and their lattice parameters were determined. In both HfO₂ and ZrO₂ systems, the lattice parameters of the solid solutions containing Ho₂0₃ were consistently greater than those containing the same amounts of Y₂O₃. This indicated that the ionic radius of Ho³⁺ was larger than that of Y³⁺ in the fluorite structure solid solutions. The effective ionic radius of Y³⁺ in eightfold coordination was estimated to be 0.1011 nm by using the measured lattice parameters and the empirical equations to predict the lattice parameters of the fluorite-type solid solutions.     

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.     

Energetics of La2O3–HfO2–SiO2 Glasses

A series of La₂O₃–HfO₂–SiO₂ glasses, approximately along the join 0.73SiO₂–0.27( x HfO₂–(1− x )La₂O₃), 0< x <0.3), was prepared using containerless processing techniques (aerodynamic levitation combined with laser heating in oxygen). The enthalpy of formation and enthalpy of vitrification at 25°C were obtained from drop solution calorimetry of these glasses and appropriate crystalline compounds in a molten lead borate (2PbO–B₂O₃) solvent at 702°C. The enthalpy of formation from crystalline oxides was exothermic and became less exothermic with increasing HfO₂ content. Heat contents were measured by transposed temperature drop calorimetry and depended linearly on the HfO₂ content. Differential scanning calorimetry showed that both the onset glass transition and the onset crystallization temperature of these glasses increased with increasing HfO₂ content. Upon slow cooling in air, the glasses crystallized to a mixture of baddeleyite, cristobalite, lanthanum disilicate, and hafnon.     

Vibrational Spectra of HfO2–ZrO2 Solid Solutions

Vibrational spectra were measured and analyzed for HfO₂–ZrO₂ solid solutions. Some Raman bands in the high–wave–number region shift almost linearly with changes in ZrO₂ concentration between the pure end–members; their band locations can be used to determine ZrO₂ concentrations in annealed solid solutions. The Raman bands of pure ZrO₂ and HfO₂ can be correlated using the band shifts of the solid solutions. Induced stress causes band shifts for the solid solutions.     

Synthesis of Ba2NaNb5O15 Powders and Thin Films Using Metal Alkoxides

Transparent and highly oriented Ba₂NaNb₅O₁₅ (BNN) thin films have been prepared by using metal alkoxides. A homogeneous precursor solution was prepared by the controlled reaction of NaOC₂H₅, Nb(OC₂H₅)₅, and barium metal. The BNN precursor included a molecular-level mixture of NaNb(OC₂H₅)₆ and Ba[Nb(OC₂H₅)₆]₂ in ethanol. The alkoxy-derived powder crystallized to a low-temperature phase, and then transformed to orthorhombic BNN (tungsten bronze) at 600°C. BNN precursor films on substrates crystallized to orthorhombic BNN at 800°C via the low-temperature phase. Highly (002) oriented BNN films of tungsten bronze structure were successfully prepared on MgO (100) substrates at 700°C by using BNN underlayer.     

High-Temperature Oxidation at 1500° and 1600°C of SiC/Graphite Coated with Sol–Gel-Derived HfO2

Oxidation of SiC compositionally graded (SCGed) graphite coated with HfO₂ derived from HfCl₄ by a sol–gel process was performed at 1500° and 1600°C in a flowing gas mixture of Ar and O₂ (80/20 kPa). SCGed graphite was produced by reaction of graphite with either molten Si or SiO gas at 1450°C. The sol–gel-derived HfO₂ precursor was deposited on SCGed graphite by a dip-coating method. Isothermal and cyclic oxidation of uncoated- and HfO₂-coated SCGed graphite was studied by monitoring overall weight change using an electro-microbalance. Scanning electron microscopy with energy-dispersive X-ray analysis was used to observe the surfaces and cross-sections of the oxidized HfO₂-coated SCGed graphite. The formation of HfSiO₄ was confirmed on the outer layer of the oxidized sample, beneath which a thin silica layer was formed. The improved oxidation resistance of SCGed graphite by coating with HfO₂ is discussed on the basis of the formation of these two layers.     

Preparation and Characterization of BaO–TeO2 Thin Films Obtained from Tellurium(VI) Alkoxide by a Sol–Gel Method

BaO–TeO₂ thin films were prepared from tellurium(VI) alkoxide by a sol–gel method and their structure was investigated by X-ray diffractometry, Fourier transform infrared spectrometry, and ¹²⁵Te static nuclear magnetic resonance. Their crystallization temperature ( T _c), optical transmittance, and dielectric constant were measured, and their refractive index was calculated from the transmission spectra. The results indicate that the BaO–TeO₂ thin films were composed of TeO₆ and TeO₄ units, and had a T _c of ∼520°C, refractive index of ∼1.79, dielectric constant of ∼20. These films had a T _c higher than the glass prepared by a melt-quench method, but their refractive index and dielectric constant were lower. These differences may be due to differences in their structural units.     

Characterization of Alkoxy-Derived Yttria-Stabilized Hafnia

High-purity submicron HfO₂−Y₂O₃ powders were prepared by the simultaneous hydrolytic decomposition of hafnium and yttrium alkoxides. Compositions of 1 to 7 mol% Y₂O₃ in HfO₂ were studied by high-temperature X-ray diffraction, electron microscopy, BET surface area measurements, emission spectrographic analysis, TGA, and DTA. X-ray diffraction and atomic absorption were used to determine the Y₂O₃ concentration. Hot-pressing or cold-pressing and sintering of the powders resulted in nearly theoretically dense bodies with uniform microstructure containing grains 1 to 5 μm in size. Surface reflection ir spectra were obtained for monoclinic, tetragonal, and cubic specimens. Fully cubic HfO₂ with 7 mol% Y₂O₃ additions was obtained at a relatively low temperature. The hot-pressed specimens, initially slightly oxygen-deficient or gray HfO_{2- x} , were reoxidized at 1000°C with no deleterious effects; thin sections were translucent to incident light.     

The System HfO2-TiO2

The system HfO₂-TiO₂ was studied in the 0 to 50 mol% TiO₂ region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO₂ was found at 1750°± 20°C. The definite compound HfTiO₄ melts incongruently at 1980°± 10°C, 53 mol% TiO₂. A metatectic at 2300°± 20°C, 35 mol% TiO₂ was observed. The eutectoid decomposition of HfO_2,ss) ( T ) → HfO_2,ss ( M ) + HfTiO_34,ssss occurred at 1570°± 20°C and 22.5 mol% TiO₂. The maximum solubility of TiO₂ in HfO_2,ss,( M ) is 10 mol% at 1570°± 20°C and in HfO_2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO₂-rich side and in the 10 to 30 mol% TiO₂ range a second monoclinic phase M of HfO₂( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO₂ region, which are based on thermal analysis techniques.     

Fracture Strength and Microstructure of β-SiAlON with Hafnia Addition

The influence of HfO₂ addition on the fracture strength and microstructure of ß-SiAlON ceramics sintered from Si₃N₄ and Al₂O₃ powders was investigated. The strength was increased by the addition of HfO₂, from ∼500 MPa to 700 MPa, and was almost constant from ambient temperature to 1300°C. Monoclinic HfO₂ grains that were distributed in the SiAlON grain boundaries had a flat shape (∼20 nm thick) and were surrounded by an amorphous phase. The aluminum concentration in ß-SiAlON in the samples with an Al₂O₃ starting composition of 15 wt% was decreased by the addition of HfO₂. The amount of secondary phases was very small at grain boundaries between the SiAlON grains; amorphous phases were observed infrequently at the triple points but were very small (∼20 nm). The effects of HfO₂ addition on the mechanism of the microstructure development and fracture strength are discussed.     

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.     

Physical Properties and Structure of Rf-Sputtered Amorphous Films in the System Ti02-Si02

Amorphous films in the system Ti0₂-Si0₂ were prepared by rf sputtering and their density, refractive index, and thermal expansion were measured. Also, the silicon Kα and titanium Kβ band emission spectra were obtained by X-ray emission spectroscopy in order to determine the coordination state of silicon and titanium ions in these amorphous films. The density and refractive index increased, but not proportionally, with increasing Ti0₂ content. On the other hand, a minimum was observed in the thermal expansion coefficient at =15 mol% Ti0₂. The coordination state of silicon ions in the amorphous films did not change with Ti0₂ content. However, the coordination number of titanium ions changed from 4 to 6, depending on Ti0₂ content. These results indicate that, in amorphous films in the system Ti0₂-Si0₂, the change of the coordination state of titanium ions has an important effect on physical properties, such as volume, molar refractivity, and thermal expansion.     

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.     

Ionic Conductivity of the Fluorite-Type Hafnia–R2O3 Solid Solutions

The ionic conductivity of the hafnia-scandia, hafnia-yttria, and hafnia-rare earth solid solutions with high dopant concentrations of 8, 10, and 14 mol% was measured in air at 600° to 1050°C. Impedance spectroscopy was used to obtain lattice conductivity. A majority of the investigated samples exhibited linear Arrhenius plots of the lattice conductivity as a function of temperature. For all investigated dopant concentrations the ionic conductivity was shown to decrease as the dopant radius increased. The activation enthalpy for conduction was found to increase with dopant ionic radius. The fact that the highest ionic conductivity among 14-mol%-doped systems was obtained with HfO₂─Sc₂O₃ suggested that the radius ratio approach should be used to predict the electrical conductivity behavior of HfO₂─R₂O₃ systems. A qualitative model based on the Kilner's lattice parameter map does not seem to apply to these systems. For the three systems HfO₂─Yb₂O₃, HfO₂─Y₂O₃, and Hf₂O₃─Sm₂O₃ a conductivity maximum was observed near the dopant concentration of 10 mol%. Deep vacancy trapping is responsible for the decrease in the ionic conductivity at high dopant concentrations. Formation of microdomains of an ordered compound cannot explain the obtained results. A comparison between the ionic conductivities of doped HfO₂ and ZrO₂ systems indicated that the ionic conductivities of HfO₂ systems are 1.5 to 2.2 times lower than the ionic conductivities of ZrO₂ systems.     

Fabrication and Structural Control of Fe/Ti Oxide Thin Films with Graded Compositional Profiles by Liquid Phase Deposition

Fe/Ti oxide films with graded composition profiles have been prepared from aqueous metal-fluoro complex solutions by the liquid phase deposition method. Films were deposited from a solution, in which the (NH₄)₂TiF₆ solution was added under a controlled flow rate into the FeOOH–NH₄F·HF solution. Transmission electron microscopy and X-ray microanalysis of the films revealed that the composition and graded profile of the deposited films in depth direction could be controlled by adjusting the mixing conditions of the reacting solution. Along with the change in composition, the microstructure of the films changes from microcrystalline β-FeOOH at the base to nanocrystalline anatase TiO₂ near the surface.     

Growth of Highly Oriented TlBa2Ca2Cu3O9—y Superconducting Films on Ag Substrates Using a Dip-Coated Barium Calcium Copper Oxide Sol—Gel Precursor

Thin films of Ba_zCa₂Cu₃O₇ a precursor of TIBa₂, Ca₂-Cu₃O_9-y, were prepared by sol-gel synthesis from an all alkoxide solution. The barium and calcium precursors were the respective metals reacted with 2-methoxyethanol, and the copper precursor was copper methoxide complexed by triethanolamine. Silver substrates were coated using the sol-gel solution by dip-coating. Subsequent processing included a low-temperature drying step (600°C), repeated coating to provide the desired thickness, heat treatment at 850°C in oxygen to remove carbon, and reaction at 860°C in a two-zone thallination furnace to produce a TIBa₂Ca₂-Cu₃O_9-y, superconducting film. These films were strongly c-axis-aligned, had a zero-resistance temperature (T₂) of 110 K, and a critical-current density (J_c) of 1.9 × 10₄ A/cmZ at 77 K and zero magnetic field.     

Preparation and Characterization of Sol—Gel-Derived Lead Magnesium Niobium Titanate Thin Films with Pure Perovskite Phase and Lead Oxide Cover Coat

A sol–gel-derived Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ (PMNT) thin film was prepared using spin coating and a PbO cover coat technique. The amount of lead excess in the precursor solution had significant effects on the phase development and microstructure of the PMNT film. The PbO cover coat proved to be effective on suppressing the formation of pyrochlore phases. PMNT thin films with a pure perovskite structure were obtained by adding 30 mol% excess lead in the precursor solution and coating the PbO layer on the top of the film. The remnant polarization ( P _r), the dielectric constant (ɛ_r), and the dissipation factor (tan δ) of these thin films, which had a thickness of 150 nm, were determined to be 9 μC/cm², 1370, and 0.031, respectively.