Hydrothermal Preparation and Characterization of Ultra-Fine BaTiO3 Powders from Amorphous Peroxo-Hydroxide Precursor

Ultra-fine BaTiO₃ powders were hydrothermally prepared by using Ba Ti-peroxo-hydroxide precursor. Amorphous Ba Ti-peroxo-hydroxide precursor were prepared by coprecipitation of Ba(NO₃)₂ and TiCl₄ aqueous solution adding in NH₄OH aqueous solution. The phase-pure BaTiO₃ powders with a cubic perovskite structure were synthesized at temperature as low as 110_C and in the pH range of 10–12. This processing method provides a simple low temperature route for producing BaTiO₃ nanoparticles. Under a TEM image and a SAD pattern analysis, it is evident that BaTiO₃ powders had spherical shape and single crystal nature. The BaTiO₃ ceramic sintered at 1200_C for 1 h had 97% of theoretical density and a relatively high dielectric constant ( _r = 3500).     

Selective Gas Detection of SnO2-TiO2 Gas Sensors

The composite, consisting of two materials with different sensing temperatures, may show the selectivity for a particular gas. In this study, the microstructural and compositional effects on the electrical conductivity and the CO and the H₂ gas sensing properties of SnO₂-TiO₂ composites were examined. SnO₂-TiO₂ composites in entire (0–100 mol%) composition range were fabricated in the form of porous pellet by sintering at 800C for 3 h. The effects of CuO-coating (or doping) on the electrical conductivity and the sensing properties to 200 ppm CO and H₂ gases were examined.With CuO-coating, SnO₂-TiO₂ composites showed the increased sensitivity to CO gas and a large difference in the sensing temperatures between CO and H₂ gases. As a result, CuO-coated SnO₂-TiO₂ composites showed the selectivity for CO gas between 100C and 190C and the selectivity for H₂ gas between 280C and 380C.     

Pb((Mg0.7Zn0.3)1/3Nb2/3)O3 Relaxor Ferroelectric Ceramics by a Reaction-Sintering Process

Pb((Mg_1/3Nb_2/3)_0.7(Zn_1/3Nb_2/3)_0.3)O₃ (PMZN) relaxor ferroelectric ceramics produced by a reaction-sintering process were investigated. Without any calcination, the mixture of PbO, Mg(NO₃)₂, Zn(NO₃)₂ and Nb₂O₅ was pressed into pellets and sintered directly. PMZN ceramics of 100% perovskite phase were obtained. Density of 8.11 g/cm³ (> 98% of theoretical value) was obtained after sintered at 1200C for 2 h. 3–9 m grain size was obtained in PMZN ceramics sintered at 1180C–1250C for two hours by reaction-sintering process.Dielectric constant at room temperature under 1 kHz reaches 18200 after sintered at 1200C for 2 h.     

Preparation of Perovskite Pb(B0.5Nb0.5)O3 (B = Rare-Earth Elements)

Pb(B_0.5Nb_0.5)O₃ (B = Sc, Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) powders were prepared from mixed nitrate solutions by a chemical coprecipitation method. This method produced very small particles (30 nm) with good compositional homogeneity. These powders were highly reactive upon calcination. The powders of the systems (B = Sc, Tm, Yb and Lu) yielded 100% perovskite phase after calcination between 800C and 1000C for 1 h. For the system with B = Er, 93% perovskite phase was formed at 900C for 1 h. For the other systems of the elements (B = Y, Nd, Sm, Eu, Gd, Tb, Dy and Ho) with a larger ionic size, perovskite compounds were not formed up to 1100C. The stability and the possible formation of the compounds with a perovskite structure for the Pb(B_0.5Nb_0.5)O₃ series were discussed.     

Partial Electronic Conductivity and Chemical Diffusivity of Li<Subscript>3<Emphasis Type="Italic">x</Emphasis></Subscript>La<Subscript>2/3−<Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> Solid Solution

Partial electronic conductivity and chemical diffusivity of Li have been measured on the system of Li_3xLa_2/3–xTiO₃ (LLT) with x = 0.13, a prospective Li⁺ electrolyte, against oxygen activity in the range of 10^–22 < a_O2 < 0.21 at 557, 610 and 663C, respectively by an ion-blocking polarization technique. It is found that the electronic conductivity of LLT, which in air is essentially an ionic conductor, varies as a_O2^–1/4 to render it mixed-conductive in reducing atmospheres, say, in a_O2 < 10^–12. The chemical diffusivity of component Li also increases from a value of the order of 10^–8 cm²/s in air atmosphere up to a maximum on the order of 10^–3 cm²/s as the electronic conductivity increases with decreasing oxygen activity. This is attributed to the variation of the electronic transference number and the thermodynamic factor with oxygen activity. The latter has been evaluated to be on the order of 10–10³.     

Ferroelectric Properties of Bi3.25Nd0.75Ti3O12 Thin Films Prepared by MOD Process

Ferroelectric Bi_{4 – x}Nd_xTi₃O₁₂(BNdT) thin films with the composition (x = 0.75) were prepared on Pt/Ti/SiO₂/Si(100) substrate by metal-organic deposition. The films were annealed by various temperatures from 550 to 650C and then the electrical and structural characteristics were investigated for the application of FRAM. Electrical properties such as dielectric constant, 2Pr and capacitance were quite dependent on the thermal heat treatment. The measured 2Pr value on the BNdT capacitor annealed at 650C was 56 C/cm² at an applied voltage of 5 V. No fatigue was observed up to 8 × 10¹⁰ read/write switching cycles at a frequency of 1 MHz regardless of annealing temperatures.     

Electrical Properties of 6H-BaTi0.95M0.05O3−δ Ceramics where M = Mn, Fe, Co and Ni

Hexagonal BaTiO₃ materials have been stabilised at room temperature according to the formula BaTi_0.95M_0.05 O_3– where M = Mn, Fe, Co and Ni. Dense ceramics (> 96% of the theoretical X-ray density) were sintered at 1450C in flowing O₂ gas from calcined powders prepared by the mixed oxide route at 1300C. All samples were single-phase and the bulk conductivity, _b, measured by Impedance Spectroscopy and Q.f measured by microwave dielectric resonance methods showed a strong dependence on the type of dopant. _b at 300C was 10^–7, 10^–5.5, 10^–5.5 and 10^–4 Scm^–1 for M = Mn, Fe, Ni and Co, respectively and Q.f at 5 GHz was 7790, 6670, 2442 and 1291 GHz, for M = Mn, Fe, Ni and Co, respectively. The correlation between _b and Q.f is attributed to the presence of oxygen vacancies and/or mixed valency of the dopant ions.     

PZN-PFW and PFN-PFW Relaxor Ferroelectric Ceramics by a Reaction-Sintering Process

Pb((Zn_1/3Nb_2/3)_0.6(Fe_2/3W_1/3)_0.4)O₃ and Pb((Fe_1/2Nb_1/2)_0.7(Fe_2/3W_1/3)_0.3)O₃ (PZNFW and PFNW) perovskite ceramics prepared by a reaction-sintering process were investigated. Without any calcination, the mixture of PbO, Zn(NO₃)₂, Fe(NO₃)₃, Nb₂O₅ and WO₃ for stoichiometric PZNFW and PFNW was pressed and sintered directly. Pyrochlore phase more than 25% were formed in PZNFW ceramics after 2 h sintering at 930–980C. PFNW ceramics of 100% perovskite phase were obtained after 4 h sintering at 930–1080C. A density of 8.13 g/cm³ (93.4% of theoretical value) was obtained after sintered at 1080C for 4 h. Dielectric constant at room temperature under 1 kHz reaches 32000 after sintered at 1080C for 4 h.     

Integration of MgO on Si(001) Using SrO and SrTiO3 Buffer Layers by Molecular Beam Epitaxy

Epitaxial MgO was deposited onto Si(001) substrates by molecular beam epitaxy using elemental metallic sources and molecular oxygen at temperatures from 150 to 400C. To facilitate epitaxy through misfit strain relaxation, epitaxial MgO layers were grown on SrO and SrTiO₃ buffer layers deposited on Si(001) substrates. The structure of the epitaxial layers was determined by X-ray diffraction, reflection high-energy electron diffraction and transmission electron microscopy. The observed orientation for the MgO/SrO/Si multilayer is cube-on-cube. The X-ray rocking curve full width half maximum of the MgO on SrO buffer layers was 2.2. SrTiO₃ buffer layers grown by recrystallization were epitaxial and exhibited improved morphology relative to those grown at a fixed growth temperature. X-ray analysis of a 5.2 nm recrystallized SrTiO₃ film indicates a fully relaxed and phase pure film. The observed orientation of MgO using SrTiO₃ buffer layers is MgO[100]SrTiO₃[100]Si[110].     

Co-Doping Effect of Mn and Y on Charge and Mass Transport Properties of BaTiO3

BaTiO₃-based multilayer-ceramic capacitors (MLCC) using base metal (Ni) electrodes normally contains Mn and Y each approximately on the order of 0.5 mol%. It is only empirically known that the co-doping of Y and Mn facilitates sintering with the base-metal electrodes as well as improves the device performance and life time. In order to understand the effect of the co-doping, we have measured the electrical conductivity and chemical diffusivity on polycrystalline BaTiO₃ that is co-doped with Y and Mn each by 0.5 mol% against oxygen partial pressure at elevated temperatures. It is found that while the n-type conductivity in reducing atmospheres (e.g., Po₂ < 10^{– 6} atm at 1000C) remains similar to that of undoped or acceptor-doped BaTiO₃, its p-type conductivity in oxidizing atmospheres (e.g., Po₂ > 10^{– 6}at 1000C) is remarkably suppressed compared to the latter. The chemical diffusivity is also similar to that of the latter in magnitude (e.g., 10^{– 2}–10^{– 5} cm²/s at 1000C), but its trend of variation with oxygen partial pressure is rather opposite. These variations of the conductivity and chemical diffusivity are mainly attributed to Mn ions changing their valence from +2 to +3 to +4 with increasing oxygen partial pressure. It is explained from a defect-chemical view why the codoping of fixed-valent donor (Y) and variable-valent acceptor (Mn) has been practiced in MLCC processing.     

Effect of B2O3 Nano-Coating on the Sintering Behaviors and Electrical Microwave Properties of Ba(Nd2 − x Sm x )Ti4O12 Ceramics

Ba(Nd_0.8Sm_0.2)₂Ti₄O₁₂ ceramics prepared by conventional solid-state sintering have a dielectric constant of about 80 and a nearly zero temperature coefficient of resonant frequency; however, the sintering temperature is above 1350_C. Doping with B₂O₃ (up to 5 wt%) promotes the densification and dielectric properties of BNST ceramics. It is found that coating BNST powder with thin B₂O₃ layer of about 180 nm reduces the sintering temperature to below 1020_C. The effects of B₂O₃ nano-coating on the dielectric microwave properties and the microstructures of BNST ceramics are investigated. Ninety-six percent of theoretical densities is obtained for specimens coated with 2 wt% B₂O₃ sintered at 960_C and the samples exhibit significant (002) preferred orientation and columnar structure.     

Nanocrystalline Alkaline Earth Titanates and Their Conductivity Characteristics Under Changing Oxygen Ambients

This work presents the first systematic study of conductivity characteristics of alkaline earth titanates in the form of polycrystalline and heteroepitaxial thin films as well as nanocrystalline ceramics as a function of temperature (between 600_C and 1000_C) and continuously adjustable oxygen partial pressures ranging from 10^{– 20} bar to 1 bar. Compared to the well-known log,-log, pO₂ profiles of single crystals, the conductivity behavior of CSD-prepared, polycrystalline SrTiO₃ thin films with a feature size of about 50 nm differs radically. The most prominent characteristics are a sharp drop under reducing conditions followed by a broad plateau region. Tailored investigations on heteroepitaxial as well as polycrystalline thin films grown by PLD and especially by studies on nanocrystalline BaTiO₃ ceramics with a mean grain size of 100 nm allowed an unambiguous assignment of the described effects to the nanocrystalline morphology of the samples.     

Low Temperature Sintering of BaTi<Subscript>4</Subscript>O<Subscript>9</Subscript>-Based Middle-<Emphasis Type="Italic">k</Emphasis> Dielectric Composition for LTCC Applications

Effect of glass addition on the low-temperature sintering and microwave dielectric properties of BaTi₄O₉-based ceramics were studied to develop the middle-k dielectric composition for the functional substrate of low-temperature co-fired ceramics. When 10 wt% of glass was added, sufficient densification was obtained and the relative density more than 98% was reached at the sintering temperature of 875C. The microwave dielectric properties were k = 32, Q × f = 9000 GHz, and t_cf = 10 ppm/C. As the added amount of glass frit with base dielectric composition, phase changes from BaTi₄O₉ to BaTi₅O₁₁ and Ba₄Ti₁₃O₃₀ was observed, which result in the modification of microwave dielectric properties.     

Dielectric and Piezoelectric Properties of Nonstoichiometric SrBi2Ta2O9 and SrBi2Nb2O9 Ceramics

Nonstoichiometric SrBi₂Ta₂O₉ (SBT) and SrBi₂Nb₂O₉ (SBN) ceramics were prepared by a solid state reaction method. X-ray diffraction analysis showed that single-phase of Bi-layered perovskite was obtained. With different Sr/Bi content ratios of SBT and SBN, Curie temperature (T_C), electromechanical factor (K_p) and mechanical quality factor (Q_m) were measured. T_C of SBN (SBT) rose from 414C (314C) to 494C (426C) when Sr/Bi content ratio was increased from 0.55/2.3 to 1.2/1.8. In the most Sr-deficient/Bi-excess ratio of 0.55/2.3, the maximum values of Q_m were obtained approximately 1013 and 3325 for SBT and SBN, respectively.     

Microwave Dielectric Properties of CuO-V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-Doped ZnNb<Subscript>2</Subscript>O<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

Microwave dielectric properties of low temperature sintering ZnNb₂O₆ ceramics doped with CuO-V₂O₅-Bi₂O₃ additions were investigated systematically. The co-doping of CuO, V₂O₅ and Bi₂O₃ can significantly lower the sintering temperature of ZnNb₂O₆ ceramics from 1150 to 870C. The secondary phase containing Cu, V, Bi and Zn was observed at grain boundary junctions, and the amount of secondary phase increased with increasing CuO-V₂O₅-Bi₂O₃ content. The dielectric properties at microwave frequencies (7–9 GHz) in this system exhibited a significant dependence on the relative density, content of additives and microstructure of the ceramics. The dielectric constant ( _r) of ZnNb₂O₆ ceramics increased from 21.95 to 24.18 with increasing CuO-V₂O₅-Bi₂O₃ additions from 1.5 to 4.0 wt%. The quality factors (Q× f) of this system decreased with increasing CuO-V₂O₅-Bi₂O₃ content and ranged from 36118 to 67100 GHz for sintered ceramics, furthermore, all Q× f values of samples with CuO-V₂O₅-Bi₂O₃ additions are lower than that of un-doped ZnNb₂O₆ ceramics sintered at 1150C for 2 h. The temperature coefficient of resonant frequency ( _f) changed from –33.16 to –25.96 ppm/C with increasing CuO-V₂O₅-Bi₂O₃ from 1.5 to 4.0 wt%     

Copper Compatible Barium Titanate Thin Films for Embedded Passives

Barium titanate thin films have been prepared by chemical solution deposition on 18 m thick, industry standard copper foils in the absence of chemical barrier layers. The final embodiment exhibits randomly oriented BaTiO₃ grains with diameters between 0.1 and 0.3 m, and an equiaxed morphology. The average film thickness is 0.6 m and the microstructure is free from secondary or interfacial phases. The BaTiO₃ films are sintered in a high temperature reductive atmosphere such that copper oxidation is avoided. Subsequent lower-temperature, higher oxygen pressure anneals are used to minimize oxygen point defects. Permittivities of 2500 are observed at zero bias and room temperature, with permittivities greater than 3000 at the coercive field. Loss tangents under 1.5% are demonstrated at high fields. The BaTiO₃ phase exhibits pronounced ferroelectric switching and coercive field values near 10 kV/cm. Temperature dependent measurements indicate a ferroelectric transition near 100C with very diffuse character. Combining the approaches of the multilayer capacitor industry with traditional solution processed thin films has allowed pure barium titanate to be integrated with copper. The high sintering temperature—as compared to typical film processing—provides for large grained films and properties consistent with well-prepared ceramics. Integrating BaTiO₃ films on copper foil represents an important step towards high capacitance density embedded passive components and elimination of economic constraints imparted by traditional noble metallization.     

Copper Cofire X7R Dielectrics and Multilayer Capacitors Based on Zinc Borate Fluxed Barium Titanate Ceramic

Copper cofired dielectrics may give new opportunities for high temperature capacitors. To demonstrate feasibility, BaTiO₃ has been formulated into X7R dielectrics with copper inner electrodes. This requires the development of a formulation that permits sintering at temperatures below 1000°C, and then firing in a reducing environment in atmospheres pO₂ 10^–8 atms. ZnO—B₂O₃ chemistries were explored with additional dopants to modify densification and the temperature coefficient of capacitance of the BaTiO₃ dielectric anomaly. X7R characteristics with relative dielectric permittivities 2750 and tan 0.01 at 1 kHz were obtained at room temperature. Multilayer capacitors were fabricated in 3.2 mm × 1.6 mm size multilayers with an acrylic binder system and oxidation resistive copper inner electrodes.     

Praseodymium-Cerium Oxide as a Surface-Effect Gas Sensor

The gas sensing behavior of praseodymium doped cerium oxide (Pr_xCe_{1 – x}O₂ or PCO) has been examined for 0–1000 ppm CO or H₂ in a 10% O₂ atmosphere at temperatures ranging from 250–350_C. Total conductivity as a function of temperature suggests that oxygen diffusion kinetics are slow below approximately 350_C. Devices with x = 0.05 and 0.10 show stable, n-type gas sensing response, while those with x = 0.20 exhibit significant drift in sensor output, presumably due to bulk oxygen migration. The response to CO is significantly stronger than that to H₂ at 300_C, and at 350_C the response to H₂ is nearly zero, resulting in a CO-selective gas sensing element. Suggestions for the source of selectivity in PCO are presented.     

Properties of CaTi1 − x Fe x O3 − δ Ceramic Membranes

CaCO₃, TiO₂ and Fe₂O₃ were mixed in the appropriate stoichiometric quantities and calcined at 1100C for 10 h. These powder mixtures were uniaxially pressed and sintered at temperatures ranging from 1350 to 1500_C for 2 h in order to obtain dense disk-shaped samples with nominal CaTi_{1 – x}Fe_xO_3– (x = 0.05, 0.15, 0.20, 0.40 and 0.60) compositions. Dilatometry and in situ high temperature powder X-ray diffraction analysis showed a good agreement on the thermal expansion behaviour of these materials between room temperature and 1000_C. The estimated linear thermal expansion coefficient is close to 13× 10^{– 6} K^{– 1} and is little affected by composition. No evidence for surface carbonation was detected in the infrared spectra collected on samples previously annealed in CO₂ atmospheres. The oxygen permeability measured at temperatures ranging from 750 to 1000_C goes through a sharp maximum for x = 0.20. This result is interpreted by structural differences related to change from disordered to ordered oxygen vacancies. The overall performance of CaTi_0.80Fe_0.20O_3– is compared to other mixed conducting materials.     

Recent Developments on MOCVD of Ferroelectric Thin Films

Ferroelectric Pb(Zr, Ti)O₃ thin films were fabricated by liquid delivery MOCVD using Pb(DPM)₂, Ti(OiPr)₂(DPM)₂ and Zr(DIBM)₄. The deposition rate of 12.3 nm/min was attained on 6-inch Pt/Ti/SiO₂/Si wafers at 550C. The average and the deviation of twofold remanent polarization were 45.5 C/cm² and ± 6.4%, respectively, over the 6-inch wafer. Step coverage was improved from 44% to 90% by decreasing deposition temperature from 550 to 400C although the deposition rate decreased by 60%. TiO₂ nanoparticles diffused to the surface of platinum bottom electrodes were effective as a seed to obtain 111 preferential oriented PZT thin films at the deposition temperature of 550C. Iridium oxide bottom electrodes were reduced to metal ones by CO and/or H₂ gases generated by decomposition of precursors. Oxide materials seem to be not the best as bottom electrodes in liquid delivery MOCVD. A cocktail source consisted of Pb(METHD)₂, Ti(MPD)(METHD)₂ and Zr(METHD)₄ was also examined. PbPt_x alloy phase existed in PZT films deposited at 500C was disappeared by post-annealing at 600C and the annealed film showed hysteresis properties with the 2P_r of 56 C/cm² and the 2E_c of 181 kV/cm.