The Defect Chemistry of Donor-Doped BaTiO3: A Rebuttal

It has recently been asserted that the donor charge in La⁺³ -doped BaTiO₃ is always compensated by Ti vacancies, and that electrons are never the primary compensating defect. It was also stated that the conductivity observed in reduced, donor-doped BaTiO₃ results from the loss of a very small amount of oxygen not directly related to the donor content. However, the observed reproducible and reversable weight loss on reduction, or gain on oxidation, is exactly that expected for a change between ionic and electronic compensation. It corresponds to the loss or gain of the excess oxygen contained in the donor oxide, e.g. LaO_1.5 vs. the BaO it replaces. The amount of this weight change is proportional to the donor concentration. This is in agreement with the observation that the equilibrium conductivity in the P(O₂)-independent region of electronic compensation is proportional to the donor concentration. Thus the conductivity observed in reduced samples is directly coupled to the donor concentration, and the carrier concentration is equal to the net donor content. In fact, the equilibrium conductivity of donor-doped BaTiO₃ conforms to the behavior expected from classical defect chemistry, and exhibits regions of both ionic and electronic compensation of the donor charge, as expected. Phase studies by TEM have shown that donor-doped BaTiO₃ sintered in air self-adjusts its composition, by splitting out a second phase if necessary, so that the appropriate amount of compensating Ti vacancies are formed. However, when sintered in a reducing atmosphere, the composition self-adjusts to accommodate charge compensation by electrons.     

Electrical Properties of Acceptor Doped BaTiO3

Electrical properties of acceptor (Mn, Mg or Mn+Mg)-doped BaTiO₃ ceramic have been studied in terms of oxygen vacancy concentration, various doping levels and electrical degradation behaviors. The solubility limit of Mn on Ti sites was confirmed to be close to or less than 1.0 mol%. Oxygen vacancy concentration of Ba(Ti_{0.995 –x}Mg_0.005Mn_x)O_{2.995 –y} (x = 0, 0.005, 0.01) was estimated to be 50 times greater than that of the un-doped BaTiO₃. The leakage current of 0.5 mol% Mn-doped BaTiO₃ was stable with time, which was much lower than that of the un-doped BaTiO₃. The BaTiO₃ specimen co-doped with 0.5 mol% Mg and 1.0 mol% Mn showed the lowest leakage current below 10^{– 10}A. It was confirmed that leakage currents of Mg-doped and un-doped BaTiO₃ under dc field are effectively suppressed by Mn co-doping as long as the Mn doping level is greater than Mg contents.     

Dysprosium Doped Dielectric Materials for Sintering in Reducing Atmospheres

Substitution of Dy rare earth ions was studied in Ba(Ti,Zr)O₃ dielectric materials, using thermogravimetry, X-ray diffraction and dielectric measurements. Dy³⁺ ions enter both the A- and the B-sites of the perovskite structure, however, the solubility on B-sites is up to 9 mol %, whereas it is only 2.5 mol% on A-sites. Dy can be easily shifted from A- to B-sites and back, using Ba or Ti excess in the material. Dy³⁺ on B-sites is a strong electron acceptor. Dy doped dielectric materials are cofired with Ni electrodes in reducing atmosphere to highly insulating BME multilayer capacitors.     

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.     

Studies of resistivity and dielectric properties of magnesium doped barium titanate sintered in pure nitrogen

This paper investigated resistivity and dielectric properties of magnesium doped BaTiO₃. Influence of variable doping concentration and Ba/Ti ratio on densification, microstructure, resistivity and dielectric properties was studied. Ceramics sintered in pure nitrogen had higher apparent density than that of ceramics sintered in air. Second phase occurred when Mg doping concentration was greater than 2 mol%. Ceramics sintered in pure nitrogen had higher resistivity than ceramics sintered in air when A/B ratio ≥1. When Ba/Ti ratio was equal to 1.01, resistivity of the samples sintered in pure nitrogen increased with doping concentration increasing. Dielectric constant changed little with doping concentration increasing, but leakage conductivity decreased much. Samples with Ba/Ti ratio of 1.005 had the largest dielectric constant irrespective of doping concentration.     

High temperature electrical characterization of nano-crystalline BaTiO3 synthesized using Ba(NO3)2 and TiO2

The reaction of Ba(NO₃)₂ with TiO₂ was studied by thermogravimetric (TG) and differential scanning calorimetric (DSC) techniques up to 1000°C and in nitrogen atmosphere. It was found that the formation of BaTiO₃ takes place above 600°C. BaTiO₃ powder was prepared by calcination of Ba(NO₃)₂ and TiO₂ precursor mixture at 800°C for 8 h. X-ray diffraction analysis of the synthesized BaTiO₃ confirmed the formation of tetragonal phase. Average crystallite size was found to be 44 nm, For the electrical and morphological characterization pellets of the obtained powder were sintered at 1000 °C for 12 h. Scanning electron micrograph (SEM) exhibits spherical and rod shaped grains. The dielectric constant, dissipation factor, complex plane impedance and ac conductivity of the sintered pellet has been measured in the temperature range of 40–600°C and frequency range of 100 Hz–2 MHz. DC conductivity of the sample was obtained from the impedance data. The conductivities (both ac and dc) and relaxation time (τ) exhibit two regions of temperature dependence, namely region I, which represents (280–450°C) and region II, which governs (450-600°C). Conduction and relaxation in both the temperature regions are explained in terms of hopping of electrons and doubly ionized oxygen vacancies (V_O^??).     

Electrical properties of BaPb1+x−y Sb y O3 ceramics derived from a modified sol-gel method

BaPbO₃ ceramics has attracted considerable interest due to the promising electrical applications. For the best control of the microstructure and phase, Sb-doped BaPbO₃ ceramics were prepared by a sol-gel route. Inorganic compounds were used as starting chemicals, citric acid and Ethylene diamine tetra-acetic acid (EDTA) as complex chelate agent, and distilled water as solvent. Experimental results demonstrate that the Sb-doped BaPbO₃ ceramics with homogeneous composition could be prepared by the method mentioned above. Influence of Pb/Ba ratio and Sb concentration on the room-temperature resistivity and the PTCR behavior of BaPb_1+x−y Sb _y O₃ (x = 0.0, 0.1, 0.2 and 0 ≤ y ≤ 0.2) compositions was investigated. The PTCR behavior was related to the Pb/Ba ratio and major compensating defect in BaPbO₃. The lowest electrical resistivity of Sb-doped BaPbO₃ was calculated to be 2.69 × 10⁻⁴ Ω·cm when the Sb concentration y = 12–13 mol%. The 0.5 mol% Sb-doped BaPbO₃ showed the best PTCR behavior, and its Curie temperature is about 850 °C.     

Mn-doped BaTiO3: Electrical Transport Properties in Equilibrium State

The electrical conductivity and thermoelectric power of Mn-doped BaTiO₃ (1 mole%) and undoped BaTiO₃ have been measured as functions of oxygen partial pressure (in the range of 10^-16 to 1 atm) and temperatures (in the range of 900 to 1200°C), and compared with each other to differentiate the effect of the Mn-addition. It is found that the isothermal conductivity of Mn-doped BaTiO₃ varies with increasing Po₂ as to to , unlike previously reported. This behavior is well explained by the shift of the ionization equilibrium, . The corresponding equilibrium constant, K_A, is determined from the Po₂ values demarcating those three different Po₂ regions as =3.19×10²² exp(–1.69 eV/kT). Basic parameters involving carrier density and mobility, and defect structure of Mn-doped BaTiO₃ are discussed in comparison with those of undoped BaTiO₃.     

Microstructure of X7R Type Base-Metal-Electroded BaTiO3 Capacitor Materials Co-Doped with MgO/Y2O3 Additives

Detailed microstructure of MgO/Y₂O₃ co-doped BaTiO₃ materials were examined using transmission electron microscopy (TEM). For the 1250_C-sintered BaTiO₃ samples possessing flat K-T characteristics, which meet the X7R specification, the granular structure is complicated. Most of the grains are very small ( 150 nm) and are highly strained. The small grains contain large proportion of Y₂O₃ species and are paraelectric, whereas the large grains contain Y₂O₃ species unevenly distributed and are of core-shell structure. In contrast, for the 1300C-sintered BaTiO₃ samples, which have K-T properties slightly off the X7R specification, the grains grew larger to around 300 nm. The core-shell structured grains are seldom observed. Apparently, it is the existence of such a non-equilibrium core-shell microstructure, which renders the dielectric properties of the BaTiO₃ materials extremely sensitive to the processing parameters.     

Effect of synthetic conditions on particle size and photo-luminescence properties of Y2O3:Eu3+ nanophosphor

In this paper, effect of synthetic conditions on the particle size, crystal structure, and the photoluminescence properties of the Y₂O₃:Eu³⁺ nanophosphor was investigated. Solvent and dispersing agent were determined as the synthetic parameters. The nanophosphor synthesized from methanol solvent showed the smaller particle size of 4 nm. The XRD analysis indicates that the crystal structure of the Y₂O₃:Eu³⁺ nanophosphor is mainly cubic crystal with orientation of (222), (440), (400), and small peak of (511) indicating monoclinic crystal. The Y₂O₃:Eu³⁺ nanophosphor synthesized by using methanol solvent and 0.1 wt.% hydroxypropyl cellulose (HPC) as a dispersing agent showed higher degree of crystallization of 10.5 of I ₍₂₂₂₎/I ₍₅₁₁₎ ratio than that without HPC. Also, the photoluminescence properties of the nanophosphor showed red color that excitation and emission wavelengths of the nanophosphor were 250 and 611 nm, respectively. Using the 250 nm UV source, the highly intensive photoluminescence peak could be achieved at 611 nm under the synthetic condition of methanol solvent adding 0.1 wt.% HPC.     

Redox behavior and electrical properties of Sr and Zr substituted BaTiO3

The oxidation and reduction behavior of Sr and Zr substituted BaTiO₃ has been studied by measuring equilibrium electrical conductivities as a function of oxygen activity. The substitution of Sr⁺² and Zr⁺⁴ for Ba²⁺ and Ti⁴⁺ in BaTiO₃ shifted the conductivity profiles to lower oxygen partial pressures. The replacement of Ba with smaller Sr ions suppresses the oxygen dissociation from the lattice, consequently increasing the enthalpy for reduction (ΔH _n ), whereas the Sr substitution requires less energy for oxidation (ΔH _p ). The increase in reduction enthalpy at lower oxygen activities suppresses the formation of oxygen vacancies, the materials becoming resistant to reduction process. Such a reduction resistant behavior gave rise to greater electrical resistivities for (Ba_0.9Sr_0.1)(Ti_0.9Zr_0.1)O₃ than for Ba(Ti_0.9Zr_0.1)O₃ when the samples were sintered at lower P(O₂).     

Photoluminescence properties of Y2O3co-doped with Eu and Bi compounds as red-emitting phosphor for white LED

Bismuth doped Y₂O₃: Eu was used as a red phosphor with a very high efficiency and an appropriate emission wavelength of around 310–400 nm. This red phosphor was synthesized by the solid state reaction which is normally used in the field of white LEDs. In this study, we synthesized Y₂O₃: Eu, Bi phosphors using a solid state reaction. We investigated the effect of the Eu^{3 +} and Bi^{3 +} concentrations and additive fluxes on the emission characteristics. The fabricated phosphors were investigated by analyzing their particle size and crystal structure with scanning electron microscopy and X-ray diffraction (XRD). Their photoluminescence (PL) spectra were also measured at room temperature.     

Effect of Sb2O3 addition on the varistor characteristics of ZnO-Bi2O3-ZrO2-MtrO (Mtr = Mn, Co)

Effect of Sb₂O₃ addition on the varistor characteristics of pyrochlore-free ZnO-Bi₂O₃-ZrO₂-M_trO (M_tr = Mn, Co) system previously proposed has been studied. With Sb₂O₃ up to 0.1 mol%, a gradual enhancement of densification and the grain growth inhibition were seen in the system sintered between 900 and 1200^∘C. In X-ray diffraction patterns, small amount of pyrochlore appeared in the specimens doped with Sb₂O₃ (>0.06 mol%), which is thought responsible for the sintering behavior. Enhanced values of non linear coefficient (α) were obtained in ZnO-Bi₂O₃-ZrO₂ (ZBZ) doped with 0.001 mol% Sb₂O₃, but was leveled off at higher concentrations. In ZBZ added with M_trO (M_tr = Mn, Co), significant increase of nonlinear coefficient (α > 30) along with low leakage current (I _L ≪ 100 μA/cm²) was attained. The α-enhancement effect of Sb₂O₃, however, was not observed in high-α ZBZ added with M_trO. As for degradation, addition of a trace amount (0.001 mol%) of Sb₂O₃ to ZBZM_tr was efficient, especially in I _L.     

A first principles investigation of new cathode materials for advanced lithium batteries

First principles calculations on the crystal and electronic structure of a layered Li(Ni_1/3Mn_1/3M_1/3)O₂ (M = Al, Ti, Cr, Fe and Mo) were undertaken as part of a search for new positive electrode materials for advanced lithium ion batteries. The formal charge of Ni, Mn and M (Ti and Mo) were estimated to be +2, +3 and +4, respectively, from electronic structures and interatomic distances. In the cases of the Al, Cr and Fe substitution, the compounds had trivalent M and tetravalent Mn ions. The solid-state redox reactions of Li(Ni_1/3Mn_1/3M_1/3)O₂ were calculated assuming a Li deinsertion scheme, and the reactions were shown to be Ni²⁺/Ni³⁺/Ni⁴⁺ and M³⁺/M⁴⁺ for the Cr and Fe substitution. Al substitution will lead to higher voltages, as fixed 3+ valence of Al forces more electron exchange with oxygen. The cases of Ti and Mo substitution, Ti and Ni ions do not participate in the redox reactions over the entire range, respectively. The substitutive cation-oxygen bonding has a more covalent character, when the redox energy of Ni is lowered, resulting in an increase in potential. As described above, the voltage profiles are very different because the types of metals are different and participate in electrochemical reactions according to the substituted.     

Effects of Mn-doping on TSDC and degradation of BaTiO3

The effects of Mn-doping on TSDC (Thermally Stimulated Depolarization Current) and electrical degradation of BaTiO₃ have been investigated. TSDCs of un-doped BaTiO₃ and Ba(Ti_1−x Mn_x)O_3−δ exhibited the three sharp TSDC peaks around phase transition temperatures. TSDC of Ba(Ti_0.995Mg_0.005)O_2.995 increased gradually from 50^∘C and this anomalous depolarization current kept going up well above the Curie temperature (∼130^∘C). TSDCs of un-doped BaTiO₃ and Ba(Ti_0.995Mn_0.005)O_3−δ decreased in the temperature range above the Curie point, whereas a slight increase in TSDC was confirmed at the specimen of Ba(Ti_0.99Mn_0.01)O_3−δ. TSDCs of Ba(Ti_0.995−y Mg_0.005Mn_y)O_3−δ (y = 0.005, 0.01) were lower than that of Ba(Ti_0.995Mg_0.005)O_2.995.     

Based on the Study on Preparation and Properties of Co(II) and La3+-Doped TiO2 Electrochromic Film

Abstract

In order to improve the optical properties and coloration effect. Co(II) and La³⁺ doped TiO₂ thin films were synthesized via a sol–gel, by using butyl titanate as precursor on the surface of ITO conductive glass substrate. Several techniques were used to characterize produces. Adopting the Sol-gel technology, [CH₃(CH₂)₃O]₄Ti, La₂O₃, CoC_l2?6H₂O as the precursor, the composite membranes with different molar ratio of Ti, Ti/La, Ti/Co to be prepared on the ITO glass substrate, the optimum technological parameters for preparation of the base material of the electro-induced discoloration glass are determined. The structure and surface morphology optical and electrochromic properties of Co(II) and La³⁺-doped TiO₂ films were examined with X-ray diffraction (XRD) analysis, energy dispersive spectrometer (EDS), scanning electron microscopy (SEM), and ultraviolet-visible spectrophotometer (UV-vis) DTA-TG analysis. It was found that:

1. The sintering temperature coating number, [CH₃(CH₂)₃O]₄Ti content of the TiO₂ film sample, and the doping category are the main factors that affect the structure and properties of the membrane based material;

2. Using two alcohol amine as chelating agent can effectively inhibit the formation of Ti (OH)₄ precipitation;

3. The properties of the TiO₂ glass film prepared by La₂O₃ and CoCl₂·6H₂O are fine;

4. Materials based on the preparation of a radiochromic film transmittance, high cyclic reversibility, cyclic voltammetry characteristics significantly, the results of the study for the electric induced radiochromic film-based material development and application provide a theoretical and experimental basis.

     

Synthesis, Microstructure and Electrical Properties of Hydrothermally Prepared Ferroelectric BaTiO3 Thin Films

Thin films of polycrystalline, tetragonal BaTiO₃ on oxidized Ti metal substrates were synthesized at 240°C under hydrothermal conditions. Microstructure and electrical properties of the films generated over a four week period of synthesis formed the focus of this study. The films displayed a smooth and shiny surface with a relatively dense structure and no observable cracks. Film thickness reached 0.5 m after two weeks of synthesis and thereafter remained constant. Diameters of the grains on the film surface were in the range of 12 m. It is proposed that initial formation of the BaTiO₃ film occurs by reaction of Ba²⁺ with solubilized titanium oxide on the Ti metal surface followed at later stages by an in-situ growth via reaction of TiO_x with Ba²⁺ diffusing through the BaTiO₃ film. X-ray diffraction and Raman spectroscopy indicated that the BaTiO₃ films are tetragonal, and the films exhibited typical ferroelectric hysteresis loops at room temperature. However, no evidence of the dielectric anomaly (Curie transition) between 30 and 200°C was observed. Dielectric constant of the films at 1 kHz at room temperature was between 400–500. Both dielectric constant and tan exhibited low dispersion as a function of frequency at temperatures below 150°C, and the dispersion increased with temperature.     

Effect of Cr additions on the microstructural stability of Ni electrodes in ultra-thin BaTiO3 multilayer capacitors

Microstructural control in thin-layer multilayer ceramic capacitors (MLCC) is one of the present day challenges to maintain an increase in capacitive volumetric efficiency. This present paper opens a series of investigations aimed to engineer the stability of ultra-thin Ni electrodes in BaTiO₃-based multilayer capacitors using refractory metal additions to Ni. Here, pure Ni and Ni–1 wt.% Cr alloy powders are used to produce 0805-type BME MLCCs with 300 active layers and with dielectric and electrode layer thickness around 1 μm. To investigate the continuity of Ni electrodes, both MLCC chips with pure and doped electrodes were sintered at different temperatures for 5 h. It is found that the continuity of Ni electrodes is improved most likely due to the effect of Cr on the low-melting point (Ni,Ba,Ti) interfacial alloy layer formation. The interfacial alloy layer is not observed when Cr is segregated at Ni-BaTiO₃ interface in the Cr-doped samples, while it is found in all undoped samples. The interfacial alloy layer is believed to increase mass-transfer along the Ni-BaTiO₃ interfaces facilitating an acceleration of Ni electrodes discontinuities.     

Microwave Tunable Properties of Ni-Doped (Ba0.5Sr0.5)TiO3 Thin Films Grown by Pulsed Laser Deposition

(Ba_0.5Sr_0.5)TiO₃ (BST) thin films were deposited by pulsed laser deposition (PLD) and investigated as a function of Ni dopant concentration in low and high frequency regions. In low frequency region (<10 MHz), the Ni-dopant concentration in BST films has a strong influence on the material properties including dielectric and tunable properties. Ni-doped (≤3 mol%) BST films showed denser, smoother morphologies and smaller grain sizes than those with 6 and 12 mol% Ni. Dielectric constant and loss of 3 mol% Ni-doped BST films were about 980 and 0.3%, respectively. In addition, tunability and figure of merit of 3 mol% doped BST films showed maximum values of approximately 39% and 108, respectively. In high frequency region (>1 GHz), the frequency tunability range at center frequency of undoped BST and 3 mol% Ni-doped BST coplanar waveguide (CPW) resonators showed 102 and 152 MHz, respectively at 30 V dc bias. The Ni-doped BST thin films are possible in applications of microwave tunable capacitors.     

Ferroelectric Properties of (Bi, Sm)4Ti3O12 (BST) Thin Films Fabricated by a Metalorganic Solution Deposition Method

Ferroelectric properties of samarium substituted Bi₄Ti₃O₁₂ films, Bi_3.15Sm_0.85Ti₃O₁₂ (BST), were evaluated for use as lead-free thin film ferroelectrics for FeRAM applications. The BST films were fabricated on the Pt/Ti/SiO₂/Si(100) substrates by a metalorganic solution deposition method. The measured XRD patterns revealed that the BST films showed only a Bi₄Ti₃O₁₂-type phase with a random orientation. The BST film capacitors showed excellent ferroelectric properties. For the film capacitor annealed at 700C, 2P_r of 64.2 C/cm² and 2E_c of 101.7 kV/cm at applied electric field of 150 kV/cm were observed. The capacitor did not show any significant fatigue up to 1.5 × 10⁸ read/write switching cycles at a frequency of 1 MHz, which suggests that the samarium should be considered for a promising lanthanide elements to make a good thin ferroelectric film for memory applications.