Microstructure effects on the local order and electronic defects in (Al,Ti, Mg) co-doped ZnO conductive ceramics

Fine particles with different sizes of (Al, Ti, Mg) oxide co-doped ZnO ceramics were realized by a suitable grinding and a mechanical ball milling, respectively. The investigations were devoted to understand the origin of the electrical conductivity evolution with the microstructure by analyzing the local order and defect involvement in the crystalline sites of ZnO ceramics. Experimental investigations of particles were conducted to probe the local order and electronic defects with an emphasis on the electrical behavior of ceramics. Particularly, Al doping is intimately correlated with the high conductivity induced from the stabilization of particular Al_Zn-Zn_i complexes. The conduction electrons are probed through the induced Knight Shift on NMR spectra and also from the particular relaxation mechanisms of paramagnetic centers revealed by EPR studies. The correlation between the electronic active defects, the microstructure in small sized particles of ZnO based ceramics and the electrical behavior are pointed out and discussed.     

Electronic active defects and local order in doped ZnO ceramics inferred from EPR and 27Al NMR investigations

Doped ZnO based ceramics were fabricated by using a solid state reaction of ZnO co-doped by TiO_2, Al₂O₃ and MgO and sintered in different atmospheres (Air, N₂, N₂ + CO). The crystalline structures consist in wurtzite ZnO and a minor spinel phase Zn₂TiO₄. The electrical conductivity is modulated by the sintering conditions with the highest value (˜10⁵ S m⁻¹) obtained in the reducing atmosphere (N₂ + CO). The role of defects and vacancies on the electrical behavior was exhaustively investigated by Raman, electron paramagnetic resonance (EPR) and solid state NMR methods. The paramagnetic centres inferred from EPR studies show a Pauli-like spin susceptibility. Their origin was assigned to shallow donors from interstitial defects (Zn_i) favored by substitutional Al ions (Al_Zn). The NMR spectral features with a characteristic 185 ppm line which correlates with the electrical conductivity are presumed to be caused by the Knight shift effect from the conduction electrons and the involved paramagnetic centres.     

Controllable microstructure tailoring for regulating conductivity in Al-doped ZnO ceramics

Structural and electrical behavior of Al₂O₃ doped ZnO-based ceramics were investigated as function of the aluminum doping ratios under reducing sintering atmosphere (N₂+CO). With Al₂O₃ doping from 0.1 mol% to 0.55 mol%, the electrical conductivity increases firstly to a maximum (1.52 × 10⁵ S·m⁻¹) at 0.25 mol%, and then decreases gradually. The increased conductivity is explained by the formation of shallow donors as Al_Zn-Zn_i complexes with doping to 0.25 mol%. As Al₂O₃ doping further increasing to 0.55 mol%, ZnAl₂O₄ spinel phase and more ZnO-ZnO grain boundaries are formed, hindering charge carriers transport, to decrease charge carrier mobility, thus to decrease the conductivity of ZnO ceramics. Therefore, the Al_Zn-Zn_i complexes, grain boundaries and ZnAl₂O₄ spinel can be adjusted by doping different Al₂O₃ amount, thus the carriers’ concentration and their mobility are optimized to increase the conductivity. Our work, as a fundamental research, is of great significance to control conductivity by regulating Al₂O₃ doping.     

High-temperature thermoelectric properties of polycrystalline Zn1−x−yAlxTiyO ceramics

The as-sintered Zn_1−xAl_xO (0 ≤ x ≤ 0.05) samples crystallized in the ZnO with a wurtzite structure, along with a small amount of the cubic spinel ZnAl₂O₄. The addition of Al₂O₃ to ZnO gave rise to a decrease in grain size, ranging from 7.3 to 2.7 μm and in relative density, ranging from 99.2 to 90.1% of the theoretical density. In the Zn_0.97Al_0.03−yTi_yO samples, as the amount of TiO₂ increased, the grain size of ZnO grains and second phases, such as Zn₂TiO₄ and ZnAl₂O₄, as well as density increased. The co-doping of Al and Ti led to a significant increase in both the electrical conductivity and the absolute value of the Seebeck coefficient, resulting in an increase in the power factor. The highest value of power factor (3.8 × 10⁻⁴ W m⁻¹ K⁻²) was attained for Zn_0.97Al_0.02Ti_0.01O at 800 °C. It is demonstrated that the Al and Ti co-doping is fairly effective for enhancing thermoelectric properties.     

Defects and microstructure of highly conducting Al-doped ZnO ceramics obtained via spark plasma sintering

Al-doped ZnO ceramics were sintered by conventional sintering method and spark plasma sintering (SPS) respectively. Electrical properties and microstructure have been investigated by various measurements. The samples sintered via SPS exhibit a huge electrical conductivity, up to 3.0 × 10⁵ S/m at room temperature, which was much higher than that of the sample sintered via the conventional sintering. Structural and morphorlogical characterizations pointed out that the further incorporation of Al ions and the absence of a secondary phase, contribute to the increase of the carrier concentration. Raman spectroscopy revealed the occurrence of structural distortions and a disorder induced by Al doping. Photoluminescence spectra were interpreted by different electronic active defects such as the defect complexes (Al_Zn-Zn_i) which play a key for the high electrical conductivity. Thus, SPS and Al doping modified the microstructure and the concentration of the electronic active defects to ensure high electrical conductivities in doped ZnO-based ceramics.     

Crystallization,mechanical, and optical properties of transparent,nanocrystalline gahnite glass‐ceramics

This paper describes the preparation of a transparent glass‐ceramic from the SiO₂‐K₂O‐ZnO‐Al₂O₃‐TiO₂ system containing a single crystalline phase, gahnite (ZnAl₂O₄). TiO₂ was used as a nucleating agent for the heat‐induced precipitation of gahnite crystals of 5‐10 nm. The evolution of the ZnAl₂O₄ spinel structure through the gradual formation of Al‐O bonds was examined by infrared spectroscopy. The dark brown color of the transparent precursor glass and glass‐ceramic was eliminated using CeO₂. The increase in transparency of the CeO₂‐doped glass and glass‐ceramics was demonstrated by UV‐visible absorption spectroscopy. EPR measurements confirmed the presence of Ce³⁺ ions, indicating that CeO₂ was effective in eliminating the brown color introduced by Ti³⁺ ions via oxidation to Ti⁺⁴. The hardness of the glass‐ceramic was 30% higher than that of the as‐prepared glasses. This work offers key guidelines to produce hard, transparent glass‐ceramics which may be potential candidates for a variety of technological applications, such as armor and display panels.     

Structural,electric modulus and complex impedance analysis of ZnO/TiO2 composite ceramics

ZnO/TiO₂ composite ceramics have been prepared by solid‐state reaction technique at 900°C. The X‐ray diffraction results revealed the formation of secondary phases referred to as spinel Zn₂TiO₄ and hexagonal ZnTiO₃. The structural analysis showed that all the composites that have been prepared have a polycrystalline nature and a hexagonal wurtzite structure. The complex modulus (M) and electric impedance of the samples have been investigated by broadband dielectric spectroscopy in a wide range of temperature (40°C‐110°C) and frequency (0.1 Hz to 10 MHz). The modulus plots (M′′, M′) illustrate the presence of non‐Debye type of relaxations attributed to the effects of interfacial and dipolar polarizations. The real and the imaginary parts of the impedance are well fitted to equivalent circuit models. At high temperatures, Z″_max varies from 0.03 × 10⁶ to 4.9 × 10⁶ Ω when the TiO₂ doping concentration increases from 1 to 7 wt%. From the obtained results, the secondary phase ZnTiO₃ plays an important role in the electrical properties.     

Band structure and thermoelectric properties of inkjet printed ZnO and ZnFe2O4 thin films

The band structure and thermoelectric properties of inkjet printed ZnO and ZnFe₂O₄ thin films have been investigated. The bulk pellets were prepared by a solid-state method and thin films were deposited using an inkjet printing method. Multiple print cycles were required to fabricate homogeneous films and the composition of the thin films can be varied by varying the relative amounts of liquid deposited. It was possible to obtain high thermoelectric properties of ZnO by controlling the ratios of dopant added and the temperature of the heat treatments. XRD analysis showed that the fabricated samples have a wurtzite structure and an additional ZnAl₂O₄ phase was formed with increasing Al content and sintering temperature. It was found that the band gap of Al doped ZnO becomes smaller with increasing Al content and thus the electrical conductivity of Al_x doped ZnO (x=0.04) thin films showed the highest electrical conductivity (114.10 S/cm). The ZnFe₂O₄ samples were compared against the ZnO samples. The formation of single phase cubic spinel structure of the sintered ZnFe₂O₄ samples was found and confirmed by X-ray diffraction technique. Secondary phase Fe₂O₃ was also detected for compositions with Zn (x≤0.4). Finally, we want to report that the electrical conductivity of Zn_xFe_3−xO₄ was lower than the conductivity of the Al-doped ZnO.     

Structural and optical characterization of Zn0.95−xMg0.05AlxO nanoparticles

The structural and optical properties of ZnO nanoparticles doped simultaneously with Mg and Al were investigated. XRD results revealed the hexagonal wurtzite crystalline structure of ZnO. The FE-SEM study confirmed the formation of nano-sized homogeneous grains whose sizes decreased monotonously with increasing doping concentrations of Mg and Al. The absorption spectra showed that band gap increased from 3.20 to 3.31 eV with Mg doping. As the Al concentration changed from x=0.01 to x=0.06 mol% at constant Mg concentration the band gap observed to be decreased. Particle sizes estimated from effective mass approximation using absorption data and these values are in good agreement with the crystallite sizes calculated from XRD data. Raman spectra of ZnO showed a characteristic peak at 436 cm⁻¹ correspond to a non-polar optical phonon E₂ (high). With increase of the Al doping concentrations, E₂ (high) phonon frequency shifted to 439 cm⁻¹ from to 436 cm⁻¹. The origin of E₂ (high) peak shift in ZnO nanoparticles is attributed to optical phonon confinement effects or the presence of intrinsic defects on the nanoparticles. PL spectra indicated that with increase of Al co-doping along with Mg into ZnO, intensity of the peak positioned at 395 nm was initially increased at x=0 and then decreased with increase of the Al concentrations from x=0.01 to x=0.06 mol%.     

Effect of ZnO on the microstructure and electrical properties of WO3–Bi4Ti3O12 ceramics

The aim of the present work is to explore the possibility of incorporate a small amount of ZnO to improve the microstructure control of W-doped BIT-based materials. Two different processing routes have been used according to previous results reported for other materials: reaction and sintering in one single step and a previous calcination step. The sintering behaviour of the samples, the obtained crystalline phases and the microstructure analysis indicate that the reaction between ZnO and Bi₂O₃ plays a critical role during sintering. Both Bi₂Ti₂O₇ and Zn₂TiO₄ secondary phases are stabilized when adding ZnO. Actually, when WO₃ and ZnO are incorporated simultaneously to BIT materials, they interact stabilizing the Bi₂Ti₂O₇ phase and avoiding the incorporation of W⁶⁺ into the BIT lattice. As a consequence, the electrical conductivity of the samples with ZnO is two orders of magnitude higher than that of the samples doped only with WO₃, suggesting that WO₃ does not form a solid solution with BIT. The curve dielectric constant vs temperature also reveals the role played by the Bi₂Ti₂O₇ phase.     

Phase transformation and dielectric properties of sputtering-prepared Zn–Ti–O thin films

Zn–Ti–O films were co-sputtered from Zn and Ti targets and then annealed at temperatures ranging from 600 °C to 900 °C for 2 h under an air atmosphere. The [Ti]/([Ti]+[Zn]) ratio decreased from 75.52 to 28.26 as the Zn-target power increased from 25 W to 75 W. The phase transition of the films strongly depended on the [Ti]/([Ti]+[Zn]) ratio. High [Ti]/([Ti]+[Zn]) ratios led to the coexistence of ZnTiO₃, Zn₂Ti₃O₈, and rutile TiO₂ phases. Zn₂Ti₃O₈ gradually became the major crystalline phase as the [Ti]/([Ti]+[Zn]) ratio and rutile TiO₂ and ZnTiO₃ phases decreased. The aforementioned phases disappeared when the [Ti]/([Ti]+[Zn]) ratio was especially low. In their place, Zn₂TiO₄ and even ZnO phases developed. The dielectric constant of the films increased with increasing [Ti]/([Ti]+[Zn]) ratio. However, extremely high [Ti]/([Ti]+[Zn]) ratios increased the dielectric loss of the films. The film mainly composed of the Zn₂Ti₃O₈ phase exhibited optimal dielectric properties, including a dielectric constant and loss equal to 40.1 and 0.0304, respectively, at 1 MHz.     

Ge-doped Li4Ti5-xGexO12 (x = 0.05) as a fast-charging,long-life bi-functional anode material for lithium- and sodium-ion batteries

We explored the doping effect of Ge⁴⁺ on the Li₄Ti_5-xGe_xO₁₂ (x = 0.0 and 0.05) anode material by looking at its electrochemical performance in both Li- and Na-ion batteries. Combined analysis using Rietveld refinement of high-resolution powder diffraction (HRPD) and transmission electron microscopy (TEM) unambiguously identified homogeneous Ge doping into the 16c octahedral Ti site of the Li₄Ti₅O₁₂ (LTO) cubic spinel structure. This Ge doping leads to a much-reduced particle size, slightly expanded lattice and increased electrical conductivity due to the increased Ti³⁺ to Ti⁴⁺ ratio, these results were verified by HRPD, scanning electron microscopy (SEM), 4-point probe and x-ray photoelectron spectroscopy (XPS) analysis. The Li₄Ti_4.95Ge_0.05O₁₂ (Ge0.05-LTO) electrode shows much-improved capacity, high-rate capability and excellent cycling stability in a Li-half cell compared with an un-doped LTO electrode. This performance improvement is due to the reduced Li⁺ diffusion path and faster Li⁺ insertion/extraction kinetics that originate from Ge doping. In addition to these results, when tested as an anode for SIBs, the Ge0.05-LTO electrode exhibits enhanced capacity and cycling stability compared to un-doped LTO electrode, demonstrating its bi-functional, advantageous features in both LIB and SIB systems.     

Replacement behavior of Ti and Mg and selective separation of MgxTi3-xO5 (x=1, 0.9, 0.75, 0.6) from Ti-bearing electric furnace smelting slag via super-gravity

Ti-bearing electric furnace smelting slag produced from direct reduction and electric furnace smelting process of vanadium titanomagnetite ore, contains a high TiO₂ content of 40–55 wt%. While a mass of Mg, Al, Ca and Si are closely mixed with Ti in the slag, which greatly limits the recovery of Ti resource from the slag. In this work, the replacement behavior of Ti and Mg in Mg_xTi_3-xO₅ was studied, and selective separation of various Mg_xTi_3-xO₅ phases from Ti-bearing electric furnace smelting slag was conducted via super-gravity. It was found that Mg could replace Ti in Ti₃O₅ and form the Mg_xTi_3-xO₅, and increasing of cooling rate greatly limited the doping of Mg into Mg_xTi_3-xO₅, the Mg_xTi_3-xO₅ was transformed from (MgTi₂)O₅ to (Mg_0.9Ti_2.1)O₅, (Mg_0.75Ti_2.25)O₅ and (Mg_0.6Ti_2.4)O₅ respectively. On this basis, various Mg_xTi_3-xO_{5 (x = 1, 0.9, 0.75, 0.6)} phases were selectively separated from the smelting slag via super-gravity, where the mass fraction of TiO₂ was increased from 78.69 to 90.32 wt% while that of MgO was decreased from 13.56 to 6.98 wt% with the increase of Ti/Mg ratio in Mg_xTi_3-xO₅. Moreover, the replacement mechanism of Mg and Ti was confirmed from characterization of crystal structure and lattice parameter of various separated high-purity Mg_xTi_3-xO₅ crystals.     

Al-doped ZnO varistors prepared by a two-step doping process

ABSTRACT

It is difficult to dope Al into main grains of ZnO varistor ceramics, especially for small doping amount. Generally, all raw materials including Al dopant are directly mixed together and sintered into ceramics. However, in this direct doping process, Al is apt to stay in grain boundaries, and almost does not enter grains. This does harm to the electrical properties of ZnO varistors. In this paper, we proposed a two-step doping process. Al₂O₃ powder was first mixed only with a part of the ZnO powder and pre-sintered. The pre-sintered powder was mixed with other additives such as Bi₂O₃ and the rest ZnO. Then ZnO varistor ceramics were prepared via solid state sintering processes. Results showed that two-step doped ZnO varistors exhibited improved electrical properties with a significant increased nonlinear coefficient and a great decreased leakage current compared to directly doped ones because more Al was incorporated into ZnO grains.     

Dependence of optical properties on doping metal, crystallite size and defect concentration of M-doped ZnO nanopowders (M = Al, Mg, Ti)

ZnO, Al-, Mg- and Ti-doped ZnO nanopowders were synthesized from CTAB-assisted oxalate intermediate by thermal decomposition method at 600 °C in air. All samples presented a hexagonal wurtzite structure. The spherical nanoparticles assembled in a porous octahedron-like shape for all samples. The size of Al-doped ZnO nanopowders increased as a function of Al ion concentration whereas the size of Mg- and Ti-doped ZnO nanopowders decreased when Mg and Ti ion concentrations were increased. The increment and reduction of their sizes can be explained by the Zener pinning effect. The E_g value of Al-doped ZnO nanopowders slightly decreased when Al ions were increased due to the crystallite size and defect concentration increased. In contrast, the E_g value of Mg- and Ti-doped ZnO nanopowders increased as a function of Mg and Ti ion concentration which can be explained by the Moss-Burstein effect.     

Effects of Ta5+ doping on microstructure evolution,dielectric properties and electrical response in CaCu3Ti4O12 ceramics

The effects of Ta⁵⁺ substitution on the microstructure, electrical response of grain boundary, and dielectric properties of CaCu₃Ti₄O₁₂ ceramics were investigated. The mean grain size decreased with increasing Ta⁵⁺ concentration, which was ascribed to the ability of Ta⁵⁺ doping to inhibit grain boundary mobility. This can decrease dielectric constant values. Grain boundary resistance and potential barrier height of CaCu₃Ti₄O₁₂ ceramics were reduced by doping with Ta⁵⁺. This results in enhancement of dc conductivity and the related loss tangent. Influence of charge compensations on microstructure and intrinsic electrical properties of grain boundaries resulting from the effects of replacing Ti⁴⁺ with Ta⁵⁺ are discussed. The experimental data and variation caused by the substitution of Ta⁵⁺ can be described well by the internal barrier layer capacitor model based on space charge polarization at the grain boundaries.     

Structural and dielectric properties,and nonlinear electrical response of the CaCu3-xZnxTi4O12 ceramics: Experimental and computational studies

CaCu_3-xZn_xTi₄O₁₂ ceramics (x = 0, 0.05, 0.10) were successfully prepared by a conventional solid-state reaction method. Their structural and dielectric properties, and nonlinear electrical response were systematically inspected. The X-ray diffraction results indicated that single-phase CaCu₃Ti₄O₁₂ (JCPDS no. 75–2188) was obtained in all sintered ceramics. Changes in the lattice parameter are well-matched with the computational result, indicating an occupation of Zn²⁺ doping ions at Cu²⁺ sites. The overall tendency shows that the average grain size decreases when x increases. Due to a decrease in overall grain size, the dielectric permittivity of CaCu_3-xZn_xTi₄O₁₂ decreases expressively. Despite a decrease in the dielectric permittivity, it remains at a high level in the doped ceramics (~3,406–11,441). Besides retention in high dielectric permittivity, the dielectric loss tangent of x = 0.05 and 0.10 (~0.074–0.076) is lower than that of x = 0 (~0.227). A reduction in the dielectric loss tangent in the CaCu_3-xZn_xTi₄O₁₂ ceramics is closely associated with the enhanced grain boundary response. Increases in grain boundary resistance, breakdown electric field, and conduction activation energy of grain boundary as a result of Zn²⁺ substitution are shown to play a crucial role in improved grain boundary response. Furthermore, the XPS analysis shows the existence of Cu⁺/Cu²⁺ and Ti³⁺/Ti⁴⁺, indicating charge compensation due to the loss of oxygen lattice. Based on all results of this work, enhanced dielectric properties of the Zn-doped CCTO can be explained using the internal barrier layer capacitor model.     

Preparation and varistor properties of new quaternary Zn–V–Mn–(La, Dy) ceramics

The La and Dy doping effects on microstructures, nonlinear electrical properties, and its stability of ZVM (Zn–V–Mn)-based ceramics were investigated. The microstructure of the quaternary ZVML (Zn–V–Mn–La)-based ceramics doped with La and quaternary ZVMD (Zn–V–Mn–Dy)-based ceramics doped with Dy commonly consisted of mainly ZnO grains and Zn₃(VO₄)₂ as a secondary phase. In addition, the quaternary ZVML- and ZVMD-based ceramics revealed the secondary phases, such as LaVO₄ and DyVO₄, respectively, in addition to Zn₃(VO₄)₂. It seems that all secondary phases act as a liquid-phase sintering aid because it has a significant effect on the sintered density and average grain size. The incorporation of La and Dy into the ZVM-based ceramics increased the breakdown field, and improved the nonlinear electrical properties, in which the nonlinear coefficient was close to 33. The ZVML-based ceramics exhibited the best stability, where the variation of breakdown field (%ΔE_B) against DC accelerated aging stress is −9.7%.     

Phase Equilibria of the Zinc Oxide–Cobalt Oxide System in Air

Phase equilibria of the zinc oxide–cobalt oxide system were studied by a combination of X‐ray diffraction and in situ electrical conductivity and thermopower measurements of bulk ceramic specimens up to 1000°C in air. Rietveld refinement of X‐ray diffraction patterns demonstrated increasing solubility of Co in ZnO with increasing temperature, which is supported by the slight increase in wurtzite (Zn_1?xCo_xO) cell volume and lattice parameter a versus temperature determined for the phase boundary compositions. Similarly, the solubility of Zn in CoO increased with increasing temperature. In contrast, the spinel phase (Zn_zCo_3?zO₄) exhibited retrograde solubility for Zn. Electrical measurements showed that the eutectoid temperature for transformation of rocksalt Co_1?yZn_yO into wurtzite and spinel is 894 ± 3°C, and the upper temperature limit of the stability of the spinel phase is 894°C–898°C for the compositions Co/(Zn+Co) = 0.82–1. The resulting composition‐temperature phase diagram is presented and compared to the earlier (1955) diagram by Robin.     

BaO–TiO2 microwave ceramics

In this paper, the structure and dielectric properties of BaO–TiO₂ system ceramics were studied. By adding ZnO and Nb₂O₅ as sintering agents to the raw materials, the BaO–TiO₂ system ceramics were sintered at a temperature of 1260 °C for 2 h and have superior dielectric properties at 1 GHz: quality factor Q=12,500, relative dielectric constant ε_r≈37, temperature coefficient of dielectric constant α_ε=0±30 ppm/°C. XRD pattern shows that the main crystal phase of the ceramics is Ba₂Ti₉O₂₀, accompanied by a small number of additional phases: BaTi₄O₉, Ba₄Ti₁₃Zn₇O₃₄, Ba₄Ti₁₃O₃₀ and Ti₂Nb₁₀O₂₉, etc. The initial Ba/Ti ratio has a great effect on the dielectric properties of the ceramics, which can be explained by the variance in the formation of phases due to different Ba/Ti ratios.