Tungstate Ceramics Research Articles

Optical properties of ZnWO4 ceramics obtained by radiation synthesis

One of the promising methods for producing ceramics from tungstate metals of the MWO4 composition is ra- diation synthesis using powerful electron fluxes. Due to the unique properties of radiation, it was possible to significantly accelerate the synthesis process. It has been demonstrated that ZnWO4 ceramics can be synthe- sized by acting on an electron flux with an energy from 1.4 to 2.5 MeV and a high power density of 10– 25 kW/cm2. In this regard, studies of the optical properties of ceramic samples of zinc tungstate ZnWO4 ob- tained by this method were carried out. The morphology of the surface, X-ray diffraction spectra and optical properties of ZnWO4 ceramics obtained by radiation synthesis were studied. X-ray diffraction measurements and EDX analysis confirmed the formation of zinc tungstate ceramics ZnWO4 as a result of radiation synthe- sis. The absorption, luminescence, and luminescence excitation spectra of synthesized samples were meas- ured. Luminescence spectra of a reference single crystal sample were also measured. X-ray diffraction con- firmed the formation of zinc tungstate (ZnWO4) as a result of radiation synthesis. The luminescent spectra of the synthesized sample coincide with the spectra of crystalline ZnWO4. A comparison of the luminescence spectra of a synthesized ceramic sample and a monocrystalline reference sample measured under different op- tical excitation conditions shows significant differences in the luminescence spectra of the synthesized sample and the reference sample and indicates possible various defects present in the compared samples.

High-temperature dimensional stability and radiation behavior of high-entropy rare earth tungstate ceramics

Materials with high-temperature dimensional stability and resistance to radiation damage are receiving increasing attention in advanced nuclear energy systems. In this research, (Sm0.2Eu0.2Gd0.2Dy0.2A0.2)2W3O12-type (A = Y, Tm,Ho) high-entropy ceramics is presented. The coefficients of thermal expansion of (Sm0.2Eu0.2Gd0.2Dy0.2Y0.2)2W3O12 (HE-RE2W3O12) is 7.21 × 10−6 K−1 at 300–1100K, which is the lowest among the three synthesized high-entropy ceramics and indicates good dimensional stability at high temperatures. After irradiation with Kr+ fluence of 4.58 × 1016 ions/cm2 at 400 °C, both HE-RE2W3O12 and Gd2W3O12 maintained crystalline, but the lattice volume expansion rate of HE-RE2W3O12 (0.71 %) is much lower than that of Gd2W3O12 (1.49 %). Additionally, in the post-irradiated HE-RE2W3O12, the Raman spectra vibration peak remained unchanged and no radiation-induced segregation was observed. Nanoindentation tests displayed that the degradation of hardness and elastic modulus in HE-RE2W3O12 is rarely degrade. Overall, these results provide new insights for research on advanced nuclear materials in the future.

Enhanced ionic conductivity of aluminum tungstate by crystallographic orientation in a strong magnetic field

AbstractThe ionic conduction of multiply charged ions, rather than singly charged ions, is beneficial for energy storage and sensor applications. The low mobility of multiply charged ions is one important obstacle to the implementation of these applications. Chemical methods, such as doping and solid solution formation, have been used to improve ionic conductivity. However, the apparent performance of ceramic electrolytes can be improved by the crystallographic alignment of anisotropic grains. In this study, crystal‐oriented aluminum tungstate ceramics were processed by slip casting in a strong magnetic field. The b‐axis‐ and c‐axis‐oriented aluminum tungstate ceramics can be produced by this technique. The orientation of grains along the b‐axis could enhance ionic conductivity by at least 1.77 times compared to that of a randomly oriented sample and 2.13 times compared to that of the c‐axis‐oriented sample. The results of this study suggest that this method can improve the ionic conductivity of an anisotropic material using polycrystalline processing instead of difficult single‐crystal synthesis techniques.

Vibrational spectroscopy and microwave dielectric properties of two novel Ca3Ln2W2O12 (Ln = La, Sm) tungstate ceramics

Two novel tungstate ceramics with a nominal composition of Ca3Ln2W2O12 (Ln = La, Sm) were prepared via standard solid-state reaction methods. According to X-ray diffraction patterns, both ceramics crystallized in hexagonal crystal systems with a space group of R3¯m (No. 166). The vibrational modes of the Raman spectra were identified, and the evolutions of the wavenumber and the full width at half maximum (FWHM) were analyzed. The optimum microwave dielectric properties with εr = 18.7, Qf = 50,500 GHz, andτf =–90 ppm/oC were realized in Ca3La2W2O12 ceramics, while moderate Qf values were obtained in the Sm composition (εr = 19.5, Qf = 15,700 GHz, andτf =–95 ppm/oC). The discrepancy in theQf values was associated with the broader FWHM of Raman spectra and smaller grain sizes. Infrared reflectivity (IR) spectra were also used to determine the intrinsic dielectric loss, where more damped phonon parameters were also identified in Ca3Sm2W2O12 ceramics.

Negative thermal expansion coefficient of Sc-doped indium tungstate ceramics synthesized by co-precipitation

Co-precipitation was successfully applied to synthesize the Sc3+ doped In2-xScx (WO4)3 (x = 0, 0.3, 0.6, 0.9 and 1.2) compounds. The composition- and temperature-induced structural phase transition and thermal expansion behaviors of Sc3+ doped In2(WO4)3 were investigated. Results indicate that In2-xScx (WO4)3 crystalizes in a monoclinic structure at 300 °C for x ≤ 0.3 and changes into hexagonal structure for x ≥ 0.6. Hexagonal In1.1Sc0.9(WO4)3 displays negative thermal expansion (NTE) with an average linear coefficient of thermal expansion (CTE) of −1.85 × 10−6 °C −1. After sintering at 700 °C and above, a phase transition from hexagonal to orthorhombic phase was observed in In2-xScx (WO4)3 (x ≥ 0.6). Sc3+ doping successfully reduce the temperature-induced phase transition temperature of In2-xScx (WO4)3 ceramics from 250 °C (x = 0) to room temperature (x = 0.9). When x = 0.9 and 1.2, the average linear CTEs of In2-xScx (WO4)3 ceramics are −5.45 × 10−6 °C−1 and -4.43 × 10−6 °C−1 in a wider temperature range of 25–700 °C, respectively.

Synthesis and Characterisation of AWO4 (A = Mg, Zn) Tungstate Ceramics

The AWO4 (A = Mg, Zn) tungstate ceramics have been prepared using conventional solid-state route and characterised. Samples were analysed X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy (IS). Phase-pure ZnWO4 and MgWO4 were successfully obtained at temperatures 1000°C for 24 and 12 hours, respectively, where both compounds had a wolframite-like structure monoclinic, space group P2/c (13). Preliminary results of electrical properties of ZnWO4 indicated that ZnWO4 is a dielectric insulator with permittivity, εr ∼ 20 at room temperature. The inhomogeneous electrical properties of ZnWO4 is observed where the grain boundary and bulk contributions effects were seen at temperature excess of 100°C.

The improvement research on microwave dielectric properties of magnesium tungstate for LTCC

The MgWO4 and Li2Mg2(WO4)3 ceramics for LTCC application were prepared by the conventional solid-state sintering method. The effect of lithium compounds addition on the densification, phase development, microstructure and microwave dielectric properties of the magnesium tungstate ceramics was investigated. Adding Li2CO3 to the MgO–WO3 in different order, the result showed that the densification and main crystalline phases were different, and the change of microstructures was conspicuous. The optimal sintering temperature of dense Li2Mg2(WO4)3 ceramic approximately ranged from 825 to 875 °C for 3 h, and the sintering temperature of MgWO4 ceramic can be reduced to 950 °C by adding Li2CO3. When the ceramics were dense enough, the MgWO4 ceramics with 9.0 wt% Li2CO3 had negative τf values of −76 ppm/°C, but the τf values of the Li2Mg2(WO4)3 ceramics were near 0 ppm/°C.

Piezoelectric and Ferroelectric Properties of Bismuth Tungstate Ceramics Fabricated by Spark Plasma Sintering

Single‐phase bismuth tungstate (Bi2WO6) ceramics with high relative density (>99%) were fabricated by spark plasma sintering. Ferroelectric, dielectric, and piezoelectric properties of Bi2WO6 ceramics were investigated. Almost saturated polarization–electric field (P–E) hysteresis loops with a remanent polarization (Pr) of ∼16.1 μC/cm2 and a coercive field (Ec) of ∼3.7 kV/mm were obtained. Curie point and second phase transition temperatures were observed at 937°±5° and 665°±5°C, respectively. The average piezoelectric constant (d33) of this high Curie point ceramic is 15±0.2 pCN−1.

MgWO 4, ZnWO 4, NiWO 4 and CoWO 4 microwave dielectric ceramics

AWO 4 (where A = Zn 2+, Mg 2+, Ni 2+ and Co 2+) tungstate ceramics were synthesised from a standard ceramic route, and their sintering behaviour and microwave dielectric properties were investigated. All AWO 4 powders formed single-phase materials, except MgWO 4, and they all had a wolframite-like monoclinic P2/c (13) structure, not the tetragonal scheelite structure found in CaWO 4. MgWO 4 had a poor Qf which deteriorated with further sintering, due to discontinuous grain growth at relatively low temperatures of 1000 °C and over. At higher temperatures, ZnWO 4, NiWO 4 and CoWO 4 have high Qf values at 1200 °C of 62,800, 24,900 and 38,600 GHz, respectively, which will all almost certainly be improved with firing at higher temperatures, particularly in the case of CoWO 4. Although not at all well sintered, ZnWO 4 and CoWO 4 have high Qf values of 34,000 and 28,900 GHz, respectively, at temperatures below the LTCC limit.

Structural, dielectric and electrical properties of lead cadmium tungstate ceramics

Polycrystalline samples of Pb(Cd 1/2W 1/2)O 3 were synthesized by a high-temperature solid-state reaction technique. Preliminary crystal structure and microstructure of the compound at room temperature were studied using X-ray diffraction (XRD) technique and scanning electron microscopy (SEM), respectively. The dielectric permittivity ( ε) and loss tangent (tan δ) of the compound were obtained both as a function of frequency (10 3–10 4 Hz) at room temperature and temperature (30–320°C) at 10 kHz. Both the ac and dc conductivity has been studied over a wide range of temperature. The activation energy ( E a) of the compound was calculated from the plot of ac conductivity versus inverse of absolute temperature. The temperature variation of resistivity shows that the compound has positive temperature coefficient of resistance (PTCR). The current–voltage ( I–V)-characteristics of the compound studied at diffierent temperatures reveals that the compound also has excellent varistor behavior.

Studies of dielectric and varistor behavior of lead manganese tungstate ceramics

Polycrystalline samples of Pb(Mn1/2W1/2)O3 were prepared by the high-temperature solid-state reaction technique. Preliminary crystal structure and microstructure of the compound at room temperature were studied using the X-ray diffraction (XRD) technique and scanning electron microscopy (SEM), respectively. The dielectric permittivity (e) and loss tangent (tanδ) of the compound were obtained both as a function of frequency (0.5×103−104Hz), at room temperature, and temperature (30 °C–320 °C), at 10 kHz. Both the a.c. and d.c. conductivities have been studied over a wide range of temperature. The forbidden energy gap (Eg) and activation energy (Ea) of the compound were calculated from the plot of d.c. resistivity and a.c. conductivity versus inverse absolute temperature, respectively. The current density–field strength (J–E) characteristics of the compound studied at different temperatures reveals that the compound has excellent varistor behavior. The change in magnitude of the current with time was also studied at different voltage.

Kinetic analysis of the serial reactions of lead magnesium tungstate ceramics using a multiple core-shell model

The solid-state reaction kinetics of a sequential reaction system were analysed theoretically and experimentally. For the reactions occurring in series, A+R→B and B+Q→C, a multiple core–shell diffusion-controlled model has been proposed. The relations between the consumption fractions of A and B and the reaction time under isothermal conditions have been quantitatively deduced. The sequential reactions of PbWO4+PbO→Pb2WO5 and Pb2WO5+MgO→Pb(Mg1/2W1/2)O3 in the formation processes of Pb(Mg1/2W1/2)O3 were utilized as the model reactions. The unidirectional diffusion of PbO into PbWO4 to form Pb2WO5 and that of MgO into Pb2WO5 to generate Pb(Mg1/2W1/2)O3 were verified by EDS. The derived equations were confirmed to elucidate accurately the kinetics of the serial reactions in the Pb(Mg1/2W1/2)O3 system. Based on the reaction model, the activation energy for the diffusion of MgO into Pb(Mg1/2W1/2)O3 was estimated to be 161.7 kJ mol-1. © 1998 Chapman & Hall

Ordering in lead iron tungstate relaxor ceramics

The ordering behavior of sodium doped lead iron tungstate ceramics, Pb 1 − x Na x [ Fe (2 − x) 3 W (1 + x) 3 ]O 3, was investigated together with their dielectric properties. The 1:1 type ordering of iron and tungsten cations, on the B sites, was evidenced by h + 1 2 , k + 1 2 , 1 + 1 2 superlattice reflections in both X-ray and transmission electron microscopy diffraction. The ordering degree increases with Na content when Na ≤ 8 at% and decreases when Na > 8 at%. Besides the perovskite PFW phase, a second phase, PbWO 4, was observed in Na doped PFW ceramics and its amount increases with increasing Na content. The dielectric permittivity decreases with the increase in the Na content, while the diffuseness increases. It is suggested that the substitution of Na + for Pb 2+ leads to a decrease in the charge imbalance between ordered and disordered ] regions, enhancing the ordering of PFW ceramics. The competition between the enhancement of ordering and the formation of the PbWO 4 phase is suggested to be responsible for the decrease of ordering, when Na > 8 at%.

Stoichiometric Dependence of the Aging Phenomena in Lead Iron Tungstate Ceramics

Lead iron tungstate, Pb(Fe2/3W1/3)O3, ceramics with different stoichiometries were used to study the isothermal aging of this relaxor ferroelectric. The tungsten‐deficient samples showed evident aging effects, more pronounced as the tungsten deficiency increased. The aging behavior showed a strong dependence on the frequency, a log‐linear function of the aging time, and a shift of the dielectric distortion with the aging temperature. Defect associations between tungsten and oxygen vacancies seemed to be responsible for this behavior. On the other hand, lead‐deficient samples showed no aging phenomena. The structural positions of point defects, tungsten, lead, and oxygen vacancies and the reorientation of defect dipole pairs have been discussed for the compositions and suggested to be responsible for the observed behavior.

Formation mechanism and relaxor ferroelectric properties of lead lithium iron tungstate ceramics

The formation mechanism and ferroelectric properties of Pb(Li1/4Fe1/4W1/2)O3 prepared by solid-state reaction have been investigated in this study. The formation processes of Pb(Li1/4Fe1/4W1/2)O3 are characterized to be an initial reaction of lead tungstates PbWO4 and Pb2WO5 at a low temperature range, followed by a subsequent reaction to produce Pb(Li1/4Fe1/4W1/2)O3 from above 650 °C. Through a two-stage calcination (700 °C/quenching-regrinding-710 °C/8 h), a nearly single phase of Pb(Li1/4Fe1/4W1/2)O3 is obtained. This compound exhibits a cubic perovskite structure (α = 8.0113 Å) with a partial ordering arrangement of B-site cations. Above 720 °C, becomes Pb(Li1/4Fe1/4W1/2)O3 thermodynamically unstable and gradually decomposes in forming elongated Pb2WO5 grains, thereby resulting in a nonhomogeneous microstructure. As the ac frequency increases, the maximum dielectric permittivity of Pb(Li1/4Fe1/4W1/2)O3 significantly decreases; in addition, the corresponding temperature increases. The strong frequency dependence of dielectric properties, as well as the critical exponent and diffuseness evaluated through a modified permittivity-temperature equation proposed in this study, verify the relaxor characteristics of Pb(Li1/4Fe1/4W1/2)O3

Fabrication of ferroelectric lead iron tungstate ceramics via a two-stage solid-state reaction

Ferroelectric Pb(Fe 2 3 W 1 3 ) O 3 was synthesized via a new two-stage solid-state reaction discussed in the present study. This method utilized Fe 2WO 6 prepared at the first-stage which subsequently reacted with a stoichiometric amount of PbO at the second stage. This process efficiently suppressed the formation of lead tungstates and liquid phases; in addition, pyrochlore phase was formed as the only intermediate compound. A substantial amount of pyrochlore phase was converted to Pb(Fe 2 3 W 1 3 ) O 3 at around 710 °C associated with an endothermic reaction. On heating up to 750 °C over 99% perovskite Pb(Fe 2 3 W 1 3 ) O 3 was formed, and this formation became complete at 840 °C. Compared with the conventional solid-state reaction, this process not only simplified the formation process, but also reduced the formation temperature of Pb(Fe 2 3 W 1 3 ) O 3 . The crystal structure of Fe 2WO 6 did not significantly influence the formation mechanism; whereas, the morphology of Pb(Fe 2 3 W 1 3 ) O 3 was markedly affected by the grain size of Fe 2WO 6. By using this process, pure Pb(Fe 2 3 W 1 3 ) O 3 with a submicron microstructure was successfully fabricated.

Synthesis and characterization of lead iron tungstate ceramics obtained by two preparation methods

Abstract Lead iron tungstate ( Pb(Fe 2 3 W 1 3 ) O 3 ) is difficult to sinter as a single phase perovskite ceramic. Side reactions lead to undesirable second phases damaging the dielectric properties of the sintered material. Understanding these reaction routes is necessary to eliminate them and to improve on the properties of these ceramics. Lead iron tungstate ceramics were sintered from powders prepared by reaction of mixtures of the three oxides, or by reaction of prereacted iron oxide and tungsten oxide with lead oxide, in an attempt to control the formation of the perovskite phase. The reaction sequences, different in both cases, lead to a higher yield of the perovskite phase when the prereacted powders were used, avoiding therefore the presence of undesirable phases. The microstructures and dielectric properties of the sintered ceramics obtained by both methodologies are reported and compared. The prereacted intermediate phase method leads to a more ordered perovskite structure with better dielectric characteristics.

Effect of excess of iron oxide and lead oxide on the microstructure and dielectric properties of lead-iron tungstate ceramics

Abstract Lead-iron tungstate Pb(Fe2/3W1/3)O3; PFW) perovskite ceramics were prepared by the conventional mixed oxides method. Additional amounts of Fe2O3 and PbO were used to examine the role of excess oxides on the phase development, densification behaviour and dielectric properties. The stoichiometric mixture of the oxides results in a residual phase (Pb2WO5) besides the perovskite. Excess Fe2O3 eliminates the Pb2WO5, with a marked increase in the dielectric constant, whereas excess PbO leads to an unexpected increase of the amount of the second phase and a consequent decrease of the dielectric constant. The densification behaviour and the microstructures obtained after firing were very dependent on the starting stoichiometry. The PFW phase obtained using PbO or Fe2O3 excesses shows various degrees of ordering which seem to be correlated with the presence of either W6+ vacancies or of Pb2+ vacancies.

Formation Process and Microstructural Evolution of Sol–Gel‐Derived Ferroelectric Lead Iron Tungstate Ceramics

The formation process of Pb(Fe2/3W1/3)O3 was simplified and the synthesis temperatures were reduced because of the improved reactivity and homogeneity in sol–gel‐derived precursors. No liquid phases appeared in the specimens formulated from clear refluxed solution, whereas they were observed in the specimens prepared from turbid solution containing lead tungstate precipitates. The mixing state of reactants in precursors greatly affected the formation of liquid phases. Pb(Fe2/3W1/3) grains with a characteristic cubic shape were formed from the sol–gel process.

Low-thermal expansion polycrystalline tantalum tungstate ceramics

The synthesis of thermal-shock-resistant materials from the system Ta2O5WO3 was investigated. Ta2WO8 had a very low unit-cell thermal expansion coefficient (+0.5 X 10−6° C−1). Ta30W2O81 also had a relatively low coefficient (+4.0 X 10−6 ° C−1) and a thermal durability over 1600° C. The thermal expansion curves of these polycrystalline ceramics were lowered because of microcracks caused by the large thermal expansion anisotropy of the crystal axes and were accompanied by hysteresis loops. The densification of Ta2WO8 ceramic was promoted by the addition of some metal oxides, and the strong ceramic of Ta30W2O81 was obtained by controlling grain growth.