Low-temperature sintering and microwave dielectric properties of MgTiO<Subscript>3</Subscript> ceramics

The effects of CuO–Bi₂O₃–V₂O₅ additions on the sintering temperature and the microwave dielectric properties of MgTiO₃ ceramics were investigated systematically. The CuO–Bi₂O₃–V₂O₅ (CuBiV) addition significantly lowered the densification temperature of MgTiO₃ ceramics from 1400 °C to about 900 °C, which is due to the formation of the liquid-phase of BiVO₄ and Cu₃(VO₄)₂ during sintering. The saturated dielectric constant (ε_r) increased, the maximum quality factor (Qf) values decreased and the temperature coefficient of resonant frequency (τ_f) shifted to a negative value with the increasing CuBiV content, which is mainly attributed to the increase of the second phase BiVO₄. MgTiO₃ ceramics with 6 wt.% CuBiV addition sintered at 900 °C for 2 h have the excellent microwave dielectric properties: ε _r= 18.1, Qf = 20300 GHz and τ_f = −57 ppm/ °C.     

Thermoelectric properties of WO<Subscript>3</Subscript>-based ceramics doped with Co<Subscript>2</Subscript>O<Subscript>3</Subscript>

Tungsten trioxide (WO₃) doped with cobalt sesquioxide (Co₂O₃) was prepared by a conventional mixed oxide processing route and the thermoelectric properties were studied from 300 up to 1,000 K. The addition of Co₂O₃ to WO₃ resulted in an increase in both the grain size and porosity, indicating that Co₂O₃ promotes the grain grown of WO₃. The magnitude of the electrical conductivity (σ) and the absolute value of the Seebeck coefficient (|S|) depended strongly on the Co₂O₃ content. As for the power factor (σS ² ), the 5.0 mol% sample has the maximum value of the power factor which is 0.12 μWm⁻¹K⁻² at 873 K.     

A novel method for synthesizing nano-crystalline MgTiO<Subscript>3</Subscript> geikielite

We report here for the first time a novel method of preparing nano-crystalline metatitanate, MgTiO₃ geikielite, with crystalline size varying from 20–40 nm. The preparation has been carried out by coprecipitation method involving titanium hydroxide and magnesium nitrate solution. The samples were characterized by using transmission electron microscopic, powder XRD and FT-IR spectroscopic studies. The sample annealed at 900 K for 2 h show ten X-ray diffraction peaks corresponding to the pure geikielite phase. FT-IR spectra of the nano-crystalline geikielite exhibit strong broad vibrational bands near 1000, 650 and 470 cm⁻¹ arising from normal vibrations of the TiO₃ group.     

Structural and dielectric properties of Bi doped Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> ceramics

The dielectric properties of 0.1–15% mol bismuth doped Ba_0.6Sr_0.4TiO₃ (BST) ceramics have been investigated systematically. The solubility limit of bismuth is determined as about 10 mol% by means of both X-ray diffraction and scanning electron microscopy, which is further verified by the fact that the lattice constant of the samples above 10 mol% is almost invariable. The temperature dependence of the dielectric permittivity suggest that the ferroelectric behavior transit to relaxor ferroelectric type when impurity concentration reaches 5 mol%, and further to relaxor behavior for samples above 10 mol% Bi content, which is verified by the absence of a hysteresis loop. Thermal expansion results show differences between 5 and 10 mol% doped samples. Dielectric tunability at room temperature decreases with bismuth content increasing. The variation of properties was attributed to the impurity induced polar regions and former long-order structure.     

Crystal chemistry and domain structure of rare-earth doped BiFeO<Subscript>3</Subscript> ceramics

Bi_(1−x)RE _x FeO₃ (BREF100x, RE = La, Nd, Sm, Gd) has been investigated with a view to establish a broad overview of their crystal chemistry and domain structure. For x ≤ 0.1, the perovskite phase in all compositions could be indexed according to the rhombohedral, R3c cell of BiFeO₃. For Nd and Sm doped compositions with 0.1 < x ≤ 0.2 and x = 0.15, respectively, a new antipolar phase was stabilised similar in structure to PbZrO₃. The orthoferrite, Pnma structure was present for x > 0.1, x > 0.15, and x > 0.2 in Gd, Sm, and Nd doped BiFeO₃, respectively. For x > 0.2, La doped compositions became pseudocubic at room temperatures but high angle XRD peaks were broad and asymmetric. These compositions have been indexed as the orthoferrite structure. It was concluded therefore that the orthoferrite phase appeared at lower values of x as the RE ferrite, end member tolerance factor decreased. However, the compositional window over which the PbZrO₃-like phase was stable increased with increasing end member tolerance factor but was not found as single phase in La doped compositions at room temperature. On heating, the PbZrO₃-like phase in BNF20 transformed to the orthoferrite, Pnma structure. T _C for all compositions decreased with decreasing A-site, average ionic polarizabilty and tolerance factor. For compositions with R3c symmetry, superstructure and orientational, and translational (antiphase) domains were observed in a manner typical of an antiphase-tilted, ferroelectric perovskite. For the new PbZrO₃-like phase orientational domains were observed.     

Microwave dielectric properties of Bi(Sc<Subscript>1/3</Subscript>Mo<Subscript>2/3</Subscript>)O<Subscript>4</Subscript> ceramics for LTCC applications

A novel microwave dielectric ceramics Bi(Sc_1/3Mo_2/3)O₄ with low firing temperature were prepared via the solid reaction method. The specimens have been characterized using scanning electron microscopy, X-ray diffraction, Raman spectroscopy and DC conductivity. The Bi(Sc_1/3Mo_2/3)O₄ ceramics showed B-site ordered Scheelite-type structure with space group C2/c. Raman analysis indicated that prominent bands were attributed to the normal modes of vibration of MoO₄ ^2? tetrahedra. The dielectric loss of Bi(Sc_1/3Mo_2/3)O₄ ceramics can be depended strongly the bulk conductivity by DC measurement. The superior microwave dielectric properties are achieved in the Bi(Sc_1/3Mo_2/3)O₄ ceramic sintered at 875 °C/4 h, with dielectric constant?~?25, Q?×?f ~?51,716 GHz at 6.4522 GHz and temperature coefficient of resonance frequency ~???70.4 ppm/°C. It is a promising microwave dielectric material for low-temperature co-fired ceramics technology.     

Synthesis of pure phase Mg<Subscript>1.2</Subscript>Ti<Subscript>1.8</Subscript>O<Subscript>5</Subscript> and MgTiO<Subscript>3</Subscript> nanocrystals for photocatalytic hydrogen production

Synthesis of pure phase Mg_1.2Ti_1.8O₅ and MgTiO₃ nanocrystals has proven to be challenging. Here, pure phase Mg_1.2Ti_1.8O₅ and MgTiO₃ nanocrystals were prepared. Furthermore, a new magnesium titanate, Mg_1.2Ti_1.8O₅, was synthesized via a solution-based route for the first time. As hydrogen evolution photocatalysts, both pure phase Mg_1.2Ti_1.8O₅ and MgTiO₃ nanocrystals exhibit excellent hydrogen production efficiency. In comparison with pure MgTiO₃ nanocrystals, the asprepared Mg_1.2Ti_1.8O₅ nanocrystals exhibited four times as much photocatalytic hydrogen production activity, up to 40 μmol·h^–1. Photoelectrochemical analysis, including linear sweep voltammetry, transient photocurrent measurement, electrochemical impedance spectroscopy, and construction of Mott-Schottky plots, demonstrated that the enhanced photocatalytic activity was attributed to the large surface area, fast photoelectron transfer, higher carrier density, and efficient charge separation of the Mg_1.2Ti_1.8O₅ nanocrystals.

Open image in new window

     

Investigation of Li<Subscript>2</Subscript>Zn<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>-based temperature stable dielectric ceramics for LTCC applications

A low temperature co-fired ceramic (LTCC) was fabricated at 910 °C /2 h from the powder mixture of Li₂Zn₃Ti₄O₁₂, TiO₂ and a B₂O₃–La₂O₃–MgO–TiO₂ glass (BLMT), and the influence of TiO₂ on microstructure and dielectric properties of the composite was investigated in the composition range (wt%) of 20BLMT–(80???x)Li₂Zn₃Ti₄O₁₂–xTiO₂ (x?=?0, 2.5, 5, 7.5, 9 and 10). The results showed that all samples consisted of Li₂Zn₃Ti₄O₁₂, TiO₂, LaBO₃ and LaMgB₅O₁₀ phase. And LaBO₃, LaMgB₅O₁₀ and a small amounts of TiO₂ were crystallized from BLMT glass during sintering process. As x increases, dielectric constant and temperature coefficient of resonance frequency of the composites demonstrated gradually increase, whereas the quality factor of the sample of x?=?0 wt% was about 41,500 GHz and the ones maintained stable at a high level of 49,000–51,000 GHz for other samples. The composite with x?=?9 wt% had an optimal microwave dielectric properties with the dielectric constant of 20.2, quality factor of 50,000 GHz and temperature coefficient of resonant frequency of ??0.33 ppm/°C.     

Dielectric properties of B<Subscript>2</Subscript>O<Subscript>3</Subscript> doped Sm(Co<Subscript>1/2</Subscript>Ti<Subscript>1/2</Subscript>)O<Subscript>3</Subscript> ceramics at microwave frequency

The microwave dielectric properties and the microstructures of Sm(Co_1/2Ti_1/2)O₃ ceramics with B₂O₃ additions (0.25 and 0.5 wt%) prepared by conventional solid-state route have been investigated. The prepared Sm(Co_1/2Ti_1/2)O₃ exhibited a mixture of Co and Ti showing 1:1 order in the B-site. Doping with B₂O₃ (up to 0.5 wt%) can effectively promote the densification of Sm(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. It is found that Sm(Co_1/2Ti_1/2)O₃ ceramics can be sintered at 1,260 °C due to the grain boundary phase effect of B₂O₃ addition. At 1,290 °C, Sm(Co_1/2Ti_1/2)O₃ ceramics with 0.5 wt% B₂O₃ addition possess a dielectric constant (ε _r) of 27.7, a Q × f value of 33,600 (at 9 GHz) and a temperature coefficient of resonant frequency (τ_f) of −11.4 ppm/ °C. The B₂O₃-doped Sm(Co_1/2Ti_1/2)O₃ ceramics can find applications in microwave devices requiring low sintering temperature.     

Microstructures and dielectric properties of Y/Zn codoped BaTiO<Subscript>3</Subscript> ceramics

The microstructures and dielectric properties of Y/Zn codoped BaTiO₃ ceramics sintered in a reducing atmosphere were investigated. XRD analysis indicated the crystal structure of samples change from tetragonal to pseudocubic with increasing Y₂O₃ and ZnO content. SEM micrographs showed Y₂O₃ can suppress grain growth more effectively compared with ZnO, which is ascribed to the presence of second phase Y₂Ti₂O₇. Proper amount of Y₂O₃ and ZnO can significantly improve the dielectric temperature characteristics due to the formation of grain core-shell structure. The high performance dielectrics meeting the X7R code were achieved by codoping 1.5 mol% Y₂O₃ and 3.0 mol% ZnO.     

Properties of Na<Subscript>0.875</Subscript>Li<Subscript>0.125</Subscript>NbO<Subscript>3</Subscript> ceramics

The structural, dielectric, and thermal properties of the Na_0.875Li_0.125NbO₃ solid solution doped with strontium and other elements have been studied in wide temperature and frequency ranges. The material has been shown to undergo a sequence of phase transitions accompanied by anomalies in its structural, dielectric, and thermal properties. The observed low-frequency dispersion of its dielectric permittivity is attributed to the effect of electrical conductivity.     

Properties of fluorine-doped PbMg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>O<Subscript>3</Subscript> ceramics

We report the first fluorine doping of lead magnesium niobate in the PbMg _{(1 + x)/3}Nb_{(2 ? x)/3}O_{3 ? x} F _x system in a wide composition range, x = 0.025 to 0.625. The fluorine content of the samples is shown to be substantially lower than the intended one because of the fluorine volatilization in the form of HF during synthesis and sintering in air. The ceramics consist of magnesium and lead oxides undetectable by x-ray diffraction, and a perovskite phase whose composition can be represented by the formula PbMg_{(1 + m)/3}Nb_{(2 ? m)/3}O_{3 ? m} F _m , where the fluorine content after sintering is m ≤ 0.12. The PbO and MgO contents of the ceramics depend on the starting mixture composition (x) and heat-treatment conditions (hydrogen fluoride and lead oxide volatilization). As a result of the low fluorine content, the diffraction patterns of the samples show no superlattice reflections, and their lattice parameter varies insignificantly with x. Data are presented on the temperaturedependent dielectric permittivity of ceramic samples sintered and annealed under different conditions.     

Structure and electrical properties of MnO doped (Ba<Subscript>0.96</Subscript>Ca<Subscript>0.04</Subscript>)(Ti<Subscript>0.92</Subscript>Sn<Subscript>0.08</Subscript>)O<Subscript>3</Subscript> lead free ceramics

In this work, (Ba_0.96Ca_0.04)(Ti_0.92Sn_0.08)O₃–xmol MnO (BCTS–xMn) lead-free piezoelectric ceramics were fabricated by the conventional solid-state technique. The composition dependence (0 ≤ x ≤ 3.0 %) of the microstructure, phase structure, and electrical properties was systematically investigated. An O–T phase structure was obtained in all ceramics, and the sintering behavior of the BCTS ceramics was gradually improved by doping MnO content. In addition, the relationship between poling temperature and piezoelectric activity was discussed. The ceramics with x = 1.5 % sintering at temperature of 1330 °C demonstrated an optimum electrical behavior: d ₃₃ ~ 475 pC/N, k _p ~ 50 %, ε _r ~ 4060, tanδ ~ 0.4 %, P _r ~ 10.3 μC/cm², E _c ~ 1.35 kV/mm, T _C ~ 82 °C, strain ~0.114 % and \(d_{33}^{*}\) ~ 525 pm/V. As a result, we achieved a preferable electric performance in BaTiO₃-based ceramics with lower sintering temperature, suggesting that the BCTS–xMn material system is a promising candidate for lead-free piezoelectric ceramics.     

Low temperature sintering and microwave dielectric properties of Li<Subscript>6</Subscript>Mg<Subscript>7</Subscript>Ti<Subscript>3</Subscript>O<Subscript>16</Subscript> ceramics with LiF additive for LTCC applications

Li₆Mg₇Ti₃O₁₆ ceramics were prepared by the conventional solid-state method with 1–5 wt% LiF as the sintering aid. Effects of LiF additions on the phase compositions, sintering characteristics, micro-structures and microwave dielectric properties of Li₆Mg₇Ti₃O₁₆ ceramics were investigated. The LiF addition could effectively lower the sintering temperature of Li₆Mg₇Ti₃O₁₆ ceramics from 1550 to 900 °C. For different LiF-doped compositions, the optimum dielectric permittivity (ε _r ) and quality factor (Q·f) values first increased and then decreased with the increase of LiF contents, whereas the temperature coefficient of resonant frequency (τ _f ) fluctuated between ??14.39 and ??8.21 ppm/°C. Typically, Li₆Mg₇Ti₃O₁₆ ceramics with 4 wt% LiF sintered at 900 °C exhibited excellent microwave dielectric properties of ε _r ?=?16.17, Q·f?=?80,921 GHz and τ _f ?=???8.21 ppm/°C, which are promising materials for the low temperature co-fired ceramics applications.     

Synthesis of MgTiO<Subscript>3</Subscript> by solid state reaction and characteristics with addition

Magnesium titanate was synthesized by the solid state reaction. Powder XRD method was used to identify the phases and content, which indicates that pure phase of MgTiO₃ can be synthesized by the solid state reaction even without the addition of excess MgO. Doped with 5 at.% CaTiO₃ and 6 wt.% ZnNb₂O₆, MgTiO₃ ceramics sintered at 1300 °C possess excellent microwave dielectric properties: a dielectric constant ε_r of 21, a τ _f value of 0 ppm/°Cand a Q × f value of 130,000 GHz (at 13 GHz).     

Dielectric characteristics of Pb(Zn<Subscript>1/3</Subscript>Ta<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> ceramics with/without PbTiO<Subscript>3</Subscript> modification

Stabilization tendencies of the perovskite structure in a Pb(Zn_1/3Ta_2/3)O₃-BaTiO₃ pseudobinary system with/without compositional modification by 20 mol% PbTiO₃ introduction were compared. In order to promote perovskite phase formation, the B-site precursor method (which is conceptually similar to the columbite process) was employed in this study. Dielectric properties of sintered samples were investigated as functions of composition and measurement frequency. Dielectric constant spectra, in the paraelectric temperature region, were further analyzed in terms of diffuseness. Microstructures of sintered specimens were also investigated and correlated with perovskite stabilization.     

Yellow and warm white light emitting Zn doped Y<Subscript>2</Subscript>O<Subscript>3</Subscript> for near UV excitable phosphor converted WLED

Optical properties of Zn doped Y₂O₃ microsheets prepared by sol–gel combustion method have been investigated and their application in phosphor converted white LED has been examined. The formation of single phase, well crystalline cubic Y₂O₃ is confirmed from powder XRD results. Effective substitution of Zn in Y₂O₃ crystal lattice is inferred from shifting of diffraction peaks. SEM images have showed that undoped as well as Zn doped Y₂O₃ formed as microsheets. Doping of Zn enhanced the growth of the sheets and its length increased from 1.5 to 19 µm. Development of structural disorder in Y₂O₃ crystal structure after Zn doping and confirmation of the conserved cubic structure of Zn doped Y₂O₃ without any secondary phase have been revealed from micro-Raman spectra. The optical band gap of Y₂O₃ has been altered after Zn doping and it is found to be decreased from 5.6 to 5.22 eV as increasing Zn concentration. Both undoped and Zn doped Y₂O₃ showed a broad visible emission from blue to green region due to various defects and impurities present in it. Broad PL excitation spectrum inferred the possibility to attain the visible emission under the excitation of light with wide range of wavelength from near UV to blue region. Excitation of pure Y₂O₃ under near UV (375 nm) LED chip lead to the emission of yellow light whereas Zn doped Y₂O₃ emitted warm white light with color coordinate of (0.42, 0.35), colour rendering index of 77.6 and correlated color temperature (CCT) of 2840 K. Hence, Zn doped Y₂O₃ discussed in the present work can be a better replacement for various rare earth doped phosphors in the application of phosphor converted WLED (pc-WLED).     

Relaxor behavior and ferroelectric properties of Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-K<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-KNbO<Subscript>3</Subscript> lead-free ceramics

Lead-free ferroelectric ceramics of (1−x) [0.88Na_0.5Bi_0.5TiO₃-0.12K_0.5Bi_0.5TiO₃]-x KNbO₃(x = 0, 0.02, 0.04, and 0.06) were prepared by the conventional ceramic fabrication technique. The crystal structure, dielectric properties and P-E hysteresis loops were investigated. XRD data showed that all compositions could form pure perovskite structure. Temperature dependence of dielectric constant ε _r and dissipation factor tanδ measurement between room temperature and 500^∘C revealed that the compounds experience phase transitions that from ferroelectric to anti-ferroelectric and anti-ferroelectric to paraelectric in the range of x = 0–0.04. The frequency dependent dielectric constant showed these compounds were relaxor ferroelectric. At low frequency and high temperature, dielectric constant and dissipation factor increased sharply attributed to the superparaelectric clusters after the KNbO₃ doped.     

Diffuse ferroelectric phase transition in SrTiO<Subscript>3</Subscript>-BiScO<Subscript>3</Subscript> system ceramics

Ceramic samples of the (1−x)SrTiO₃-(x)BiScO₃ system with x = 0, 0.05, 0.1, 0.2, 0.3, 0.4, and 0.5 have been synthesized for the first time. X-ray diffraction data showed that the samples with x = 0.2, 0.3, and 0.4 at room temperature comprise the mixture of two phases: (i) cubic nonpolar phase with Pm3m structure and (ii) tetragonal polar phase with P4mm structure. The temperature dependences of permittivity and dielectric loss tangent of these samples exhibit anomalies characteristic of ferroelectrics with diffuse phase transitions.