Search

Abstract The effects of two-step sintering on the microstructure, mechanical and thermal properties of aluminum nitride ceramics with Yb 2 O 3 and YbF 3 additives were investigated. AlN samples prepared using different sintering methods achieved almost full density with the addition of Yb 2 O 3 –YbF 3. Compared with the one-step sintering, the grain sizes of AlN ceramics prepared by the two-step sintering were limited, and the higher flexural strength and the larger thermal conductivity were obtained. Moreover, the electrochemical impedance spectroscopy of AlN ceramic was associated with thermal conductivity by analyzing the defects and impurities in AlN ceramics. The fitting grain resistance and the activation energy for the grain revealed the lower concentrations of aluminum vacancy in the two-step sintered AlN ceramics, which resulted in the higher thermal conductivity. Thus, mechanical and thermal properties for AlN ceramics were improved with Yb 2 O 3 and YbF 3 additives sintered using two-step regimes. [ABSTRACT FROM AUTHOR]

(Ba 0.6 Sr 0.4 ) 0.85 Bi 0.1 TiO 3 (abbreviated as BSBT) ceramics doped with x wt% Zn 2 P 2 O 7 ( x  = 0, 1, 5, 10, 15) were synthesized. The influence of Zn 2 P 2 O 7 on microstructures and electrical properties of BSBT ceramics were studied. Microstructural observation indicated that Zn 2 P 2 O 7 doping could inhibit the growth of grain and improve the microstructure homogeneity of BSBT ceramics. Dielectric measurements exhibited a relaxor-like characteristic for all samples. Breakdown strength measurements indicated that undoped BSBT ceramics owned a breakdown strength of 122 kV/cm. However, with increasing Zn 2 P 2 O 7 content, the breakdown strength enhanced monotonously, and the maximum breakdown strength reached 326 kV/cm at x  = 15. Meanwhile, the energy storage efficiency was improved greatly by Zn 2 P 2 O 7 addition. The results suggest that BSBT ceramics doped with Zn 2 P 2 O 7 could be taken for a candidate for energy storage applications. In addition, the correlation between energy storage efficiency and interface polarization was qualitatively discussed by complex impedance spectra. [ABSTRACT FROM AUTHOR]

The effect of yttrium fluoride (YF 3) on the densification behavior, microstructure, phase composition and thermal conductivity of aluminium nitride (AlN) ceramics with yttrium oxide (Y 2 O 3) and YF 3 additives were studied. Since YF 3 provided liquid phases and promoted densification at a lower temperature, the sintering temperature required to reach the full density of AlN samples decreased with the increase in YF 3 content. Appropriate addition of YF 3 could improve the thermal conductivity of AlN ceramics, but the values of thermal conductivity decreased as YF 3 increased further. It is attributed to the ability of YF 3 to react with oxygen impurity was worse than that of Y 2 O 3. Moreover, the reducing atmosphere significantly affected the phase composition, and the oxygen content in grain boundary phases decreased at 1750 °C and 1800 °C. Therefore, the proper proportion of Y 2 O 3 –YF 3 additives could simultaneously improve densification and the thermal conductivity of AlN samples at a low sintering temperature. [ABSTRACT FROM AUTHOR]

The ion valence state, phase composition, microstructure, and microwave dielectric properties of Sr(1-1.5x)CexTiO3 (x = 0.1-0.67, SCT) ceramics were systematically investigated. Sr(1-1.5 x)CexTiO3 ceramics were produced with gradual structural evolution from a cubic to a tetragonal and turned to an orthorhombic structure in the range of 0.1 &#8804 x ≤ 0.67. Above a critical Ce proportion (x = 0.4), microstructural changes and normal grain growth initially occurred. On the basis of chemical analysis results, the reduction of Ti4+ ions was hastened by tetravalent ions (Ce4+). By contrast, this reduction was inhibited by trivalent ions (Ce3+). The observed dielectric behavior was strongly influenced by phase composition, oxygen vacancies (V...O), and defect dipoles, namely, (Ti'- V...O) and (V'Sr - V...O). Temperature stable ceramics sintered at 1350°C for 3 h in air yielded an intermediate value of dielectric constant (εr = 40), with the smallest reported value of temperature coefficient of resonant frequency (sf = +0.9 ppm/°C), and quality factor (Q × f = 5699 GHz) at x = 0.6. [ABSTRACT FROM AUTHOR]

The effects of eutectic addition of 0.8LiF–0.2CaF 2 on microwave dielectric properties and structure of MgO ceramics were investigated. Addition of LiF–CaF 2 decreased the sintering temperature of MgO ceramics to 900 °C but did not affect the dielectric properties of the ceramics. The dielectric constant ( ε r ) and temperature coefficient of the resonant frequency ( τ f ) of MgO ceramics were not influenced by the LiF–CaF 2 eutectic content within the range of 0.02–0.08. However, the quality factor ( Q×f ) was dependent on the amount of LiF–CaF 2 added. Moreover, open or closed sintering condition influenced the microstructure and microwave dielectric properties of LiF–CaF 2 -doped MgO ceramics. The X-ray diffraction and impedance spectroscopy analyses indicated that Li and Ca substitution for Mg and F substitution for O occurred in MgO ceramics, and a high amount of F was incorporated into the MgO lattice under closed sintering condition. Overall, 0.96MgO–0.04(0.8LiF–0.2CaF 2 ) ceramics sintered at 950 °C in an open crucible exhibited excellent microwave dielectric properties of ε r =9.18, Q×f =130198 GHz and τ f =−65 ppm/°C. This study shows that eutectic addition of LiF–CaF 2 effectively decreases the sintering temperature of MgO without deteriorating the ceramic performance. [ABSTRACT FROM AUTHOR]

Abstract: Non-aqueous gelcasting and dry pressing were used to prepare 45wt% Ba0.6Sr0.4TiO3–55wt% MgO (BSTM) ceramics. The effects of different forming methods on the microstructures and dielectric properties of the BSTM ceramics were investigated. The densities of the BSTM ceramics prepared by non-aqueous gelcasting are lower but more uniform than that of the BSTM ceramics prepared by dry pressing. The XRD analysis illustrates that phase compositions are completely the same no matter what forming method is adopted. The SEM results show that the BSTM green samples and sintered ceramics prepared by non-aqueous gelcasting are more uniform than that prepared by dry pressing. Furthermore, it is found that the BSTM ceramics prepared by non-aqueous gelcasting have higher and more uniform dielectric constant, tunability and loss tangent (measured at 10kHz and 20°C). Meanwhile, the BSTM ceramics prepared by non-aqueous gelcasting have higher dielectric constant and lower Q × f value (namely more loss) when they are measured at microwave frequencies. [Copyright &y& Elsevier]

The phase composition, microstructure, dielectric properties and breakdown strength of (Mg 0.9 Zn 0.1 ) 2 (Ti 1− x Mn x )O 4 ceramics synthesized using a conventional solid-state reaction were investigated. The substitution of Ti 4+ by Mn 2+ can form a single spinel phase and result in a homogenous microstructure, which improves breakdown strength and reduces dielectric loss. Impedance spectroscopy results show that grain and grain boundary exhibit similar impedance properties and that the Schottky barrier in the grain boundary cannot be observed. When the x value is equal to 0.01, (Mg 0.9 Zn 0.1 ) 2 (Ti 1− x Mn x )O 4 ceramics with a high breakdown strength of 102 kV/mm (at 0.2 mm) and a low dielectric loss of 5.3×10 −5 (at 6 GHz) can be obtained, which are beneficial in the development of high voltage solid-state pulse-forming lines. [ABSTRACT FROM AUTHOR]

The crystal structure and microwave (MW) dielectric characteristics of the compound in the perovskite type series (1- x ) (Sr 0.25 Ce 0.5 TiO 3 )- x La(Mg 0.5 Ti 0.5 )O 3 , x  = 0.0 to 0.6 have been studied. The structural phase evolutions from orthorhombic (for x  = 0.0) to cubic (for x  ≥ 0.15) as a function of La(Mg 0.5 Ti 0.5 )O 3 have been investigated and are related to variation in the dielectric characteristics. For x  = 0.0, the electron diffraction (TEM) revealed that Sr 0.25 Ce 0.5 TiO 3 compound were tilted in both anti-phase and anti-parallel displacement of A-site cation. The nonlinear trend of the temperature coefficient of resonant frequency ( τ f ) vs dielectric constant resulted from the tilted (for x  = 0.0) to nontilted (for x  ≥ 0.15) structure transition. The value of τ f decreases markedly from +194 ppm/°C ( x = 0.0) to + 0.8 ppm/°C ( x  = 0.6), but the Q  ×  f value (Q-factor) increases from 19,238 GHz to 37,508 GHz. Low loss microwave (MW) dielectric properties: ε r  = 47.0, Q  ×  f  = 37508 GHz and τ f = +15 ppm/°C and ε r  = 39, Q  ×  f  = 34098 GHz and τ f = +0.8 ppm/°C were achieved for compositions with x  = 0.45 and 0.6. [ABSTRACT FROM AUTHOR]

A perovskite solid solution (1 −x )(Sr 0.5 Ce 0.5 )TiO 3+δ − x NdAlO 3 , x = 0.1 to 0.4 was prepared by conventional solid state method. X-ray diffraction spectra revealed a single phase with tetragonal structure, indicating that doping of NdAlO 3 significantly stabilized the perovskite-like structure. The addition of NdAlO 3 facilitated the formation of large plate-like grains with porous microstructure. The dielectric constant ( ε r ) decreased with increasing x because of the small ionic polarizability of NdAlO 3 . The Q × f value was strongly dependent on the microstructure of these ceramics. The temperature coefficient of resonant frequency ( τ f ) gradually shifted to near zero with a rise of x , which resulted from the decrease in tolerance factor ( t ). The solid solution with x = 0.4 sintered at 1550 °C for 4 h showed a good combination of dielectric properties: ε r = 72, Q × f = 12052 GHz and τ f = +5 pmm/°C. [ABSTRACT FROM AUTHOR]

Phase formation, chemical structure and microwave (MW) dielectric behavior of A-site Ce-doped SrTiO 3 ceramics sintered in close aluminum pipe have been investigated. Samples with x = 0.4 appeared cubic by X-ray diffraction (XRD) but exhibited splitting and superstructure reflections in the range of 0.45 ≤ x ≤ 0.6. X-ray photoelectron spectroscopy (XPS) analysis revealed the sample possesses mixed-valent states of Ti 3+ /Ti 4+ upon oxygen vacancy formation. As the x value increases, the Q × f value increases slightly from 7569 to 22,682 GHz, but the temperature coefficient of resonant frequency ( τ f ) decreases markedly from +372 to +5 ppm/°C. A novel combination of microwave dielectric properties: ε r = 61, Q × f = 22,682 GHz and τ f = +5 pmm/°C were achieved for composition Sr 0.1 Ce 0.6 TiO 3 at 1300 °C for 3 h. [ABSTRACT FROM AUTHOR]