Vacuum heat treatment of LiAlO2 densified silicon nitride ceramics

The aim of the present work has been to produce high-dense Si₃N₄ ceramics by a cheaper pressureless sintering method and then to attain vacuum heat treatment to remove residual grain boundary glass in gaseous form. LiAlO₂ was used as a sintering additive rather than using Li₂O, since its grain boundary glass is not stable above 1200 °C. LiAlO₂ was synthesised from 42% Li₂CO₃ and 58% Al₂O₃ powder mix reacting together at 1450 °C for 3 h in a muffle furnace. X-ray analysis showed that 95% LiAlO₂ was obtained. LiAlO₂ was milled and added to silicon nitride powder as a sintering additive. Hot-pressing and pressureless sintering of LiAlO₂ containing Si₃N₄ compacts were carried out at temperatures between 1450–1750 °C. The sintered samples were vacuum heat-treated at elevated temperatures under high vacuum to remove intergranular glass and to increase refractoriness of Si₃N₄ ceramics. Scanning electron microscope images and weight loss results showed that Li in grain boundary glass (Li–Al–Si–O–N) was successfully volatilised, and oxidation resistance of the sintered samples was increased.     

Conduction analysis of Li2O doped 0.2[Pb(Mg1/3Nb2/3)]–0.8[PbTiO3–PbZrO3] ceramics fabricated by columbite precursor method

We have fabricated 0.2Pb(Mg_1/3Nb_2/3)O₃–0.8Pb(Zr_0.475Ti_0.525)O₃ [PMN–PZT] ceramics doped with various amounts of Li₂O (0, 0.05, 0.1, 0.2, 0.3 wt.%) using the columbite precursor method. The effects of Li-doping on the conduction behavior of PMN–PZT ceramics are discussed in relation to the low frequency dielectric dispersion and frequency domain measurement. The Li-doped PMN–PZT ceramics sintered at 950 °C showed a sufficient densification with large dielectric constant and low dielectric loss. The incorporation of Li⁺ ion in PMN–PZT ceramics led to an appreciable reduction in electrical conductivity and further enhanced the ferroelectric and piezoelectric properties. The activation energies of PMN–PZT + xLi₂O (x = 0, 0.05, 0.1, 0.2, 0.3 wt.%) ceramics calculated from ac conductivity measurement using the Arrhenius relation were 1.05, 1.25, 1.27, 1.38 and 1.41 eV, respectively. The conduction behavior is examined in the low frequency and high temperature region and the results are discussed in detail through crystal defect mechanism.     

Microwave dielectric characteristics of Ba(Mg1/3Ta2/3)O3 ceramics sintered at low-temperatures

The microwave dielectric properties and microstructures of Ba(Mg_1/3Ta_2/3)O₃ (BMT) ceramics sintered at low temperatures with 2–3 wt.% NaF additives were investigated. BMT ceramics sintered at 1340 °C for 3–12 h showed dielectric constants (_r) of 25.5–25.7, Qf values of 41 500–50 400 GHz and temperature coefficients of the resonator frequency (τ_f) of 10.9–21.4 ppm °C⁻¹. The variation of sintering time almost had no effect on the dielectric constant. The Qf value increased and the τ_f decreased with increasing sintering time. The ordering degree of Mg²⁺ and Ta⁵⁺ at B-sites increased with increasing sintering time.     

Microwave dielectric properties and microstructures of MgTa2O6 ceramics with CuO addition

The effect of CuO addition on the microstructures and the microwave dielectric properties of MgTa₂O₆ ceramics has been investigated. It is found that low level-doping of CuO (up to 1 wt.%) can significantly improve the density of the specimens and their microwave dielectric properties. Tremendous sintering temperature reduction can be achieved due to the liquid phase effect of CuO addition observed by scanning electronic microscopy (SEM). The sintered samples exhibit excellent microwave dielectric properties, which depend upon the liquid phase and the sintering temperature. With 0.5 wt.% CuO addition, MgTa₂O₆ ceramic can be sintered at 1400 °C and possesses a dielectric constant (_r) of 28, a Q × f value of 58000 GHz and a temperature coefficient of resonant frequency (τ_f) of 18 ppm/°C.     

Sintering behavior and microwave dielectric properties of Bi3(Nb1−xTax)O7 solid solutions

Solid solutions of Bi₃(Nb_1−xTa_x)O₇ (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)⁻¹ between 0 ≤ x ≤ 1 and Bi₃(Nb_1−xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1−xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm⁻³, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from −113 ppm °C⁻¹ of Bi₃NbO₇ to −70 ppm °C⁻¹ of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi₃(Nb_1−xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.     

Microstructures and Dielectric Properties of Ba1-xSrxTiO3 Ceramics Doped with B2O3-Li2O Glasses for LTCC Technology Applications

Ba1-xSrxTiO3 ceramics, doped with B2O3-Li2O glasses have been fabricated via a traditional ceramic process at a low sintering temperature of 900 ℃ using liquid-phase sintering aids. The microstructures and di- electric properties of B2O3-Li2O glasses doped Ba1-xSrxTiO3 ceramics have been investigated systemat- ically. The temperature dependence dielectric constant and loss reveals that B2O3-Li2O glasses doped Ba1-xSrxTiO3 ceramics have di?usion phase transformation characteristics. For 5 wt% B2O3-Li2O glasses doped Ba0.55Sr0.45TiO3 composites, the tunability is 15.4% under a dc-applied electric field of 30 kV/cm at 10 kHz; the dielectric loss can be controlled about 0.0025; and the Q value is 286. These composite ceramics sintered at low temperature with suitable dielectric constant, low dielectric loss, relatively high tunability and high Q value are promising candidates for multilayer low-temperature co-fired ceramics (LTCC) and potential microwave tunable devices applications.     

Nanocrystallization of SrBi2Nb2O9 from glasses in the system Li2B4O7---SrO---Bi2O3---Nb2O5

Transparent glasses in the system (100−x)Li₂B₄O₇–x(SrO---Bi₂O₃---Nb₂O₅) (10≤x≤60) (in molar ratio) were fabricated by a conventional melt-quenching technique. Amorphous and glassy characteristics of the as-quenched samples were established via X-ray powder diffraction (XRD) and differential thermal analyses (DTA) respectively. Glass–ceramics embedded with strontium bismuth niobate, SrBi₂Nb₂O₉ (SBN) nanocrystals were produced by heat-treating the as-quenched glasses at temperatures higher than 500 °C. Perovskite SBN phase formation through an intermediate fluorite phase in the glass matrix was confirmed by XRD and transmission electron microscopy (TEM). Infrared and Raman spectroscopic studies corroborate the observation of fluorite phase formation. The dielectric constant (_r) and the loss factor (D) for the lithium borate, Li₂B₄O₇ (LBO) glass comprising randomly oriented SBN nanocrystals were determined and compared with those predicted based on the various dielectric mixture rule formalism. The dielectric constant was found to increase with increasing SBN content in LBO glass matrix.     

Al2O3掺杂BaTi0.85Sn0.15O3陶瓷的制备及性能表征

通过传统固相二次烧结法来制备x wt% Al₂O₃（x=0、1.0、1.5）/BaTi_0.85Sn_0.15O₃（BTS）陶瓷。研究了掺杂不同含量Al₂O₃对BTS陶瓷的微观结构、介电性能及挠曲电性能的影响。结果表明,掺杂Al₂O₃的BTS陶瓷不改变陶瓷的晶体结构,仍为标准钙钛矿结构晶型;Al₂O₃的掺入能够有效降低晶粒尺寸,具有明显的细晶作用。随着Al₂O₃含量的增大,Al₂O₃/BTS陶瓷的介电常数减小,介电损耗得到明显改善,居里峰逐渐宽化且向温度高的方向偏移。Al₂O₃/BTS陶瓷的挠曲电系数随着Al₂O₃含量的增加和测试环境温度的升高均减小。此外,Al₂O₃/BTS陶瓷的挠曲电系数和介电常数之间存在一种近线性关系,但当温度非常接近于居里温度时,这种线性关系减弱。     

(K0.44,Na0.52,Li0.04) (Nb0.84,Ta0.10,Sb0.06)O3 Ferroelectric Ceramics

Recent progress in (K_0.44,Na_0.52,Li_0.043-based ceramics (KNN) with special emphasis on (K_0.44,Na_0.52,Li_0.040.84,Ta_0.10,Sb_0.06))O₃ (KNN-LT-LS) is reviewed concisely. The base KNN and its compositional derivatives are analyzed in terms of dopant-property relationships, which are then extended to the ternary derivatives. The effects of processing conditions such as humidity, precursor purity, and oxygen partial pressure during sintering are elaborated on from a phenomenological perspective. It is also shown that the spontaneous polarization is sensitive to the processing route chosen for synthesis (mixed oxide versus perovskite routes). Special attention is devoted to the discussion of the morphotropic phase boundary (MPB) dilemma in the KNN-LT-LS system, where it is shown that the origin of high piezoelectric activity is actually due to a polymorphic transition at room temperature. It is shown that prototype transducers based on pure and 1 mol% Ba²⁺ doped KNN-LT-LS exhibit performance metrics comparable to those fabricated using PZT-5H. Overall, KNNLT- LS ceramics show great promise for lead-free applications, although issues such as temperature dependence of properties and strong sensitivity to processing conditions remain as the 2 major challenges.     

Ferroelectric, dielectric and piezoelectric properties of potassium lanthanum bismuth titanate K0.5La0.5Bi4Ti4O15 ceramics

The Aurivillius type bismuth layer-structured compound potassium lanthanum bismuth titanate (K_0.5La_0.5Bi₄Ti₄O₁₅) is synthesized using conventional solid-state processing. The phase analysis is performed by X-ray diffraction (XRD) and the microstructural morphology is conducted by scanning electron microscopy (SEM). The ferroelectric, dielectric and piezoelectric properties of K_0.5La_0.5Bi₄Ti₄O₁₅ (KLBT) ceramics are investigated in detail. The remnant polarization (P_r) and coercive field (E_c) are found to be 8.6 μC cm⁻² and 60 kV cm⁻¹, respectively. The Curie temperature T_c and piezoelectric coefficient d₃₃ are 413 °C and 18 pC N⁻¹, respectively.     

Novel soft solution synthesis and characterization of submicromic LiCoVO4

A novel soft solution process has been used to prepare LiCoVO₄ by reacting Co(CH₃CH₂COO)₂, Li₂CO₃, NH₄VO₃ and citric acid. LiCoVO₄ powders were successfully prepared at as low as 450 °C in 4 h. Compared to the solid-state reaction processes, the soft solution process greatly reduced the temperature and the time for preparing LiCoVO₄. The inverse spinel structure and high crystallinity of the synthesized product has been confirmed by X-ray diffraction. Thermal analysis proves that the phase formation of the compound occurs at about 450 °C. The results of the IR investigations show that the band located at 820 cm⁻¹ corresponds to the stretching vibration mode of VO₄ tetrahedron with the A symmetry. SEM examination reveals a spherical grain distribution, the average particle size being typically lower than 1 μm. The quantitative result from ICP-AES analysis is Li_0.967Co_0.994VO₄.     

Electrical properties of garnet-like lithium ionic conductors Li5+xSrxLa3--xBi2O12 fabricated by spark plasma sintering method

A typical approach involving Pechini method and spark plasma sintering (SPS) method was presented for the preparation of high density Li_5+xSr_xLa_3--xBi₂O₁₂ (x = 0, 1) ceramics. Phase formation, microstructure, grain size and electrical properties of the specimens were examined using XRD, SEM and alternating current impedance spectroscopy (ACIS). Dense Li₅La₃Bi₂O₁₂ and Li₆SrLa₂Bi₂O₁₂ ceramics with pure garnet-like phase, relative density of 97% and average grain size of about 5 μm were fabricated using this approach. The total conductivities at 298 K of Li₅La₃Bi₂O₁₂ and Li₆SrLa₂Bi₂O₁₂ ceramics prepared by the SPS method are 5.1×10^-5 and 6.8×10^-5 S/cm, respectively, 2 times higher than that of samples prepared by the conventional sintering method.     

Piezoelectric and dielectric properties of PZT–PZN–PMS ceramics prepared by molten salt synthesis method

Piezoelectric powders and ceramics with the composition of Pb_0.95Sr_0.05(Zr_0.52Ti_0.48)O₃–Pb(Zn_1/3Nb_2/3)O₃–Pb(Mn_1/3Sb_2/3)O₃ (PZT–PZN–PMS) were prepared by molten salt synthesis (MSS) and conventional mixed-oxide (CMO) methods, respectively. The influence of synthesis process on the properties of powders and ceramics were investigated in detail. The results show that the MSS method significantly improved the sinterability of PZT–PZN–PMS ceramics, resulting in an improvement of dielectric and piezoelectric properties compared to the CMO method. The optimum values of MSS samples are as follows: _r = 1773; tan δ = 0.0040; T_c = 280 °C; d₃₃ = 455 pC/N; k_p = 0.70; Q_m = 888; E_c = 10.3 kV/cm; and P_r = 28.2 μC/cm², at calcination of 800 °C and sintering of 1120 °C temperature.     

Influence of A-site nonstoichiometry on sintering, microstructure and electrical properties of (Bi0.5Na0.5)TiO3 ceramics

Lead-free (Bi_0.5Na_0.5)_1+xTiO₃ ceramics (x = −0.02, −0.01, −0.005, 0, 0.005 and 0.01) were prepared by ordinary sintering. The effect of A-site stoichiometry on the densification, microstructure, dielectric properties, high-temperature impedances, and piezoelectric properties was explored. It was found that the high conductivity of (Bi_0.5Na_0.5)TiO₃ (BNT) ceramics should be mainly attributed to the formation of A-site cation vacancies during sintering. Improved physical and electrical properties can be achieved in the sample with A-site cation excess. The control of the stoichiometry proves to be an effective way to improve BNT ceramics for possible application.     

Templated grain growth of SrBi2Ta2O9 ceramics: Mechanism of texture development

Textured SrBi₂Ta₂O₉ (SBT) ceramics were fabricated via templated grain growth (TGG) technique using platelet-like SBT single crystal templates. The templates (5 wt%) were embedded in a fine-grain SBT powder matrix containing 3 wt% of Bi₂O₃ excess that were subjected to uniaxial pressing and sintering at 1000–1250 °C for up to 24 h. Microstructural characterization by SEM was performed to establish the effect of sintering parameters on the grain growth and texture development. It was found that the ceramics developed a bimodal microstructure with notable concentration of large (longer than 90 μm) aligned grains with c-axis oriented parallel to the pressing direction. The mechanism controlling the texture development and grain growth in SBT ceramics is discussed.     

ZnO掺杂对Ca0.25(Li0.43Sm0.57)0.75TiO3陶瓷烧结性能和微波介电性能的影响

采用溶胶-凝胶法制备Ca_0.25(Li_0.43Sm_0.57)_0.75TiO₃(CLST)微波介质陶瓷纳米粉体, 研究了ZnO掺杂量和烧结温度对CLST+ xmol% ZnO陶瓷烧结性能和微波介电性能的影响。XRD分析结果表明: 随着ZnO掺杂量x的增加, 陶瓷的晶体结构从正交相变为伪立方相, 并在x≥1.5的样品中出现了杂相。CLST+ xmol% ZnO陶瓷的致密化烧结温度随x的增加而降低, x=1.0的样品的致密化烧结温度比x=0的降低了200 ℃。介电常数ε_r和频率品质因数Qf 随x增加和烧结温度的升高具有最优值, 频率温度系数则单调降低。x=1.0的样品在1100 ℃烧结时具有优异的综合性能: ρ = 4.85 g/cm³, ε_r =102.8, Qf = 5424 GHz, τ_f = -8.2×10^-6/℃。表明ZnO掺杂的CLST陶瓷是一种很有发展潜力的微波介质陶瓷。     

High electrical properties of W-additive Mn-modified PZT–PMS–PZN ceramics for high power piezoelectric transformers

The ceramics were prepared successfully by the addition of WO₃ to the Mn-modified Pb(Zr_0.52Ti_0.48)O₃–Pb(Mn_1/3Sb_2/3)O₃–Pb(Zn_1/3Nb_2/3)O₃ (PZT–PMS–PZN) for high power piezoelectric transformers application. XRD analysis indicated that the ceramics were mainly composed of a tetragonal phase in the range of 0–1.0 wt.% WO₃ addition. The grain size of the ceramics significantly decreased from 10.0 to 2.9 μm by addition of WO₃. Moreover, the addition of WO₃ promoted densification of the ceramics and increased mechanical quality factor (Q_m), planar coupling factor (K_p) and piezoelectric constant (d₃₃) kept high values, whereas, dielectric loss (tan δ) was low. Δf (=f_a − f_r) slightly changed when WO₃ addition was above 0.5 wt.%. The ceramics with 0.6 wt.% WO₃ addition, sintered at 1150 °C showed the optimized piezoelectric and dielectric properties with Q_m of 1852, K_p of 0.58, d₃₃ of 243 pC/N and tan δ of 0.0050. The ceramics are promising candidates for high power piezoelectric transformers application.     

Microwave dielectric properties of W-doped Ba0.6Sr0.4TiO3 thin films grown on (001) MgO by pulsed laser deposition with a variable oxygen deposition pressure

The microwave dielectric properties of Ba_0.6Sr_0.4TiO₃ 1 mol% W-doped thin films deposited using pulsed laser deposition, are improved by a novel oxygen deposition profile. The thin films were deposited onto (001) MgO substrates at a temperature of 720 °C. A comparison is made between three different oxygen ambient growth conditions. These include growth at a single oxygen pressure (6.7 Pa) and growth at two oxygen pressures, one low (6.7 Pa) and one high (46.7 Pa). Films were deposited in a sequence that includes both a low to high and a high to low transition in the oxygen deposition pressure. Following deposition, all films were post-annealed in 1 atm of oxygen at 1000 °C for 6 h. The dielectric Q (defined as 1 / tanδ) and the dielectric constant, ε_r, were measured at room temperature, at 2 GHz, using gap capacitors fabricated on top of the dielectric films. The percent dielectric tuning (defined as (ε_r(0 V) − ε_r(40 V)) / ε_r(0 V) × 100) and figure of merit (FOM) (defined as percent dielectric tuning × Q(0 V)) were calculated. The film deposited using the two-stage growth conditions, 6.7 / 46.7 Pa oxygen, showed a maximum Q(0 V) value with high percent dielectric tuning and gave rise to a microwave FOM twice as large as the single stage growth condition. The improved dielectric properties are due to initial formation of a film with reduced interfacial strain, due to the formation of defects at the film/ substrate interface resulting in a high Q(0 V) value, followed by the reduction of oxygen vacancies which increases the dielectric constant and tuning.     

Sm2O3掺杂(Ba0.7Ca0.3)TiO3-Ba(Zr0.2Ti0.8)O3无铅压电陶瓷的制备与性能

采用固相合成法制备了Sm2O3掺杂的(Ba0.7Ca0.3)TiO3-Ba(Zr0.2Ti0.8)O3(BCZT)无铅压电陶瓷.借助XRD、SEM等手段对该陶瓷的显微结构与电性能进行了研究.结果表明,Sm2O3的掺杂降低了BCZT无铅压电陶瓷的烧结温度并使居里温度点Tc从85℃提高到95℃.当Sm2O3掺杂量为0.02wt%～0.1wt%时,样品具有典型ABO3型钙钛矿结构.Sm2O3掺杂量为0.02wt%时,所得陶瓷样品具有最优综合电性能,其压电常数d33、机电耦合系数kp、机械品质因子Qm、介电损耗tanδ和介电常数εr分别为590 pC/N、0.52、43、1.3%和3372.     

Consolidation behavior of L12 phase (Al + 12.5 at.% Cu)3Zr powder with nanocrystalline structure during CIP and subsequent sintering

The mechanically alloyed (Al + 12.5 at.% Cu)₃Zr powders were consolidated by cold isostatic pressing (CIP) and subsequent sintering. Effects of CIP pressure and sintering temperature on the stability of metastable L1₂ phase and nanocrystalline structure were investigated. Before sintering, the powders were CIPed at 138, 207, 276, and 414 MPa. The relative densities of the CIP compacts were not greatly affected by the CIP pressure. However, the L1₂ phase of the specimen CIPed at pressures greater than 276 MPa was partially transformed into D0₂₃. The optimum consolidation conditions for maintaining L1₂ phase and nanocrystalline microstructure were determined to be CIP at 207 MPa and sintering at 800 °C for 1 h for which the grain size was 34.2 nm and the relative density was 93.8%. Full density specimens could be prepared by sintering above 900 °C, however, these specimens consisted of L1₂ and D0₂₃ phases. The grain sizes of all the specimens were confirmed by TEM and XRD, and were found to be less than 40 nm. This is one of the smallest grain sizes ever reported in trialuminide intermetallic compounds.