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Nanostructuring is an effective approach to improve thermoelectric (TE) performance, which is caused by the interface and quantum effects on electron and phonon transport. For a typical layered structure such as sodium cobalt (NCO), a highly textured ceramic with nanostructure is beneficial for the carrier transport properties due to the strong anisotropy. In this paper, we established a textured NCO ceramic with highly oriented single crystals in nanoscale. The Na0.6CoO2 platelet crystals were prepared by a one-step hydrothermal method. The growth mechanism was revealed to involve dissolution-recrystallization and exchange reactions. NCO TE ceramics fabricated by a press-aided spark plasma sintering method showed a high degree of texturing, with the platelet crystals basically lying along the in-plane direction perpendicular to the press direction. TE properties of the textured NCO ceramics showed a strong anisotropic behavior. The in-plane electrical conductivity was considerably larger than the out-of-plane data because of fewer grain boundaries and interfaces that existed in the in-plane direction. Moreover, the in-plane Seebeck coefficient was higher because of the anisotropic electronic nature of NCO. Although the in-plane thermal conductivity was high, a prior ZT value was enabled for these NCO ceramics along this direction because of the dominant electrical transport. This finding provides a new approach to prepare highly oriented ceramics. [ABSTRACT FROM AUTHOR]

In this work, Pb(Zr, Ti)O (PZT) powders were prepared by traditional hydrothermal method and solid state reaction. Subsequently, PZT ceramics were sintered and their electric properties (including piezoelectric coefficient d , electromechanical coupling coefficient k and mechanical quality factor Q ) as a function of sintering temperature (1125-1250 °C) were comparably studied. Given that the Pb deficiency may exist in the hydrothermally synthesized PZT powders, effects of Pb excess (0, 20 and 80 mol%) in the hydrothermal precursor on their sinterability were also involved. The hydrothermally synthesized PZT powders with 80 mol% Pb excess showed the highest sintering activity, and their electric properties peaked at 1175 °C ( d = 225 pC/N, k = 0.446, Q = 300). It's demonstrated that A-site deficiency may degrade the sintering activity of PZT powders, and hydrothermal synthesis of PZT powders with appropriate Pb excess was the compromising pathway to increase their sinterability, decrease the sintering temperature and lessen the volatilization of Pb ion in the fabrication of widely applied PZT ceramic. [ABSTRACT FROM AUTHOR]

Sodium was used to substituted for potassium in Pb(Zr,Ti)O-Sr(K,Nb)O (PZT-SKN) based on the lead-free (NaK)NbO (KNN) piezoelectric ceramic and a new ceramic material Pb(Zr,Ti)O-Sr(Na,Nb)O (PZT-SNN) was developed in order to improve the humidity resistance. PZT-SKN and PZT-SNN ceramics have been fabricated by the conventional solid-state reaction method at the sintering temperature of 1,150-1,225 °C for 2 h. The effects of SKN and SNN on the microstructure, piezoelectric properties and humidity resistance of the prepared ceramics have been systematically investigated and compared. The phase structures of the two ceramics are both tetragonal. With increasing of SKN/SNN, both the grain size and Curie temperature decrease. Among all compositions studied, the 0.99PZT-0.01SKN and 0.98PZT-0.02SNN sintered at 1,175 °C exhibited optimal comprehensive properties. Especially, the PZT-SNN has a higher humidity resistance than PZT-SKN. The optimal values of d, kp, ε and Tc for 0.98PZT-0.02SNN is 448pC/N, 0.63, 2,126.32 and 354 °C, respectively. [ABSTRACT FROM AUTHOR]

(KNa)NbO piezoelectric ceramics can be sintered at a temperature as low as 750 °C for 5 h by incorporating LiCO + BiO + ZnO as the sintering aid, whereas the conventional sintering temperature is around 1,100 °C. The optimal 'soft' piezoelectric properties are obtained for ceramics sintered at 850 °C for 5 h. The dielectric permittivity ( ε), piezoelectric coefficient ( d), electromechanical coupling ( k) and mechanical quality factors ( Q) of (K, Na)NbO modified with 5.5 wt% sintering aids are 1,436, 90 pC/N, 0.3 and 10, respectively. These values are similar to the values obtained for (KNa)NbO ceramics sintered above 1,100 °C. The underlying mechanism for abrupt change of dielectric permittivity is explained. [ABSTRACT FROM AUTHOR]

In this work, microstructure characteristics, dielectric, piezoelectric and ferroelectric properties of lead-free (KNaLi)(NbTaSb)O (KNLNST) doped with 1 mol% copper oxide (CuO) piezoelectric ceramics prepared by a conventional solid-state reaction route are investigated with an emphasis on the influence of sintering temperature. The introduction of CuO could significantly improve the sinterability of KNLNST ceramics. It is found that the tetragonality of the ceramics increases with raising sintering temperature. A dense microstructure with increased grains is developed, probably due to liquid-phase sintering. Both the piezoelectric constant d and planar electromechanical coupling k increase with increasing relative density and grain size. The Curie temperature T values increase slightly when the sintering temperature is increased. In addition, the KNLNST ceramics doped with 1 mol% CuO show obvious dielectric relaxor characteristics, and the relaxor behavior of ceramics is strengthened by increasing the sintering temperature. The improved piezoelectric and dielectric properties of d = 241 pC/N, k = 0.437, dielectric loss tan δ = 0.0087, mechanical quality factor Q = 138, dielectric constant ε = 1,304 can be obtained for specimens sintered at 1,080 °C for 3 h. [ABSTRACT FROM AUTHOR]

KNaNbO- x ZnO (KNN- xZn) lead-free ceramics have been prepared using the conventional sintering technique and the effects of ZnO addition on the phase structure and piezoelectric properties of the ceramics have been studied. Our results reveal that a small amount of ZnO can improve the density of the ceramics effectively. Because of the high density and ZnO doping effects, the piezoelectric and dielectric properties of the ceramics are improved considerably. The good piezoelectric and dielectric properties of d = 114 pC/N, k = 0.36, ε = 395, and Q = 68 were obtained for the KNN ceramics doped with 1 mol% ZnO. Therefore, the KNN-1.0 mol%Zn ceramics is a good candidate for lead-free piezoelectric application. [ABSTRACT FROM AUTHOR]

K0.5Na0.5NbO3 (KNN) lead-free piezoelectric ceramics containing 1 mol% ZnO (KNNZ) were prepared by optimizing the sintering conditions using the newly developed K5.70Li4.07Nb10.23O30 (KLN) as a sintering aid. The phase structure and electrical properties of the KNNZ ceramics were investigated as a function of the KLN concentration. The results show that a small amount of KLN was incorporated into the lattice and formed a single-phase perovskite structure. The KLN modification lowered the phase transition temperature of the orthorhombic–tetragonal structure and increased the Curie temperature. Interestingly, an appropriate amount of KLN can simultaneously improve the sinterability of the KNNZ ceramics and the ferroelectric and piezoelectric properties. Good piezoelectric and dielectric properties (piezoelectric constant, d 33  =  131 pC/N; planar coupling coefficient, k p  =  0.42; dielectric constant, ϵ r  =  634; and mechanical quality factor, Q m  =  20) were obtained for the KNNZ ceramics doped with 1 mol% KLN. Therefore, KNNZ + 1.0 mol% KLN ceramics containing an appropriate amount of KLN are good candidate materials for the preparation of lead-free piezoelectric ceramics. [ABSTRACT FROM PUBLISHER]

Lead-free (KNaLi)(NbSbTa)O (abbreviated as KNLNST) piezoelectric ceramics with x = 0.045-0.075 have been prepared by an ordinary sintering technique with sintering temperature at 1,120 °C. The results of X-ray diffraction reveal that Ta has diffused into the perovskite lattice to form a solid solution. The grain growth of the ceramics was inhibited by substituting Ta for Nb. It has been shown that the substitution of Ta decreases Curie temperature T and orthorhombic-tetragonal phase transition temperature T. Besides, the dependence of the ceramics with different Ta content on the dielectric, piezoelectric and ferroelectric properties has been investigated. The results indicate that Ta substitution provides 'soft' piezoelectric characteristics, owing to improvements in d, k and ε and a decease in Q. For the ceramics with x = 0.06, the piezoelectric coefficient d becomes maximum at a value of 270 pC/N, while the other electrical properties remain reasonably high: dielectric constant ε = 1,577, planar mode electromechanical coupling factors k = 0.4, Curie temperature T = 252 °C and the remanent polarization P = 16.03 μC/cm. These results show that (K, Na, Li) (Nb, Sb)O ceramics with small amount of Ta ( x <8 mol%) are good lead-free piezoelectric ceramic. [ABSTRACT FROM AUTHOR]

Abstract: BaTiO3 (BT) powders were synthesized by the hydrothermal method for fabricating lead-free barium titanate piezoelectric ceramics. The obtained powders were washed by distilled water and 0.01N acetic acid solutions separately, and utilized to obtain piezoelectric ceramics by traditional sintering. BT ceramics with the highest piezoelectric properties (d 33 value is over 190 pC/N) was obtained from the BT powder synthesized at 250°C and washed by acetic acid solution. The influence of washing method and sintering temperature on the piezoelectric properties of BT ceramics were studied. The reasons were investigated by comparing the properties of BT powders and their compacts sintered at different temperature. [Copyright &y& Elsevier]

Sodium niobate nanopowders (N-NaNbO 3 ) and submicron powders (M-NaNbO 3 ) were synthesized by the solvothermal and hydrothermal route, respectively. Subsequently, their sintering activities were comparably investigated through a modified solid-state sintering process. It's found that N-NaNbO 3 green bodies presented an apparent shrinkage after the calcination at 650 °C, and dense N-NaNbO 3 ceramics can be obtained at 950 °C. Besides, the optimized sintering temperature of N-NaNbO 3 ceramics was 100 °C lower, yet the shrinkage was four times larger when compared with M-NaNbO 3 samples. Importantly, N-NaNbO 3 samples showed improved dielectric properties in the whole frequency range, and the room temperature relative dielectric constant (ε r ) of N-NaNbO 3 samples sintered at 950 °C was as high as ~8000. The improved permittivity of N-NaNbO 3 samples was attributed to the compact microstructure, unique crystalline structure and less oxygen vacancies. At the same time, the effect of abnormal phase transition behavior originated from the metastable structure was considered to explain the enhanced room temperature permittivity. It's believed that the solvothermal route has great potentials in the synthesis of alkali niobate ceramics powders with high activity for low-temperature sintering. [ABSTRACT FROM AUTHOR]

Designed component of 0.95Al 2 O 3 –0.015Yb 2 O 3 –0.01MgO–0.025SiO 2 (95Al 2 O 3 –Yb/Mg/Si) ceramics were prepared by the traditional mixed-oxide method in the sintering temperature range of 1450–1700 °C. The influence of sintering temperature on the crystal structure, densification, microstructure, mechanical, friction and wear properties of 95Al 2 O 3 -YbMgSi ceramics were systematically investigated. XRD and SEM analysis results revealed that the increase in sintering temperature was very beneficial to eliminate the pores, increase the density and grain size, which obeys the common grain growth law. Both the flexural strength and hardness of obtained samples were increased almost linearly when the sintering temperature increased from 1450 °C to 1700 °C. The ceramics sintered at 1650 °C showed the optimum properties: H v = 1812.3, σ = 151.3 MPa, μ = 0.41, ρ = 3.72 g/cm3 and K c = 8.06e−5 mm3/N·m, respectively. Furthermore, the results of friction and wear experiments suggested that the 95Al 2 O 3 –Yb/Mg/Si ceramic prepared at the optimizing sintering process exhibited more stable friction state and lower wear degree under non-lubricated conditions. The enhanced mechanical properties could be attributed to their structure densification, pore elimination and gain growth with the increase of sintering temperature. [ABSTRACT FROM AUTHOR]

Abstract: Two-step sintering was applied to manufacture a typical Li- and Ta/Sb-modified alkaline niobate-based lead-free piezoelectric ceramics. The sintering condition dependences of dielectric constants and piezoelectric properties were investigated. Under the optimal condition, dense specimens have an average grain size of approximately 5μm, and show good dielectric and piezoelectric properties. Significantly, the (K0.4425Na0.52Li0.0375) (Nb0.8925Sb0.07Ta0.0375)O3 ceramics sintered under the condition “1130/10/1020/15” show the peak values of the piezoelectric coefficient (d 33), electromechanical coupling coefficient (k p), and dielectric constant (ɛ), which are 281pC/N, 50% and 1500, respectively, owing to the densest microstructure of typical bimodal grain size distributions. Furthermore, the KNLNST ceramics maintain relatively low dielectric loss over wide temperature ranges. The result indicates that the KNLNST ceramics sintered by two-step sintering exhibit good temperature stability. [Copyright &y& Elsevier]

Abstract: Lead-free (K0.4425Na0.52Li0.0375) (Nb0.9625−x Sb x Ta0.0375)O3 piezoelectric ceramics were prepared by the conventional sintering method. The effects of the Sb content on the phase structure, microstructure, dielectric, piezoelectric, and ferroelectric properties of the (K0.4425Na0.52Li0.0375) (Nb0.9625−x Sb x Ta0.0375)O3 ceramics were investigated. The much higher Pauling electronegativity of Sb compared with Nb makes the ceramics more covalent. By increasing x from 0.05 to 0.09, all samples exhibit a single perovskite structure with an orthorhombic phase over the whole compositional range, and the bands in the Raman scattering spectra shifted to lower frequency numbers. The grain growth of the ceramics was improved by substituting Sb5+ for Nb5+. Significantly, the (K0.4425Na0.52Li0.0375) (Nb0.8925Sb0.07Ta0.0375)O3 ceramics show the peak values of the piezoelectric coefficient (d 33), electromechanical coupling coefficient (k p ), and dielectric constant (ɛ), which are 304pC/N, 48% and 1909, respectively, owing to the densest microstructure of typical bimodal grain size distributions. Besides, the underlying mechanism for variations of the electrical properties due to Sb5+ substitution was explained in this work. [Copyright &y& Elsevier]

Abstract: 0.55Pb(Ni1/3Nb2/3)O3–0.45Pb(Zr0.3Ti0.7)O3(PNN–PZT) ceramics with different concentration of xFe2O3 doping (where x =0.0, 0.8, 1.2, 1.6mol%) were synthesized by the conventional solid state sintering technique. X-ray diffraction analysis reveals that all specimens are a pure perovskite phase without pyrochlore phase. The density and grain size of Fe-doped ceramics tend to increase slightly with increasing concentration of Fe2O3. Comparing with the undoped ceramics, the piezoelectric, ferroelectric and dielectric properties of the Fe-doped PNN–PZT specimens are significantly improved. Properties of the piezoelectric constant as high as d 33 ~956 pC/N, the electromechanical coupling factor k p ~0.74, and the dielectric constant ε r ~6095 are achieved for the specimen with 1.2mol% Fe2O3 doping sintered at 1200°C for 2h. [Copyright &y& Elsevier]

• Sintering BFO-BT piezoelectric ceramics at low temperature below 900 °C. • Low-temperature sintered BFO-BT ceramics maintain superior electrical properties. • Good thermal stability of BFO-BT ceramic with high T d around 300 ℃. • High piezoelectricity of d 33 > 119 pC/N in wide temperature range. Piezoelectric ceramics usually have high sintering temperature, which cannot meet the low-temperature co-firing requirements of ceramics and metal electrodes in production of micro devices. Here, we studied the effects of CuO-B 2 O 3 binary composite sintering aids on the sintering behavior, phase composition, microstructure, and electrical properties of the bismuth ferrite-barium titanate (0.7BiFeO 3 -0.3BaTiO 3 , BFO-BT) based lead-free piezoelectric ceramics. We found that the CuO-B 2 O 3 sintering aids can effectively lower the sintering temperature of the BFO-BT ceramics to 880 ℃, which is far below the melting point of metal electrodes. The BFO-BT based ceramics with 0.5 wt% sintering aids exhibited excellent dielectric, ferroelectric, and piezoelectric properties (ε m = 83029, P m = 38.4 μC/cm2, P r = 33.8 μC/cm2, d 33 = 119 pC/N) with high Curie temperature (T C = 460 ℃). Meanwhile, this ceramic possessed high depolarization temperature (T d) near 300 ℃, demonstrating a good temperature stability of piezoelectricity. This study provides a feasible solution to improve the compatibility of lead-free piezoelectric materials and low-melting point metal electrodes for fabrication of micro piezoelectric devices. [ABSTRACT FROM AUTHOR]