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Preparation of single-phase Sr 2 CrWO 6 ceramics is difficult since inevitable SrWO 4 and Sr 2 WO 5 parasitic phases are hard to remove. In this paper, pure Sr 2 CrWO 6 (S1) ceramics were easily synthesized by changing the mixing procedure of raw materials of the solid-state reaction method. Furthermore, Sr 2 CrWO 6 (S2) ceramics with parasitic phase SrWO 4 were also prepared for comparison. The crystal structure, chemical states, and magnetic and transport properties of S1 and S2 were investigated. Although there were no distinct differences in the crystal structure and chemical states of S1 and S2, S1 exhibited a higher magnetization and Curie temperature than S2, and this is because of the suppressed magnetic interaction in S2 caused by the large amount of Cr/W anti-site defect content. Smaller grain size and looser grain connectivity resulted in a higher resistivity of S1 than that of S2. Moreover，S2 responded more sensitively to a magnetic field of 2 T at low temperature ( T ≤ 50 K ). It is interesting to note that an appreciable low-field magnetoresistance (LFMR; H = 0.5 T) was observed, reaching − 46.5% and − 34.6% at 3 K and − 20.1% and − 21.8% at 50 K for S1 and S2, respectively. This remarkable LFMR property indicates that Sr 2 CrWO 6 ceramics may be a good candidate for practical applications in magnetic storage devices. It is possible that our work may not only provide an efficient experimental process for preparing other single-phase double perovskites that are difficult to synthesize but also provokes further investigation into the fundamental physical properties of magnetic double perovskites.

The itinerant electron density (n) near the Fermi level has a close correlation with the physical properties of Sr2FeMoO6. Two series of single-phase Sr(2−y)NayFeMoO6 (y = 0.1, 0.2, 0.3) and Sr(2−y)NayFe(1−x)Mo(1+x)O6 (y = 2x; y = 0.1, 0.2, 0.3) ceramics were specially designed and the itinerant electron density (n) of them can be artificially controlled to be: n = 1 − y and n = 1 − y + 3x = 1 + 0.5y, respectively. The corresponding crystal structure, magnetization and the ferromagnetic Curie temperature (TC) of two subjects were investigated systematically. The X-ray diffraction analysis indicates that Sr(2−y)NayFeMoO6 (y = 0.1, 0.2, 0.3) have comparable Fe/Mo anti-site defect (ASD) content in spite of decreased n. However, a drastically improved Fe/Mo ASD can be observed in Sr(2−y)NayFe(1−x)Mo(1+x)O6 (y = 2x; y = 0.1, 0.2, 0.3) caused by the intrinsic wrong occupation of normal Fe sites with excess Mo. Magnetization–magnetic field (M–H) behavior confirms that it is the Fe/Mo ASD not n that dominantly determines the magnetization properties. Interestingly, approximately when n ≤ 0.9, TC of Sr(2−y)NayFeMoO6 (y = 0.1, 0.2, 0.3) exhibits an overall increase with decreasing n, which is contrary to the TC response in electron-doped SFMO. Such abnormal TC is supposed to relate with the ratio variation of n(Mo)/n(Fe). Moreover, when n ≥ 1, TC of Sr(2−y)NayFe(1−x)Mo(1+x)O6 (y = 2x; y = 0.3) exhibits a considerable rise of about 75 K over that of Sr(2−y)NayFe(1−x)Mo(1+x)O6 (y = 2x; y = 0.1), resulting from improved n caused by introducing excess Mo into Sr(2−y)NayFeMoO6. Maybe, our work can provide an effective strategy to artificially control n and ferromagnetic TC accordingly, and provoke further investigation on the FeMo-baseddouble perovskites.

Series of Sr2Fe(1+x)MoO6 (x = 0, 0.03, 0.05, 0.08) ceramics have been synthesized by introducing metal Fe at grain boundaries. The increasing of Fe segregation changes the Fe/Mo anti-site defect (ASD) and decreases grain boundary (GB) strength significantly. Fe/Mo ASD contributes to low-field magnetoresistance (LFMR) negatively, but GB positively. When x ≤ 0.03, the ASD content of the maternal Sr2FeMoO6 (10%, x = 0) decreases to 8% (x = 0.03), which compensates the lowering of GB strength partially, and hence a slightly improved LFMR behavior is observed. However, when x ≥ 0.03 (x = 0.05, 0.08), Fe impurity bring about heavy ASD content and decreasing GB strength, so the LFMR property is suppressed greatly. The magnetoresistance behavior has been explored by a fitting model further. We believe that, our work may provide a facile and effective strategy for adjusting magnetoresistance response by tuning B-site ordering and GB strength of the double perovskite.

This paper reports the significant improved piezoelectric properties of high temperature bismuth titanate niobate (Bi 3 TiNbO 9 , BTN) polycrystalline ceramics. The piezoelectric performance of BTN ceramics is significantly enhanced by cerium modifications. The dielectric measurements indicate that the Curie temperature T c gradually decreases over the temperature range of 907–889 °C with cerium contents increasing up to 0.7 wt%. The BTN-5Ce (BTN+0.5 wt% CeO 2 ) exhibits optimized piezoelectric properties with a piezoelectric constant d 33 of 16 pC/N, which is five times the value of unmodified BTN ( d 33 ~3 pC/N), while BTN-5Ce maintains a high Curie temperature T c of 894 °C. The temperature-dependent electrical impedance and electromechanical coupling factors ( k p , and k t ) reveal that the BTN-5Ce exhibits thermally stable electromechanical coupling characteristics up to 500 °C but significantly deteriorates at 600 °C due to high conductivity at a higher temperature. The thermally stable electromechanical properties in combination with the ceramics׳ high electrical resistivity (10 6 Ω cm at 500 °C) and high Curie temperature (~900 °C) demonstrate that cerium-modified BTN ceramics are good materials for high temperature sensing applications.

Low field magnetoresistance (LFMR) of double perovskite Sr 2 FeMoO 6 (SFMO) is low at room temperature. In this paper, a low-cost, facile, and effective way to enhance the LFMR is developed. Single-phase SFMO ceramics with 10% Fe/Mo anti-site defects have been prepared, and then soaked in glycerin/water (1/1, V/V) mixture for 0–14 days. The insulating glycerin tends to segregate into the SFMO grain boundary to form organic/inorganic composite. Such a post-treatment has no effects on magnetic properties, but can increase the resistivity by ∼500 times, as the result, remarkable enhanced LFMR is observed in composites. With 0.5 T magnetic field, the LFMR of the SFMO soaked for 14 days reaches −29.5% at 10 K and −11.0% at 300 K, more than 2 times larger than that of the SFMO without soaking, which are −12.0% at 10 K and −4.7% at 300 K. Based on the experimental results, a possible mechanism is proposed to explain the relation between the composite structure and the LFMR response. We believe our work not only provides a novel method to enhance the magnetoresistance in SFMO ceramics, but also sheds light on tuning transport/magnetoresistance in other perovskite ceramics.

In order to clarify the influence of excess ions in A or B sites on perovskite (K,Na)NbO3 ceramics, various compositions of (K0.48Na0.52)Nb1+x%O3 (ABO3) ceramics where x is in the range of ±1 % were prepared by conventional solid state method and their densification, structure, dielectric and piezoelectric properties were investigated. Results showed that a small amount of excess A-site ions could compensate for the deficiency of K and Na ions in A-sites caused by volatilization resulting in good piezoelectric properties. The ceramics with x = −0.1 exhibited optimum piezoelectric properties with d 33 = 127pC/N and k p = 0.41. However, presence of too much alkali elements (x

Sr 2 Fe 1−0.5 x Mo 1−0.5 x Ir x O 6 ceramics ( x =0, 0.02, 0.04, 0.06, and 0.08) have been synthesized and the structures, magnetic and transport properties have been investigated. X-ray diffraction reveals a very low solution limit of x =0.06, which however, results in significantly decreased Fe/Mo ordering degree and thus decreased magnetization value. On the other hand, the Curie temperature ( T c ) rises from 390 K ( x =0) to 400 K ( x =0.02) firstly, and then decreases to 360 K and 340 K for the compositions with x =0.04 and 0.06, respectively. In addition, the resistivity and magnetoresistance drop drastically with increasing x . The composition with x =0 shows semiconductor transport behavior in the temperature range of 10–400 K, whereas the other compositions show a minimum resistivity around 120 K. These results are discussed in detail by considering the effects of Ir substitution.

Two kinds of plain fabrics, made of silver plating yarn, were tested on conductivity under weft extension to investigate the relationships between tensile property and resistance. The results are following: the resistances of two kinds of fabrics have a linear relationship with strain within a definite range; the strain sensitivity of the inelastic plain fabric is smaller than that of the elastic plain fabric; the changes of contact resistance of the overlapped yarns cause the inelastic plain fabric sensitive to strain; when two kinds of fabrics were stretched repeatedly to simulate heartbeat signals, their resistance remained stable at a value. So they satisfy stability requirement.

MgTiO3-modified (Na0.53K0.41Li0.06)Nb0.91Sb0.09O3 ceramics were synthesized using solid-state reaction method. The addition of MgTiO3 makes the structure of the ceramics changes from the coexistence of orthorhombic and tetragonal phases to a single tetragonal phase. The orthorhombic-tetragonal phase transition temperature T O-T decreases to below room temperature, while the Curie temperature increases with the addition of MgTiO3. As a result, the tetragonal phase region is expanded, and good thermal stability of electrical properties is obtained in MgTiO3 modified ceramics. The 0.5mol% MgTiO3 modified ceramics exhibit outstanding properties of d 33∼258pC/N, ∼1431, k p∼0.435, and P r∼18.1 μC/cm2, with improved thermal stability.

Gelcasting is widely used in ceramic material preparation. A disadvantage of this technology is inhomogeneous structure and inner stress caused by temperature gradient owing to the heating of slurry. Theoretical analysis and experiments prove that gelling process of ultrafine ZrO2 can be completed at room temperature. When premixed solution/initiator/catalyst ratio is 100/1/0.5 (by volume), the gelcasting ZrO2 body with homogeneous structure and bending strength of 50.09MPa is prepared.

Na0.5Bi4.5Ti4O15-based materials with A-site vacancy were synthesized using conventional solid state processing. The (Li,Ce) modification of Na0.5Bi4.5Ti4O15-based materials resulted in the obvious improvement of the piezoelectric activity and dielectric permittivity. The dielectric and piezoelectric properties of Na0.5Bi4.5Ti4O15-based ceramics exhibiting a very stable temperature behavior, together with its high TC ~641oC, excellent piezoelectric coefficient ~28pC/N and very low temperature coefficient of resonant frequency, making the (Li,Ce) modified Na0.5Bi4.5Ti4O15-based ceramics a promising candidate for high temperature applications.

The effect of K content on the properties of LiCe co-substituted (Na 1―x K x ) 0.5 Bi 4.5 Ti 4 O 15 -based ceramics was investigated. The Curie temperature (T C ) gradually decreases from 641 to 551 °C with increasing the K content. The piezoelectric coefficient d 33 , thickness coupling factor k t , and mechanical quality factor Q of (Na 0.5 Bi 0.5 ) 0.80 (Li 0.5 Ce 0.5 ) 0.10 [] 0.10 Bi 4 Ti 4 O 15 (x = 0.00) ceramic were found to be 28 pC/N, 25%, and 2693, respectively, together with the high T C (∼641 °C) and stable piezoelectric properties, demonstrating that the (LiCe) modified Na 0.5 Bi 4.5 Ti 4 O 15 -based material a promising candidate for high temperature applications.

The dielectric, piezoelectric, and electromechanical properties of the (NaCe)-substituted calcium bismuth niobate (CaBi 2 Nb 2 O 9 , CBN) were investigated. The piezoelectric activities of CBN ceramics were significantly enhanced by (NaCe) co-substitution. The dielectric and electromechanical properties of CBN ceramics at elevated temperature were studied in detail. Compared to the pure CBN ceramics, the substituted CBN ceramics possess a broadened peak. The nearly temperature independent dielectric and electromechanical behavior, coupled with ultrahigh Curie temperature ( T c ∼900 °C), make CBN ceramics promising for high-temperature applications.

(Na0.48–xK0.48–xLi0.04Srx)Nb0.89Ta0.05Sb0.06O3 (abbreviated as NKLSrx NST) lead-free ceramics are fabricated by conventional solid state reaction. The material with perovskite structure contains both tetragonal and orthorhombic phases at room temperature when x ≤ 0.02, and it changes into cubic phase gradually as x increases. A second phase KSr2Nb5O15 begins to appear at x = 0.03 and becomes obvious with increasing Sr content. Dielectric studies show that the Sr2+ modification lowers the Curie temperature and induces the diffuseness of cubic–tetragonal phase transition. The addition of Sr2+ improves the density and piezoelectric properties of NKLNST. The ceramic modified with 0.008 mol SrO shows the optimum piezoelectric properties (d33 = 278 pC/N, kp = 0.504, tan δ = 0.025). Degradation of kp is found after the first thermal cycling due to the tetragonal–orthorhombic phase transition near room temperature. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

(K0.5Na0.5)NbO3 (KNN) based lead free ceramics have been fabricated by a solid state reaction. In this work, LiSbO3 (LS) modified KNN based ceramics were sintered at atmospheric pressure and high density (>96% theoretical) was obtained. The detailed elastic, dielectric, piezoelectric and electromechanical properties were characterized by using the resonance technique combined with the ultrasonic method. The full set of material constants for the obtained polycrystalline ceramics were determined and compared to the pure hot pressed KNN counterpart. KNN-LS polycrystalline ceramic was found to have higher elastic compliance, dielectric permittivity and piezoelectric strain coefficients, but lower mechanical quality factor, when compared to pure KNN, exhibiting a “softening” behavior. However, a high coercive field (∼17 kV/cm) was found for the LS modified KNN material. The properties as a function of temperature were determined in the range of −50–250 ∘C, showing a polymorphic phase transition near room temperature, giving rise to improved piezoelectric behavior.

The nonlinear electrical properties of TiO2-based varistor doped with 0.25mol% Ta2O5 and different contents of Sc2O3 were investigated. It was found that the TiO2 varistor ceramic doped with 0.10mol% Sc2O3 exhibited an optimal nonlinear coefficient of 7.8, a breakdown electrical field of 16.0V/mm, and relative dielectric constant of 1.27 × 105 (measured at 1 kHz). In order to analyze the effect of Sc2O3 on TiO2 varistors, studies were made on the capacitance versus voltage characteristics. A Schottky-type barrier, which is assumed as the origin of varistor behavior, was inferred from the C-V measurement. The barrier height and donor concentration were obtained as 0.41eV and 1.21 × 1026cm-3, respectively, for sample doped with 0.10mol% Sc2O3. Analogized to the ZnO varistors, the formation mechanism of Schottky-type barrier was discussed in this paper by the theory of defect in crystal lattice.

The effects of Dy 2 O 3 on the microstructure and electrical properties of the SnO 2 based varistor system were investigated. It was found that Dy 2 O 3 significantly affects the grain size and electrical properties of the SnO 2 -based varistors. The sample with 1.00 mol% Dy 2 O 3 has the best nonlinear electrical property and the highest nonlinear coefficient ( α = 26.3) among all samples. By SEM, TEM and EDS images, it was found dysprosium segregates at the grain boundary, which is responsible for “ p -type characteristics” of the grain boundaries. The reason that grain size decreases with increasing Dy 2 O 3 concentration is given. The reason why the varistor sample has the excellent nonlinearity is explained.

Er3+–Yb3+ co-doped glasses and glass ceramics containing LaF3 nanocrystals were prepared and their absorption spectra obtained. The Judd–Ofelt parameters Ω t (t = 2, 4, 6) for f–f transition of Er3+, as well as spontaneous emission probabilities, branching ratios and radiative lifetimes for stimulated emission of each band were determined. Addition of Er3+ and Yb3+ ions into low-phonon energy LaF3 nanocrystals makes the upconversion emission of Er3+–Yb3+ co-doped glass ceramic much stronger than that of Er3+–Yb3+ co-doped glass.

The microstructure development of SnO2·CuO based ceramic material was analyzed by XRD and SEM and the electrical properties were investigated by J-E relation. The secondary phases of copper oxide were found by the XRD. Copper oxide could make tin oxide densify and advance the grain growth, while tantalum oxide would retard the grain growth. Excess copper would centralize at the grain boundaries and prevent the mass transport. The high nonlinear coefficient (α = 27.3) and low leakage current density (JL = 16 μA cm−2) for the 0.05 mol% Ta2O5-doped SnO2·CuO based varistor sample were obtained. The modified defect barrier model for CuO and Ta2O5-doped SnO2 based varistors was introduced.

The electrical properties of CaCu3Ti4O12 ceramic materials, showing an enormously large dielectric constant, were investigated. It was found that the grain boundary plays an important role in the giant dielectric behaviour of these ceramics. Measurement of the electrical current density (J) versus the electrical field (E) was carried out. A good linear relationship between lnJ and E1/2 was found, which demonstrates that the Schottky barrier should exist at the grain boundary. A double Schottky barrier model composed of a depletion layer and a negative charge sheet was proposed, analogous to the barrier model for ZnO varistors. An activation energy value of about 0.6 eV was obtained from the data of the characteristic frequency corresponding to the peak of the imaginary part of the dielectric permittivity versus temperature, which may be attributed to the activation of to in the depletion layer.

The varistor system SnO2·CuO· Ta2O5 with different Ta2O5 dopants sintered at 1300°C for 80 min was investigated. It is found that Ta2O5 significantly affects the grain size and the electrical properties. The average grain size decreases from 6.8 to 1.9 µm with an increase in Ta2O5 concentration from 0.05 to 1.00 mol%. The sample with 0.05 mol% Ta2O5 has the best nonlinear electrical property and the highest nonlinear coefficient (α = 37.9) and the sample with 0.50 mol% Ta2O5 possesses the highest densification (98.7% of the theoretical density) among all the samples. The reason for the nonlinearity of SnO2 ceramics is explained.

The effect of Co2O3 on the microstructure and electrical properties of Ta-doped SnO2 varistors was investigated. It was found that a sample doped with 0.1 mol% Co2O3 had the highest nonlinear coefficient α = 33, the highest breakdown electrical field EB = 872 V mm−1 and the lowest relative dielectrical constant er = 598 (measured at 1 kHz). However, 0.1 mol% Co2O3 is not sufficient for densification of SnO2 ceramics, and the relative density of the sample doped with 0.1 mol% Co2O3 (85.8%) is much lower than that of the samples doped with 0.3, 0.5, 0.8 and 1.2 mol% Co2O3 (about 98%). The highest breakdown electrical field and lowest relative dielectric constant of the sample doped with 0.1 mol% Co2O3 are mainly the result of the loose microstructure and the smallest average grain size. The measurements of grain boundary barrier height, ΦB, and grain boundary resistance, RGB, indicate that CoSn× should be located at the depletion layer and is important to the formation of the grain boundary barrier.

The varistor properties of Sc 2 O 3 -doped SnO 2 ·Co 2 O 3 ·Nb 2 O 5 ceramics were investigated. It was found that the non-linear coefficient presents a peak of 17.1 at the concentration of 0.06 mol% Sc 2 O 3 , the average grain size decreases from 10 to 5 μm, the breakdown electrical field increases from 199 to 1790 V/mm and relative electrical permittivity decreases from 2600 to 60 as Sc 2 O 3 concentration was increased up to 0.09 mol%. The increase of the breakdown electrical field with increasing Sc 2 O 3 concentration is mainly attributed to the decrease of the average grain size. The reason why the permittivity decreases with increasing Sc 2 O 3 concentration was originated from the ratio of the grain size to the barrier width. To illustrate the grain-boundary barrier formation of (Sc, Co, Nb)-doped SnO 2 varistors, a modified defect barrier model was introduced, in which the negatively charged acceptors substituting for Sn ions should be located at SnO 2 lattice sites in the depletion layers, instead at the grain interfaces.

The nonlinear electrical behavior and dielectric properties of TiO 2 -based ceramics with various WO 3 contents have been investigated. It was found that the 0.25% WO 3 +99.75% TiO 2 has an optimal nonlinear coefficient α of 9.6, a breakdown electrical field of 44.5 V mm −1 , and an ultrahigh relative dielectric constant of 7.41×10 4 (measured at 1 kHz). The theory of defects in the crystal lattice was introduced to explain the nonlinear electrical behavior of the TiO 2 ·WO 3 system. A grain boundary barrier model that is analogous to the grain boundary defect model for ZnO varistors are introduced in the article. From this point, the nonlinear electrical behavior of the TiO 2 system is explained.

The nonlinear electrical behavior and dielectric properties of TiO2-based ceramics with various WO3 contents have been investigated. It was found that 0.25% WO3+99.75% TiO2 has an optimal nonlinear coefficient of α=9.6, a breakdown electrical field of 44.5 V/mm, and ultrahigh relative dielectric constant of 7.41×104 (measured at 1 kHz). The theory of defects in the crystal lattice was introduced to explain the nonlinear electrical behavior of the TiO2⋅WO3 system. Schottky potential barriers at the grain boundaries that are analogies to the grain boundary defect model for ZnO varistors are introduced in the article. From this point, the nonlinear electrical behavior of the TiO2 system is explained.

Via optimizing the experimental strategy, a series of Sr2Mn(1+x)W(1−x)O6 (x = 0,1/7, 1/5, 1/3, 4/11, 13/27 and 3/5) double perovskites were simply synthesized in either 1%H2/99%Ar or Ar streams. The x-ray diffraction patterns indicate that, with the increasing Mn-content (x), a gradual phase transition from monocline (P21 /n) to tetragonal (I4/m) occurs. Comparative investigations, based on x-ray photoelectron spectroscopy and titrimetric analysis, confirm that the chemical states of the Mn cation are extremely sensitive to x values, and can present +2, +2/+3, +3 and +3/+4 chemical states in different compositions, but this is not the case for the W cation, which shows a stable +6 state in all compositions. The induced magnetic cations pairs Mn2+/Mn3+or Mn3+/Mn4+, along with the antiferromagnetic (AFM) order affect the magnetic behaviours significantly, giving rise to the change of dominant AFM interactions to considerable ferromagnetic (FM) interactions. Maybe our results not only provide an effective way for preparing Sr2MnWO6-based double perovskites, but also may be helpful for further development of Mn-based magnetic oxides.

The effect of Sb2O3 on the electrical and physical properties of SnO2-based ceramics were investigated by measuring the densities, permittivities, the properties of V–I and boundary barriers. SnO2·Co2O3 ceramics cannot exhibit electrical nonlinearity. A little amount of Sb2O3 can improve the nonlinear properties of the samples greatly. The height and width of the defect barriers were calculated. It was found that the sample doped with 0.01 mol% Sb2O3 exhibits the highest density (ρ=6.90 g/cm3), the highest reference electrical field and the best electrical nonlinearity (α=12.9), which is consistent to its highest and narrowest defect barriers. The effect of Sb2O3 dopants can be explained by the substitution and segregation of antimony ions. The effect of cooling rate was also investigated. The samples cooled at 2°C/min exhibit better nonlinear electrical properties. A grain-boundary defect barrier model of SnO2·Co2O3·Sb2O3 varistor was also introduced.

The nonlinear electrical properties of TiO2–Y2O3–Nb2O5 ceramics were investigated as a new varistor material. It was found that an optimal doping composition of 99.75%TiO2–0.60%Y2O5–0.10% Nb2O5 was obtained with low breakdown voltage of 8.8 V mm−1, high nonlinear constant of 7.0 and ultrahigh relative dielectric constant of 7.6×104, which is consistent with the highest and narrowest grain boundary barriers in the composition. Samples doped with 0.10 mol.% Nb2O5 exhibit the highest permittivitty and resistivity at low frequencies and comparatively lower values at high frequencies in comparison with other samples studied. In view of these electrical characteristics, the ceramics of 99.75%TiO2–0.60%Y2O3–0.10%Nb2O5 is a viable candidate for capacitor-varistor functional devices. The performance of the ceramics as a function of Nb-doping depends primarily on the extent of substitution of Ti4+ with Nb5+. In order to illustrate the role of grain boundary barriers for high Nb-doping co-concentrations in TiO2–Y2O3–Nb2O5 varistors, a grain-boundary defect barrier model was introduced.

The reason that the (Ni, Nb)-doped SnO2 varistors exhibit poorer densification and electrical nonlinearity than the (Co, Nb)-doped SnO2 varistors is explained. The effect of Mn2+ on the electrical nonlinear properties of SnO2 based ceramics were investigated. The sample doped with 0.10 mol% MnCO3 exhibits the highest reference electrical field of 686.89 V/mm, the highest electrical nonlinear coefficient of 12.9, which is consistent with the highest grain-boundary defect barriers. It can be explained by the effect of the substitution of Sn4+ for Mn2+, which facilitate the formation of the defect barriers, and the maximum of the substitution. The shrinkage rates increase with the doping of MnCO3, although the sample doped with 0.5 mol% MnCO3 appears the highest density (ρ=6.87 g/cm3). In order to illustrate the grain boundary barriers formation in SnO2.Ni2O3.Nb2O5.MnCO3 varistors, a grain-boundary defect barrier model was introduced.

The piezoelectric, dielectric, and electromechanical properties of the (KCe) co-substituted calcium bismuth niobate (CaBi2Nb2O9, CBN) were investigated. The piezoelectric activities of CBN ceramics were significantly enhanced and the dielectric loss tan δ decreased by (KCe) substitution. The Ca0.9(KCe)0.05Bi2Nb2O9 ceramics possess the optimal piezoelectric properties, and the piezoelectric coefficient (d33), Curie temperature (TC), and electromechanical coupling factors (kp and kt) were found to be 16 pC/N, 868 °C, 8.6%, and 23.8%, respectively. The excellent dielectric and electromechanical spectra, together with the high piezoelectric activities and ultrahigh Curie temperature, make CBN ceramics promising candidates for high temperature piezoelectric applications. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The piezoelectric properties of the cobalt-modified sodium bismuth titanate (Na0.5Bi4.5Ti4O15, NBT) piezoelectric ceramics were investigated. The piezoelectric properties of NBT ceramics were significantly enhanced by cobalt modification. The Curie temperature TC and piezoelectric constant d33 for the 0.3 wt% cobalt-modified NBT ceramics (NBT-C3) were found to be 663 °C and 30 pC/N, respectively. Thermal annealing studies presented that the cobalt-modified NBT ceramics possess stable piezoelectric properties, demonstrating that the cobalt-modified NBT-based ceramics are promising candidates for high temperature piezoelectric applications. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report that the Fe-rich single-phase Sr2Fe1+x Mo1−x O6(x = 1/9, 1/6 and 1/3) ceramics can be synthesized in a N2 stream by the solid-state reaction method, while the usual Sr2FeMo2O6 (i.e. x = 0) can only be synthesized in a H2+Ar stream. X-ray diffraction shows that all ceramics are well crystallized with tetragonal symmetry. X-ray photoelectron spectroscopy results confirm that the chemical states of interior Fe and Mo cations are significantly more complex than those of the surface cations. With the increase in x, the materials change from ferrimagnetic (x = 0, 1/9, 1/6) to spin-glass-like behaviour (x = 1/3) and the magnetism decreases dramatically. The Curie temperature (T c) and the magnetoresistance (MR) at 10 K with a field of 2 T (T c, MR) are (400 K, −20.9%), (390 K, −10.3%) and (385 K, −4.5%) for x = 0, 1/9 and x = 1/6, respectively. The results are further analysed by Monte Carlo simulations. Our results show that high-T c ferrimagnetic double perovskites with good magnetic and transport properties can be obtained conveniently in a N2 stream instead of conventional H2+Ar, which not only provides a safe and low-cost way to synthesize these materials, but may also be helpful in decreasing the oxygen vacancy concentration.

The strong temperature dependence of dielectric and piezoelectric properties is a big 'bottleneck' for practical applications of KNN-based piezoceramics. In order to resolve this problem, Ca0.5Sr0.5TiO3 (CST) is doped into (Na0.53K0.407Li0.063)Nb0.937Sb0.063O3 (NKLNS) ceramics. The thermal stability of sE11 and k31 in the temperature range from −50°C to 200°C for ceramics with 1.0 and 1.5 mol% CST is raised in comparison with ceramics without CST. Ceramics with 1.5 mol% CST exhibit high piezoelectric properties (d33 = 202 pC/N, kp = 44%) and low dielectric loss (2%) at room temperature. The excellent piezoelectric properties, which are comparable to conventional PZT ceramics, indicate that these ceramics are promising candidates for lead-free piezoelectric applications.

The Aurivillius-type bismuth layer-structured (NaBi)0.46(LiCe)0.04Bi4Ti4O15 (NBT-LiCe) piezoelectric ceramics were synthesized using conventional solid-state processing. Phase analysis was performed by x-ray diffraction and microstructural morphology was assessed by scanning electron microscopy. The dielectric, piezoelectric, ferroelectric, and electromechanical properties of NBT-LiCe ceramics were investigated. The piezoelectric activities were found to be significantly enhanced compared to NBT ceramics, which can be attributed to the lattice distortion and the presence of bismuth vacancies. The dielectric and electromechanical properties of NBT-LiCe ceramics at elevated temperature were investigated in detail. The excellent piezoelectric, dielectric, and electromechanical properties, coupled with high Curie temperature (Tc=660 °C), demonstrated that the NBT-LiCe ceramics are the promising candidates for high temperature applications.

The effect of (Sc,Ta,Ce) doping on the properties of Bi3TiNbO9 (BTNO)-based ceramics was investigated. The cerium modification greatly improves the piezoelectric activity of Bi3(Ti0.96Sc0.02Ta0.02)NbO9-based ceramics and significantly decreases the dielectric dissipation. The d33 of Bi3Ti0.96Sc0.02Ta0.02NbO9+x wt %CeO2 (x=0.35) was found to be 18 pC/N, the highest value among the BTNO-based ceramics and almost three times as much as the reported d33 values of the pure BTNO ceramics (∼6 pC/N). The modification increased the resistivity ρ of the samples extremely, resolving the low resistivity problem for high temperature applications. The dielectric spectroscopy shows that the TC for all the ceramics is higher than 900 °C. The mechanical quality factor Q and planar coupling factors kp and kt of Bi3Ti0.96Sc0.02Ta0.02NbO9+0.35 wt %CeO2 ceramic were found to be 2835, 9%, and 23%, respectively, and it has high TC and stable piezoelectric properties, demonstrating that the (Sc,Ta,Ce) modified BTNO-based material is a wonderful candidate for high temperature applications.

Lead-free alkaline niobate based (Na0.52K0.48−xLix)Nb1−x−ySbxTayO3 piezoceramics have been prepared by the conventional mixed oxide method without using other techniques. An experimental formula for producing a set of ceramics with high piezoelectric properties is obtained while cutting down the Ta content and maintaining a high Curie temperature. The highest piezoelectric constant d33 is 308pC∕N, with a dielectric loss tanδ of about 2.0% and a Curie temperature of 339°C. The samples also possess outstanding high-field piezoelectric strain effects. The high-field piezoelectric strain coefficient d33* is as high as 490pm∕V. (Li, Sb, Ta) modified (Na,K)NbO3 shifts the orthorhombic to tetragonal phase transition to near room temperature, which plays an important role in the improvement of the piezoelectric properties.

The effect of (Li,Ce) substitution for A site on the properties of Na0.25K0.25Bi2.5Nb2O9-based ceramics was investigated. The piezoelectric activity of Na0.25K0.25Bi2.5Nb2O9-based ceramics is significantly improved by the modification of lithium and cerium. The Curie temperature (TC) gradually increases from 668to684°C with increasing the (Li,Ce) modification. The piezoelectric coefficient d33 of the [(Na0.5K0.5)Bi]0.44(LiCe)0.03[ ]0.03Bi2Nb2O9 ceramic was found to be 28pC∕N, the highest value among the Na0.25K0.25Bi2.5Nb2O9-based ceramics and also almost 50% higher than the reported d33 values of other bismuth layer-structured ferroelectric systems (∼5–19pC∕N). The planar coupling factors kp and kt were found to be 8.0% and 23.0%, together with the high TC (∼670°C) and stable piezoelectric properties, demonstrating that the (Li,Ce) modified Na0.25K0.25Bi2.5Nb2O9-based material a promising candidate for high temperature applications.

Lead-free piezoelectric ceramics, with the nominal composition of 0.948(K0.5Na0.5)NbO3–0.052LiSbO3 (KNN-LS5.2), were synthesized by conventional solid-state sintering, and the piezoelectric and electromechanical properties were characterized as a function of temperature. The Curie temperature of the KNN based perovskite material was found to be 368°C with an orthorhombic-tetragonal polymorphic phase transition (TO-T) temperature at approximately ∼35°C. The room temperature dielectric permittivity (e33T∕e0) and loss were found to be 1380 and 2%, respectively, with piezoelectric properties of k33∼62% and d33∼265pC∕N and k31∼30% and d31∼−116pC∕N. The temperature dependence of the properties mimicked the compositional variation seen in the proximity of a morphotropic phase boundary [e.g., lead zirconate titanate (PZT)], with a maxima in the dielectric and piezoelectric properties and a corresponding “softening” of the elastic properties. Unlike that found for PZT-type materials, the modified KNN material exhi...

The effect of (LiCe) substitution for A site on the properties of (NaBi)0.48◻0.04Bi2Nb2O9 (NB◻N)-based ceramics was investigated. The coercive fields (EC) of NB◻N)-based ceramics were significantly decreased from 61.0to32.5kV∕cm and the Curie temperature (TC) gradually decreases from 820to803°C with increasing the (LiCe) modification. The piezoelectric coefficient d33, planar coupling factor kp, and mechanical quality factor Q of (NaBi)0.38(LiCe)0.05◻0.14Bi2Nb2O9 ceramic were found to be 27pC∕N, 11.2%, and 2600, respectively, together with the high TC (∼809°C) and stable piezoelectric properties, demonstrating that the (LiCe) modified NB◻N-based material a promising candidate for high temperature applications.