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High density Ba4Sm2Fe2Nb8O30 (BSFN) multiferroics ceramics with tetragonal tungsten bronze structure had been prepared by microwave sintering (MS) for 30min and conventional sintering (CS) methods for 4 h at 1275 °C. Single tungsten bronze phase and equiaxial grains are obtained for the MS BSFN ceramics, while a small amount secondary phase of SmNbO4 is observed in the CS BSFN ceramics with columnar grains. Compared to Ba4Sm2Fe2Nb8O30 ceramics prepared by CS method, enhanced dielectric, ferroelectric and magnetic properties are achieved for the MS BSFN ceramics. The values of electric polarization Pr and coercive electric field Ec are 2.11 μC/cm2 and 7.14 kV/cm for the MS BSFN ceramics, respectively. Meantime, the magnetic polarization Mr of 0.410emu/g and coercive magnetic field Hc of 2930Oe are also obtained for the MS BSFN ceramics. Based on the density, crystal structure, point defect and grain, the reasons of enhanced dielectric, ferroelectric and magnetic properties are discussed for the MS BSFN ceramics It is indicated that Ba4Sm2Fe2Nb8O30 is an intrinsic room temperature multiferroic materials.

A novel photochromic (PC) ferroelectric material, Nd3+-doped K0.5Na0.5NbO3 translucent ceramic, was fabricated by a conventional solid method. The ceramic shows moderate optical transparency due to its dense microstructure, as well as good photoluminescence (PL) properties of up-conversion (UC) emission in the visible region and down-shifting (DS) emission in the near-infrared region. Simultaneously, the colors of all the ceramics turn darker after 365 nm illumination, and then recover to their initial states upon thermal stimulus (210 °C, 10 min), exhibiting a typical PC phenomenon. In particular, the effect of the PC behavior on PL lifetimes of Nd3+ was investigated. Multi-mode reversible modulations of various optical behaviors were realized based on the PC behavior, especially for the regulation of near-infrared emission. And the maximum modulation ratios of transmittances, UC or DS PL intensities and luminescence lifetimes are 43.0%, 66.7%, 66.4% and 12.8%, respectively. Owing to high-temperature sintering and Nd3+ substitution in the ceramics, cation and oxygen vacancies appear to produce F′ and V1 color centers, which are in charge of the PC behavior and corresponding modulations. These results suggest that the ceramics are a promising PC material in visible and near-infrared optical data storage, photo-switches, and optoelectronic devices. Meanwhile, they also provide insights into the applications of rare-earth Nd3+ in other inorganic PC ferroelectric materials.

Er3+-doped xK0.5Na0.5NbO3-(1-x)Ba0.85Ca0.15Ti0.9Zr0.1O3 (x = 0.04, 0.05, 0.06 and 0.07) translucent ceramics have been first prepared by conventional pressureless sintering. Owing to the effective suppression of grain growth induced by both Er3+ and K0.5Na0.5NbO3, the ceramics possess dense and fine-grained microstructure. The XRD patterns demonstrate cubic-like phase structure with minimal optical anisotropy. The ceramics perform good photochormic (PC) properties under 407-nm light irradiation and superior reproducibility after heat treatment (200 °C, 5 min). And PC reaction is responsible for coloration and decoloration processes. The up-conversion photoluminescence before and after irradiation have been studied. With increasing the K0.5Na0.5NbO3 content or elevating the sintering temperature of the ceramics, the luminescent quenching degree (ΔRt) gradually increases. The color centers induced by irradiation can be reversibly created or eliminated, resulting in invertible luminescent modulations. This work can guide other multifunctional PC transparent ferroelectric materials for optoelectronic applications.

In this paper, cold sintering was served as a forming method to assist the conventional sintering, which is so-called cold sintering assisted sintering (CSAS) method. Lead-free K0.5Na0.5NbO3 piezoelectric ceramics were prepared by the CSAS method, and the effects of the different procedures on the sintering behaviors and electrical properties of KNN ceramics were studied. Compared with conventional sintering (CS), cold sintering process can induce potassium-rich phase on the KNN particle surface, and remarkably increase both the green and sintering density of KNN ceramics. Meanwhile, the potassium-rich phase would transform to K4Nb6O17 second phase on the grain surface, and subsequently suppress the volatilization of potassium element. The sinterability and electrical properties were greatly improved, and KNN piezoelectric ceramics with high performance can be manufactured in a wide sintering temperature range (1055 °C–1145 °C), which proves that CSAS has the potential to be an excellent sintering technique for producing KNN based ceramics.

The lead-free Er3+-doped (K0.48Na0.48Li0.04)(Nb0.96Bi0.04)O3 (KNLNB-Er-x) ceramics were fabricated by conventional pressureless sintering. They possess a tetragonal perovskite phase with dense microstructure. High transmittances of the ceramics are obtained both in the visible and infrared regions. The optical band gap energies of them are 3.07–3.11 eV, close to other KNN-based materials. The relaxor-like characteristics, good dielectric, ferroelectric and piezoelectric properties of the ceramics have been obtained. The up-conversion photoluminescence spectra have been studied and obvious color-tunable emissions have been observed by modulating temperature. The fluorescence intensity ratio (FIR) value based on green emissions at 533 and 555 nm in the temperature ranging from 300 to 750 K have been investigated, giving the maximum sensitivity of ~ 0.0038 K−1. Furthermore, the FIR technique opens up a method to detect the Curie temperature of the ceramics. Owing to both optical and electrical properties, the KNLNB-Er-x transparent ceramics could be a promising candidate in the application of color-tunable solid-state lightings, optical temperature sensors, and electrical-optical coupling devices.

Effect of 35BaO–35ZnO–30B2O3 (BZB) addition on the sintering behaviors, phase evolution and microwave dielectric properties of Ti-modified Ba3Ti5Nb6O28 (Ba3Ti5.1Nb5.9O27.95, BTNO) ceramics had been investigated. BZB addition effectively reduced the sintering temperature of BTNO from 1250 °C to about 900 °C. With increasing BZB addition, the crystal phase of the present ceramics changed from single phase Ba3Ti5Nb6O28 to mixing phases of Ba3Ti5Nb6O28 and Ba3Ti4Nb4O21. And the major phase gradually became from Ba3Ti5Nb6O28 to Ba3Ti4Nb4O21. Dielectric constant and temperature coefficient increased with the rising of BZB addition. But the Qf value gradually declined from 28000 to about 6000 GHz. The BTNO ceramics with 15% BZB addition exhibited excellent microwave dielectric properties as following: e = 46.3, Qf = 5887 GHz and τf = 35.5 ppm/°C, which was potential of candidate materials for LTCC application. And the variation of microwave dielectric properties had been also discussed with the phase and microstructure evolution.

2.5 mol% Ni2+-modified Bi0.5Na0.5TiO3 (BNT–Ni-0.025) ceramic has been fabricated based on the sol–gel method. The BNT–Ni-0.025 ceramic possesses a rhombohedral perovskite structure with denser microstructure than that of pure BNT ceramic. The introduction of certain amount of Ni2+ (e.g. 2.5 mol%) in BNT can reduce the sintering temperature (50 °C lower), effectively suppress the grain growth, greatly decrease the dielectric loss and then produce multifunctionality. The BNT–Ni-0.025 ceramic also exhibits good ferroelectric properties with a large remanent polarization of ~26 μC/cm2. The observed weak ferromagnetism of the ceramic can be ascribed to the exchange interaction between free carriers (oxygen vacancy induced delocalized electrons) and the localized Ni2+ ions. Under the excitation of 370 nm, the BNT–Ni-0.025 ceramic shows broadband NIR luminescence centered at ~1480 nm from Ni2+ with long emission lifetime (28 µs), which can cover the optical communication window and be applied in broadband optical amplifiers. The multifunctional BNT–Ni-0.025 ceramic shows potentials in the application of optical–electrical–magnetic areas.

Effects of ZnO–B2O3 (ZB) addition on the densification, phase evolution and microwaves dielectric properties of Ba4Sm9.33Ti18O54 (BST) ceramics for low-temperature fired applications have been investigated. The sintering temperature of BST ceramics can be effectively lowered to about 1000°C with introduction of ZB. Tungsten bronze like single phase is observed in the BST ceramics with 0.5 and 1.0% ZB. However, Sm2Ti2O7 secondary phase appears when ZB addition reaches 2%, and Sm2Ti2O7 phase gradually increases with the increase ZB addition. Microwave dielectric properties of the present ceramics are strongly dependent on phase constitution and density. Optimal microwave dielectric properties of e = 63.4, Qf = 2830 GHz, τf =–8.8 ppm/°C is obtained for BST ceramics with 1% ZB addition.

Structure and dielectric properties have been investigated for CaCu3Ti4O12 (CCTO) ceramics prepared by rapid sintering with relatively high sintering temperature and very short holding time. A very small amount of CuO is observed in XRD patterns of CCTO ceramics. Surface SEM of CCTO ceramics exhibits large grain and intergranular phase. Complex impedance spectra indicate that the internal barrier layer capacitor (IBLC) microstructure has been formed in the CCTO ceramics. And a giant dielectric permittivity of 8.5 × 103 at 1 kHz is achieved for the CCTO ceramics sintered at 1130 °C for 30 min. The rapid sintering method may be effective in applying these ceramics, which are expected to facilitate IBLC microstructure.

Effects of BiVO4 addition on the sintering behavior, crystal structure, microstructure and microwave dielectric properties of CaLa4Ti4O15 ceramics were investigated. When BiVO4 addition is below 3 %, the sintering temperature of CaLa4Ti4O15 ceramics is reduced from 1,550 °C to about 1,400 °C. And only phase CaLa4Ti4O15 is observed in the CaLa4Ti4O15 ceramics. However, the secondary phase CaLa4Ti5O17 appears when BiVO4 addition reached 3 %. It is important that a small amount of BiVO4 addition can not only lower the sintering temperature but also improve the microwave dielectric properties of CaLa4Ti4O15 ceramics. The CaLa4Ti4O15 ceramics with 1.0 % BiVO4 addition, which is sintered at 1,425 °C, exhibits excellent microwave dielectric properties of e = 44.3, Qf = 51,377 GHz, τ f = −9.0 ppm/°C. The variation of microwave dielectric properties had been also discussed with the structure evolution.

The effects of Ti-doping on the decomposition behavior, crystal structure, sintering behavior and dielectric properties have been investigated for the Ti-doped BiFeO3 ceramics derived from molten salt method. XRD reveals the almost pure phase BiFeO3 is synthesized in the Ti-doped BiFeO3 powders. And the particle size of Ti-doped BiFeO3 powers obviously decreases with the increase of Ti content. However, the sintering temperature elevates significantly after Ti-doping. The DC resistivity can enhance by up to four orders of magnitude (from 104 to 108 Ωm) with only 5 at% Ti doping. But the dielectric constant is suppressed from 104 to 102, and dielectric loss obviously reduces with a small amount of Ti doping. The variation of dielectric properties has been discussed from the decomposition of BiFeO3 phase. The Ti-doping can effectively suppress the decomposition reaction in Ti-doped BiFeO3 ceramics.

BiFeO3 powder was synthesized in NaCl media at temperature range from 700 to 800 °C, using Bi2O3 and Fe2O3 as raw materials. Effects of calcining temperature and salt ratios on the synthesis of BiFeO3 powder had been investigated. It was found that NaCl effectively promoted the formation of BiFeO3. Almost pure BiFeO3 phase with a very small amount of Bi2Fe4O9 phase was synthesized at 750 °C with salt weight ratios of 1:1. A large amount of BiFeO3 phase decomposed to Bi2Fe4O9 and Bi25FeO39 phase when the temperature was up to 800 °C. In the present method, the calcining temperature played an important role in the formation of BiFeO3 phase. BiFeO3 ceramics derived from molten salt method were prepared and exhibited the higher dielectric constant.

CaO–SrO–Li2O–Ln2O3–TiO2 ceramics were prepared by solid state reaction method, where Ln2O3 consists of equal proportions of Nd2O3 and Sm2O3. Dielectric properties and crystal structure were investigated with respect to the content of TiO2. Single phase with an orthorhombic perovskites structure was formed within the composition range of investigation. The frequency dependence of dielectric properties of the present ceramics was extensively investigated. Dielectric constant was less sensitive to frequency. However, dielectric loss and temperature coefficients were both very sensitive to frequency and gradually decreased with increasing frequency, such as the variation was more than ten times between 1 MHz and several GHz. The relationship between the temperature coefficient and dielectric loss was also discussed at different frequencies. And the mechanism of the frequency dependence was discussed in term of the role of Li ions.

Titanium coating was deposited on carbon fibre reinforced silicon carbide matrix (C/SiC) composite by an inexpensive molten salt reaction method. The microstructure and composition of the coating were investigated through optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. The effect of titanium coating as a transition layer in protective systems on the oxidation of C/SiC composite was studied. The results show that titanium coating can reduce the open porosity of C/SiC composite, and prevent carbon diffusion through the coating. The outer MgO-Al2O3–SiO2 (MAS) glass ceramics layer is dense and crack free owing to the titanium coating. Titanium coating alone or MAS glass–ceramics coating do not protect the composite from oxidation effectively. The weight loss of the composite coated with Ti/MAS multilayers was 0˙0039 g cm–2 after 12 h of thermal cycling in air.

Due to the continuous demand for miniaturization of electronics, there is an interest in developing dielectric materials with higher permittivity. Recently, the CaCu3Ti4O12 (CCTO) has been reported [1–3], has a very high permittivity (~10) at room temperature and very small temperature dependence in a broad temperature range. However, it appears that the dielectric properties of CCTO are very sensitive to processing. [4–7] Permittivities from 478 to 300,000 have been obtained for CCTO prepared by different routes. The original work, done by Subramanian et al. [1] and Ramirez et al. [2] using conventional mixed-oxide processing via mortar and pestle, showed that room temperature permittivities ranged from 10,000 to 20,000. Other researchers using the same technique have reported permittivities of 478, 2400 and 18,700 [8–10]. The highest permittivity reported as derived from impedance spectroscopy measurements was 300,000 for CCTO made from powder that was mixed via ball milling and sintered for 24 h at 1100 C [11]. Though different hypotheses have been put forth to explain the high permittivity of CCTO, its origin is still not fully justified. At present, the grain boundary barrier layer capacitance model of extrinsic mechanism is the most favorable one [12–19]. It is believed that insulating surfaces form on semi-conducting grains during the sintering of CCTO ceramics, which results in an electronically heterogeneous material very similar to the nature of internal barrier layers capacitors (IBLC). The dielectric properties of these IBLCS are very sensitive to different processing parameters. In the present work, the CCTO ceramics are prepared by the solid-state reaction method. The sintering effects on the phase evolution, structure and dielectric properties of CCTO ceramics have been investigated.

The dielectric behavior and the features of the diffused phase transition of the SrpSm6−pTi8−pNb2+pO30(p=4,5) ceramics were investigated together with their structure. Both Sr5SmTi3Nb7O30 and Sr4Sm2Ti4Nb6O30 had tungsten bronze structure and weak ferroelectric nature. Though Sr5SmTi3Nb7O30 showed the diffused ferroelectrics phase transition, it was concluded that the transition was mainly the displacive ferroelectric phase transition. In Sr4Sm2Ti4Nb6O30, the Sr2+ ions and Sm3+ ions did not form an order structure and it showed a typical displacive ferroelectric phase transition. The dielectric anomaly was attributed to the different BO6 octahedral distortion.

Modification of Ba5NdTi3Ta7O30 dielectric ceramics was investigated through introducing Bi4Ti3O12. With increasing of Bi4Ti3O12 content, the dielectric constant increased, and the temperature coefficient of the dielectric constant changed from negative to positive. The small temperature coefficient (τe < 50 ppm/°C) combined with high dielectric constant (e = 178) and low dielectric loss (tan δ = 0.007 at 1 MHz) was achieved in the composition x = 0.6.

Co-modification of Ba5NdTi3Ta7O30 dielectrics ceramics was investigated through Pb substitution for Ba and introducing Bi4Ti3O12 secondary phase. The dielectric constant increased from 150 to 283, the temperature coefficient of the dielectric constant decreased from −2500 ppm/°C to −1279 ppm/°C, and the dielectric loss decreased to 0.0007 at 1 MHz. Meanwhile, the bi-phase ceramics were investigated to achieve temperature stable ceramics with high dielectric constant and low dielectric loss. As the composition x varied from 0.4 to 0.7 for (1 − x)(Ba0.8Pb0.2)5NdTi3Ta7O30/xBi4Ti3O12, the temperature coefficient of the dielectric constant changed from negative to zero to positive.