•(Cr1/2Nb1/2)4+ substitution for Ti4+ would low down the τf of the samples.•The Cr3+ substitution for Ti4+ would promote the quality factor.•(Cr1/2Nb1/2)4+ substitution for Ti4+ makes the permittivity maintain a high value.•The high Q × f value and high permittivity were the advantages of the samples.The microwave dielectric properties and the morphology of Ba6−3xNd8+2xTi18−y(Cr1/2Nb1/2)yO54 (x = 0.75, 0 ≤ y ≤ 3.0) ceramics prepared under different sintering conditions were investigated in this work. The effects of substitutions on the microstructure and microwave dielectric properties were discussed. The X-ray diffraction (XRD) patterns of the sintered samples revealed a single-phase formation BaNd2Ti4O12 with a tungsten bronze type structure in the system. The results of energy dispersive spectrometer (EDS) and lattice parameters calculated on XRD data could confirm the substitution at B-site. A small amount of substitutions improved quality factor value (Q × f) and the temperature coefficient of resonant frequency (τf) but led to a decrease of the permittivity. The temperature coefficient of resonant frequency (τf) was found to decrease with increasing substitutions because of the declination of tolerance factor (t). And the τf could be adjusted from +62.4  ppm/°C to −7.3  ppm/°C with increment of substitutions. Finally, excellent dielectric properties were obtained as y was 0.5 sintered at 1400 °C for 2 h in air: εr = 88.6, Q × f = 11486 GHz, τf = +37.1  ppm/°C.It was evident that the tendency of the τf was consistent with the variation of tolerance factor (t). The τf was much affected by the titling of Ti–O octahedral that bigger ionic radius of (Cr1/2Nb1/2)4+ substituted for Ti4+ would significantly reduce the temperature coefficient of resonant frequency.

•The (1−x) MgTiO3–xMg2.05SiO4.05–0.06CaTiO3 composite ceramics were prepared.•The results indicated that exceeding Mg in Mg2.05SiO4.05 not only hold back the formation of MgSiO3 but also inhibits the formation of second phase MgTi2O5.•A new microwave dielectric material has optimal dielectric properties.The (1−x) MgTiO3−xMg2.05SiO4.05–0.06CaTiO3 ceramic system composite dielectric ceramics with different amounts of Mg2.05SiO4.05 addition were prepared by solid state reaction method. The results indicated that exceeding Mg in Mg2.05SiO4.05 not only hold back the formation of MgSiO3 but also inhibits the formation of second phase MgTi2O5. A new microwave dielectric material, 0.8MgTiO3–0.2Mg2.05SiO4.05–0.06CaTiO3 ceramics sintered at 1380 °C for 4 h had optimal dielectric properties (εr=15.4, Q×f=72,705 GHz and τf=−1.45 ppm/°C) which satisfied microwave applications in resonators, filters and antenna substrates.

Low-loss ceramics (Zn0.3Co0.7)Ti1−xSnxO3 (ZCTS) (x = 0, 0.02, 0.05, 0.09, 0.14) were prepared by the conventional solid-state route. The effects of Sn ratio on phase composition, microstructure, and the microwave dielectric properties of (Zn0.3Co0.7)Ti1−xSnxO3 materials were investigated using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDS). The results revealed that the ZCTS (x = 0.02, 0.05, 0.09, 0.14) composites consisted of three crystalline phases: ZnTiO3-type phase, Zn2TiO4-type phase, and TiO2 phase. Due to the compensating effect of rutile TiO2, the temperature coefficient of resonant frequency (τf) for (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0.02) ceramic was tuned to near zero value. It was found in our experiment that with the increase of Sn ratio, the microwave dielectrics showed a great promotion in Q × f when Ti4+ was partially substituted by Sn4+. Typically, the (Zn0.3Co0.7)Ti1−xSnxO3 (x = 0.02) ceramic sintered at 1,220 °C for 4 h exhibited good microwave dielectric properties of εr = 24, Q × f = 66,700 GHz and τf = −5.43 ppm/°C.

Microwave dielectric ceramics in the Ba0.2Sr0.8La4Ti4+xO15 composition series were prepared through a solid state mixed oxide route. The stoichiometric Ba0.2Sr0.8La4Ti4O15 ceramics can be well densified in the temperature of 1,450 °C with a high compact microstructure. The phase composition was found to be sensitive to nonstoichiometry of TiO2 in this system because the major crystal phase of excess TiO2 samples was transformed from SrLa4Ti4O15 to La2TiO5. As the amount of TiO2 increased, the εr value of the Ba0.2Sr0.8La4Ti4+xO15 ceramics increased sharply first, and then showed a slightly decline after reaching the maximum value at x = 0.10, the Q × f value decreased almost linearly from 62,800 to 36,100 GHz, while the τf value varied gradually toward positive direction from −20.0 to −2.6 ppm/°C. Optimum dielectric properties were achieved for stoichiometric Ba0.2Sr0.8La4Ti4O15 and nonstoichiometric Ba0.2Sr0.8La4Ti4.20O15 ceramics which exhibited an excellent Q × f value and a near zero τf value with other considerable microwave dielectric properties, respectively.

The effects of magnesium and niobium substitution for titanium on the microwave dielectric properties of Ba3.75Nd9.5Ti18−z(Mg1/3Nb2/3)zO54 (0 ≤ z ≤ 3) ceramics were studied. The temperature coefficient of resonant frequency (τf) decreased from about +60 ppm/°C to +17 ppm/°C when z ≤ 1. Excellent quality factor (Qf = 7300 GHz) as well as high dielectric constant (εr = 80.96) were obtained. For z ≥ 1.5, Nd2(Ti,Mg,Nb)2O7 secondary phase appeared which would obviously influence the microwave dielectric properties. As z varied from 0 to 3, matrix grain size degraded which would obviously deteriorate the microwave dielectric properties by conducting more pores, especially the Qf value. The τf value was found to be related to b-site bond valence (VB-O) and unit cell volume (Vm). Average ionic polarizability (αD) and relative density evidently influenced the dielectric constant.

The microstructures and the microwave dielectric properties of the (1 − x)(Mg0.97Co0.03)2(Ti0.95Sn0.05)O4–xCaTiO3 ceramic system prepared by the conventional solid-state route were investigated. (Mg0.97Co0.03)2(Ti0.95Sn0.05)O4 possesses high dielectric constant (εr = 14.23), high quality factor (Q × f = 188,760 GHz), and negative τf value (τf = −55.48 ppm/°C) at 1,390 °C for 4 h. In order to achieve a temperature stable material, CaTiO3, having a large positive τf value of 800 ppm/°C, was added to (Mg0.97Co0.03)2(Ti0.95Sn0.05)O4. Two phase system was confirmed by the X-ray diffraction patterns and the energy-dispersive X-ray analysis. Although the εr of the specimen could be boosted by increasing amount of CaTiO3, it would instead render a decrease in the Q × f. By appropriately adjusting the x value in the (1 − x)(Mg0.97Co0.03)2(Ti0.95Sn0.05)O4–xCaTiO3 ceramic system, zero τf value can be achieved. A new microwave dielectric material, 0.91(Mg0.97Co0.03)2(Ti0.95Sn0.05)O4–0.09CaTiO3 ceramic sintered at 1,390 °C had optimal dielectric properties (εr = 18.13, Q × f = 87,562 GHz, τf = 3.75 ppm/°C) which satisfied microwave applications in resonators, filters and antenna substrates.

(Mg0.97Zn0.03)2(Ti0.95Sn0.05)O4 ceramics by adding CaTiO3 have been prepared via the solid-state reaction method. The microstructures of samples are systematically studied in order to establish the effects of sintering temperature and additives on microwave dielectric properties of (Mg0.97Zn0.03)2(Ti0.95Sn0.05)O4 ceramics by X-ray diffraction and scanning electron microscopy. A fine combination of microwave dielectric properties (εr = 14.57, Q × f = 183,468 GHz, τf = −43.7 ppm/°C) was achieved for (Mg0.97Zn0.03)2(Ti0.95Sn0.05)O4 ceramics sintered at 1,390 °C for 4 h. CaTiO3, as a τf compensator, was added to form a temperature-stable ceramic system. For (1−x) (Mg0.97Zn0.03)2(Ti0.95Sn0.05)O4−xCaTiO3 system, 0.93(Mg0.97Zn0.03)2(Ti0.95Sn0.05)O4−0.07CaTiO3 ceramic sintered at 1,390 °C had optimal dielectric properties (εr = 18.32, Q × f = 94,715 GHz, τf = −4.1 ppm/°C) which satisfied microwave applications in resonators, filters and antenna substrates.

The effects of Co2O3 addition on the sintering behavior, phase formation, microstructure and microwave dielectric properties of 0.95MgTiO3–0.05CaTiO3 ceramics have been investigated. The structure and microstructure of the ceramics were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. These results suggested that Co2O3 addition not only contributes in lowering the sintering temperature but also inhibits the formation of second phase MgTi2O5 in 0.95MgTiO3–0.05CaTiO3 ceramics. Moreover, the decomposition of Co2O3 would inhibit the Ti4+ from being restored to Ti3+, which had the positive effect on the Q × f value. A fine combination of microwave dielectric properties (εr = 20.48, Q × f = 76,485 GHz, τf = 2.43 ppm/°C) was achieved for 0.95MgTiO3–0.05CaTiO3–0.01Co2O3 ceramics sintered at 1300 °C for 4 h, which satisfied microwave applications in resonators, filters and antenna substrates.

The effects of Ta2O5/Y2O3 codoping on the microstructure and microwave dielectric properties of Ba(Co0.56Zn0.40)1/3Nb2/3O3-xA-xB (A = 0.045 wt.% Ta2O5; B = 0.113 wt.% Y2O3) ceramics (x = 0, 1, 2, 4, 8, 16, 32) prepared according to the conventional solid-state reaction technique were investigated. The x-ray diffraction (XRD) results showed that the main crystal phase in the sintered ceramics was BaZn0.33Nb0.67O3-Ba3CoNb2O9. The additional surface phase of Ba8CoNb6O24 and trace amounts of Ba5Nb4O15 second phase were present when Ta2O5/Y2O3 was added to the ceramics. The 1:2 B-site cation ordering was affected by the substitution of Ta5+ and Y3+ in the crystal lattice, especially for x = 4. Scanning electron microscopy (SEM) images of the optimally doped ceramics sintered at 1340°C for 20 h showed a compact microstructure with crystal grains in dense contact. Though the dielectric constant increased with the x value, appropriate addition would result in a tremendous modification of the Q × f and τf values. Excellent microwave dielectric properties (εr = 35.4, Q × f = 62,993 GHz, and τf = 2.6 ppm/°C) were obtained for the ceramic with x = 0.4 sintered in air at 1340°C for 20 h.

BaTiO3 (BT) based X9R ceramics with high permittivity about 1700 were prepared by doping and pre-sintering technique. Pure Bi0.5Na0.5TiO3 (BNT) dopant was synthesized by the conventional solid state reaction first. Using this new approach, high performance BTBNT (BT doped with BNT) materials, owning high Curie temperature (139 °C), flat ferroelectric transition region and large permittivity at room temperature, were obtained. The effects of several dopants on dielectric properties of BTBNT ceramics were measured by the LCR meter. The suppression effect for the peaks in the dielectric constant at Curie temperature of these dopants have been ranked as follows: BiNbO4 > CaZrO3 > Nb2O5 > BNT.

The influences of CaZrO3 on the dielectric properties and microstructures of BaTiO3 (BT)-based ceramics have been investigated. The experiment results showed that the dielectric constant at room temperature increased with the addition of CaZrO3 in the range of 0–3.0 mol%, which could be explained by the growth of BT grains. XRD analysis revealed that the tetragonality declined as CaZrO3 concentration increased. XRD patterns of BT ceramics with different amounts of CaZrO3 doping were analyzed by a recently developed procedure-materials analysis using diffraction (MAUD), which was based on the Rietveld method combined with Fourier analysis. The results depicted that the high temperature peak of temperature-capacitance characteristics (TCC) was largely dependent on the micro-strain of samples. Furthermore, more CaZrO3 doping resulted in lower porosity and higher density. It was revealed that proper usage of CaZrO3 could improve the dielectric properties significantly, which was benefit to develop X8R multi-layer ceramic capacitors.

The effects of tetragonality and secondary phase on the Curie point of barium titanate (BaTiO3) ceramics were investigated in BaTiO3–MnNb2O6 system using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscope (SEM). In the case of MnNb2O6 addition up to 0.50 mol%, the Curie point decreased as the room temperature tetragonality decreased. With MnNb2O6 content greater than 1.00 mol%, a secondary phase Ba2Ti3Nb4O18 was formed, resulting in a shift of the Curie point towards higher temperatures. The chemical defect reaction and the internal stress effect on the Curie point have also been discussed.

In this paper, Mn2+ and calcium borosilicate glass (CBS) were used as effective dopants to prepare environmentally friendly dielectric materials satisfying EIA X8R specification. The effects of various Mn2+ concentrations on the dielectric properties of BaTiO3 (BT) ceramics were investigated when CBS amount was fixed. The dielectric measurements showed that the permittivity was decreased continuously with increasing Mn2+ concentrations, resulted from the existence of paraelectric regions caused by Mn2+ doping. The dielectric loss was decreased at the beginning and then increased with increasing Mn2+ concentrations, whereas the resistivity showed an opposite movement. The SEM images and X-ray diffraction curves corresponding to BT ceramics doped with different Mn2+ amounts proved that Mn2+ can promote the crystallization of CBS glass to form the secondary phase Ca4Mn4Si8O24, which determines the intensity of the high temperature peak (at about 125 °C) of the variation rate curve of capacitance. Moreover, the modification effects of Mn2+ on the dielectric properties of BT have been investigated. It is revealed that a proper usage of Mn2+ can improve the dielectric properties significantly, and Mn2+ and CBS co-doped BT ceramics are promising to develop X8R MLCs.

•Na0.5Sm0.5TiO3 + Cr2O3 ceramics were prepared by one process using the solid state method.•The XPS results showed that Cr3+ substitution restrained the formation of Ti3+ ions.•The substitution of Cr3+ for Ti4+ would effectively lower the τf.•The substitution of Cr3+ for Ti4+ improved the quality factor and maintained a high permittivity.The Na0.5Sm0.5TiO3 + x mol Cr2O3 ceramics (x = 0, 0.25%, 0.5%, 0.75%, 1%, 1.25% and 1.5%) were conventionally prepared using solid-state reaction method by one synthesizing process. All specimens were identified as a structure of orthorhombic perovskite with Pnma space group. Both of XRD patterns and refinement results showed that the Cr3+ had incorporated into the lattice at Ti sites, leading to the expansion of cell volume. The densification can be improved by adding a proper amount of Cr2O3 (0.25% ≤ x ≤ 1%). The permittivity (εr) slightly increased with x ≤ 0.25%, later decreased with further addition of Cr2O3, which showed a similar varying trend to dielectric polarizabilities. The temperature coefficient of the resonant frequency (τf) was reduced from 199.3 ppm/°C to 160.8 ppm/°C due to an increase of B-site bond valence. The experimental XPS results confirmed that there was a significant increase of quality factor (Q×f) from 8993 GHz at x = 0–11854 GHz at x = 1% due to the restraint of Ti4+ reduction to Ti3+. When added with 1% mol Cr2O3, the Na1/2Sm1/2TiO3 ceramic sintered at 1450 °C for 2 h exhibited high microwave dielectric properties of εr = 96, Q×f = 11854 GHz and τf = 171.2 ppm/°C.The experimental XPS results showed that Cr3+ substitution restrained the formation of Ti3+ ions in the Na0.5Sm0.5TiO3 + Cr2O3 ceramics by one synthetic process. Fig. 6. (a) The experimental spectrum of Ti 2p for pure Na0.5Sm0.5TiO3 and NST ceramics doped with 1% mol Cr2O3, and (b) the experimental and deconvoluted Ti 2p XPS spectrum for pure Na0.5Sm0.5TiO3 ceramics sintered at 1450 °C for 2 h.

Microwave dielectric properties of Li2Mg3−xCaxTiO6 (x=0–0.18) ceramics were studied using a conventional solid-state route to find temperature stable and high Q microwave ceramics. As the calcination temperature was 500 °C, the Li2TiO3 phase with monoclinic rock salt structure in C2/c space group started to form. When the samples were calcined from 600 °C to 900 °C, the XRD patterns exhibited a remarkable chemical reaction between the MgO and Li2TiO3 phases, which eventually formed the Li2Mg3TiO6 phase. The results indicated the Li2Mg3TiO6and CaTiO3 co-existed with each other and formed a stable composite system when the calcium content was added. The SEM photographs indicated that the pores caused by the Li evaporation could be effectively reduced due to the appearance of CaTiO3. As x was increased from 0 to 0.18, the relative density was significantly improved due to the elimination of pores. As the Ca content increased, the dielectric constant (εr) increased from 14.8 to 20.6; the quality factor (Q×f) decreased from 148,713 GHz to 79,845 GHz, and the temperature coefficient of resonant frequency (τf) significantly increased from −42.4 to +10.8 ppm/°C due to the increased amount of CaTiO3. Therefore, at x=0.12, the LMCxT ceramics sintered at 1280 °C for 6 h displayed excellent comprehensive properties of εr=17.8,Q×f=102,246 GHz and τf=−0.7 ppm/°C.