Lead-free piezoelectric single crystals with nominal composition of 99.3K0.5Na0.5NbO3-0.4LiBiO3-0.3CaZrO3(KNN-LB-CZ) doped with MnO2 were prepared by a seed-free, solid-state crystal growth method. The effects of MnO2 content on the growth, structure and piezoelectric properties of KNN-LB-CZ single crystals were investigated.The results show that the addition of MnO2 inhibits the crystal growth at different growth temperatures. When the MnO2 content is in the range of 0.125%~0.5%, the single crystal area on the upper surface of the sample decreases from 95% to 30% gradually, and when the MnO2 content is over 0.625%, there is almost no large grains visible to the human eye.The addition of MnO2 does not change the perovskite structure of the crystals or produce second phase in the crystal, but it is beneficial to the densification of the single crystal microstructure.An appropriate amount of MnO2 doping is beneficial to improve the piezoelectric properties of single crystal. The piezoelectric constant d33 and dielectric loss tan δ of single crystal without MnO2 doping are 200 pC/N and 0.06, respectively. When the MnO2 doping contend is 0.5%, the d33 of the single crystal increases to 450 pC/N, and its dielectric loss also decreases to 0.03.

In this review article, the principle of Fresnel magnetic imaging and together with the manipulation methods by electric currents, magnetic fields, and temperatures in Lorentz transmission electron microscopy (L-TEM) is introduced at the beginning. The manipulation method has the advantages of in-situ characterization topological skyrmions with high-resolution in real space. The experimental results by using the manipulation method in centrosymmetric, symmetry-broken bulk magnets and ferromagnetic multilayers have been respectively reviewed. The realization of high-density, zero-field, nonvolatile topological states in broadened temperature range broke through the bottleneck of requiring the magnetic fields to stabilize skyrmions as information bits just near the phase transition temperature in previous studies. The generation mechanism of topological skyrmion has been revealed by studying the critical role of magnetic anisotropy on the spin configuration texture and the density of topological domains. The advantages of L-TEM in characterizing the magnetic domains accelerate the discovery of various topological states such as magnetic vortex, meron, skyrmion and magnetic bubble and new magnetic materials.

As one of lead-free piezoelectric and ferroelectric ceramics, structure and electrical properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics have been widely studied. Effects of point defects in (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics, are summarized in this work. The contents include introduction of crystallite structure and electrical properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics, point defects in (Na0.5Bi0.5)0.94Ba0.06TiO3, reported methods for characterizing point defects, effects of ion nonstoichiometry, ion doping, technology parameters on structure and electrical properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics. Ion nonstoichiometry, ion doping, and technology parameters can cause changes in type and concentration of point defects, which will affect structure and electrical properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics. Based on the overview about point defect effects of structrue and electrical properties of (Na0.5Bi0.5)0.94Ba0.06TiO3 ceramics, it is hoped to provide reference for the design and control structrue and electrical properties of functional ceramics via the point defect effects.

BiScO3-PbTiO3 (BS-PT) piezoelectric ceramics with high Curie temperature and excellent electrical properties have become one of materials for working in high-temperature environments. The 0.36BS-0.64PT and 1wt%Pb-0.36BS-0.64PT piezoelectric ceramics were prepared by the solid-state reaction. The influences of different sintering processes on the phase structure and properties of the two ceramic components were studied. The optimal sintering and polarization technological were determined. The results showed that the phase composition of ceramic such as increased tetragon and decreased rhombohedral phase content strongly depends on the sintering temperature and soaking time. Through the adjustment of sintering and polarization process, the excellent properties, including Curie temperature TC=454 ℃, piezoelectric constant d33=365 pC/N, electromechanical properties kp=51% were obtained in 1wt%Pb-0.36BS-0.64PT composition ceramics.

In order to obtain the atomic arrangement at picometer scale without artifacts, a series of morphological processing of the high resolution atomic scale images are carried out with numerical program.The center of each atoms is fitted and quantitatively calculated in order to obtain the displacements of polarization and error analysis at picometer level. By using this program, the 90-degree domain structure in ferroelectric thin film PbTiO3 is quantitatively analyzed, and the polarization of single domain and the "head to tail" domain wall at unit cell scale are calculated and illustrated as well. In addition, this approach is further applied to quantitatively analyze the annular bright-field image, and the oxygen octahedral tilting angles in the LaCoO3 thin film can be measured.The present method can obtain microstructure features of materials at picometer scale and calculate them quantitatively.

Cold sintering process is an extremely low temperature process, with the sintering temperature typically below 300 ℃, which uses a transient liquid phase (such as water, acid solutions and aqueous alkali) and an applied pressure to assist the densification of ceramic powders. The low sintering temperature and short holding time of cold sintering process can effectively solve the difficulties encountered in the interface control, phase stability and composite sintering. This article mainly introduces the sintering mechanism of cold sintering process and the applications in dielectric materials, focusing on microwave dielectrics, ferroelectrics, piezoelectrics, ceramic based composites and multilayer materials. The cold sintering process has realized the densification of various dielectric materials with good properties at extremely low temperatures, which has greatly promoted the progress and development of low temperature sintering technology. It provides a new route for the development of dielectric materials, and is expected to make a breakthrough in the field of high performance dielectric materials with low CO2 emission. However, it is worth noting that cold sintering process is in the early stage. The sintering mechanism is relatively complex and large varieties of material systems are available, so there is still a lot of room for improvement.

Multi-rare-earth La/Ce ions modified CaBi2Nb2O9（CBN）ceramics were prepared by a conventional solid state sintering method. The effects of different sinter temperatures on crystal structure, microstructure and electrical properties were investigated. The X-ray analysis data show that the specimens sintered in the temperature range of 1 000-1 150 ℃ have a single bismuth-layered structural phase. A secondary phase corresponding Ca2Nb2O7 was detected for the ceramics sintered at 1 200 ℃. The grain size significantly increases with an increase of sinter temperatures. The microstructure of ceramic grains changes from spherical to disk like morphologies. Moreover, the dielectric, dc resistivity and piezoelectric properties at different sintering temperature were studied. The Curie temperature of modified-CBN ceramics decreases from 935 ℃ to 915 ℃ with increasing sinter temperatures. Meanwhile, the dc resistivity at 500 ℃ decreases from 3 MΩ·cm to 0.76 MΩ·cm. La/Ce-CBN ceramics sintered at 1 150 ℃ for 2 h possess good piezoelectric properties, with piezoelectric coefficient d33 of 16.6 pC/N. The d33 value decreases to 15.1 pC/N after annealing at 850 for 2 h, exhibiting a good piezoelectric temperature stability.

MgTiTa2O8 microwave dielectric ceramic material has moderate dielectric constant and high Q×f value, but its τf value is too large to be applied in practical devices. Mg(Ti1-xZrx)Ta2O8(x=0.2,0.4,0.6,0.8) microwave dielectric ceramics were prepared by the traditional solid-state reaction method, and the effects of replacing Ti4+ with Zr4+ on the microstructure and microwave dielectric properties of MgTiTa2O8 dielectric ceramics were studied. With the gradual increase of Zr4+ doping, the internal structure of ceramics changes in the following order: tri-rutile structure coexistence of tri-rutile and wolframite structure wolframite structure, and the change of ceramic internal structure will affect the microwave dielectric properties of the materials. Compared with pure MgTiTa2O8 microwave dielectric ceramics, the temperature coefficient of the material is improved to 9.6×10-6 /℃ when x=0.4.MgTi0.6Zr0.4Ta2O8 microwave dielectric ceramics were prepared at the sintering temperature of 1 350 ℃, showing excellent microwave dielectric properties: εr=31，Q×f=30 544 GHz，τf=9.6×10-6/℃.

The lead-free ceramics Bi0.44Na0.50Ba0.06TiO3-δ and Bi0.50Na0.44Ba0.06TiO3+δ were prepared via the conventional sintering method. The two kinds of ceramics were sintered at various temperatures. Their crystallite structure, microstructure, dielectric and ferroelectric properties were studied comparatively. The ceramics sintered at 1 150 ℃ have pure perovskite structure and dense microstructure with relative densities higher than 97%. The dielectric measurement shows that the dielectric spectra are different from each other obviously. Dielectric constant of the Bi0.44Na0.50Ba0.06TiO3-δ ceramics increases greatly with increasing temperature. The two kinds of ceramics show diffuse phase transition character. The values of maximum polarization, remanent polarization, and coercive field are 34.1 μC/cm2, 28.5 μC/cm2 and 53.7 kV/cm for Bi0.44Na0.50Ba0.06TiO3-δ. The corresponding values are 42.8 μC/cm2, 31.8 μC/cm2 and 48.2 kV/cm for Bi0.50Na0.44Ba0.06TiO3+δ. The ceramics Bi0.50Na0.44Ba0.06TiO3+δ shows better ferroelectric properties.

K0.5Na0.5NbO3 precursor powders with pure phase and mean grain size of 100 nm were obtained via normal milling and high-energy ball milling methods. As one liquid phase, deionized water was added into the precursor powders. K0.5Na0.5NbO3 ceramics with the simple composition were prepared by means of cold sintering and post-annealing. Effects of cold-sintering temperature, cold-sintering time, pressure on phase and densification of the cold-sintered pellets were studied. The cold-sintered pellets were post-annealed at various temperatures. Effect of annealing temperature on dielectric and ferroelectric properties of the post-annealed ceramics was studied. The ceramics prepared at cold-sintering temperature 180 ℃ and annealing temperature 1 100 ℃ show dielectric constant 345, dielectric loss 0.03 at room temperature, remanent polarization 25.8 μC/cm2, maximum polarization 31.4 μC/cm2, and coercive field 10.2 kV/cm. The dielectric and ferroelectric properties are better than those of the ceramics prepared via the normal solid-state sintering method.

In order to improve the transmittance of potassium sodium niobate based lead free ceramics, a series of 0.95(K1/2Na1/2)NbO3-0.05Ba(Zn1/3Nb2/3)O3-x mol %LiBiO2（0.95KNN-0.05BZN-x mol%LB） transparent ceramics were prepared by a traditional solid-state method and the transmittance was characterized. It was found that when x=1, the transmittance of ceramics was the highest (65%, 780 nm), while the porosity began to increase and the transmittance gradually decreased when the doping amount continued to increase. The results indicated that the introduction of LiBiO2 not only reduces the porosity of ceramics, but also increases the grain size, reducing the light scattering from pores and grain boundaries, and effectively improves the transmittance of ceramics.

Sr1-xLaxFe12O19 (x=0, 0.05, 0.1 and 0.15) ceramics were successfully synthesized by solid state reaction method. La doping can affect the crystal lattice parameters, but it has little effect on the morphology. The dielectric constant of the doped ceramics is significantly higher than that of the pure phase ceramics, but the dielectric loss is relatively large. The dielectric properties of the doped ceramics are mainly caused by the Maxwell-Wagner interface polarization effect. At the same time, the magnetic properties are improved after doping lanthanum ions. The change of the dielectric constant with magnetic field shows that the temperature transition from positive to negative magnetodielectric effect is different. The transition temperature for the pure SrFe12O19 is between 50 and 75 K, and the transition temperature for samples with x=0.05, 0.1 and 0.15 is between 25 and 50 K. The magnetodielectric coefficient is relatively large between 75 and 200 K due to the polarization effect of the Maxwell-Wagner interface, which is an extrinsic phenomenon.The results show that the dielectric constant of La doped ceramics is greatly improved, while the magnetic properties are well maintained.