For crystalline materials, atomistic local structures are deviations from their long-range average crystal structures. With the recent development of experimental techniques and in-depth investigation methods for probing local structures, it has been realized and understood that local structures have profound influences on the physical properties of functional materials, and the understanding of local structures provides theoretical instructions for the improvements of materials performances as well as development of novel applications. This review summarized the well-established X-ray and neutron scattering techniques, data analysis and model refinements mothods in the local-structure investigation researches; in addition, examples of local structural investigations for several classical ferroelectric perovskites are reviewed.

The ceramics of 0.96K_0.48Na_0.52Nb_(1-x)Ta_xO₃ 0.04BaZrO₃ 0.3%MnCO₃ were prepared via solid-state reaction and subsequent sintering in air and reducing atmospheres. The effects of Ta doping and sintering atmosphere on the microstructure and piezoelectric properties of ceramics were investigated. The results demonstrate that all the ceramics sintered in different atmospheres exhibit a pure perovskite structure. The phase structure of KNN-based ceramics is the coexistence of rhombohedral and orthorhombic phases. Also, the generation of oxygen vacancies is suppressed as Ta doping content is increased. When x = 0.1, the ceramics sintered in a reducing atmosphere exhibit the optimum electrical properties (i.e., d₃₃=172 pC/N, , k_p=24.9, , and tanδ=0.021), which are superior to the counterparts sintered in air atmosphere (i.e., d₃₃=142 pC/N, pm/V, k_p=21.1, , and tanδ=0.022). It is indicated that sintering in a reducing atmosphere can suppress the grain growth, thus facilitating the densification of ceramics and improving the piezoelectric properties.

3-1 type and 3-3 type porous PZT ceramics were fabricated by the alginate ionotropic gelation process and gelcasting technology respectively. The resultant samples were sintered at different temperatures and characterized in terms of both microstructure and piezoelectric properties. It was noted that the porosity and grain size of porous PZT ceramics increased with the sintering temperature increasing from 1 150 to 1 250 ℃, which resulted in the increase of relative permittivity, piezoelectric efficient, thickness coupling coefficient k_t and compressive strength, the corresponding decrease of hydrostatic voltage coefficient and hydrostatic figure of merit (HFOM). Under the joint effects of unidirectionally aligned channels and the incorporation of Ca²⁺ into PZT matrix, the 3-1 type PZT ceramics possessed higher ε_r and compressive strength, lower d₃₃ and HFOM than that of 3-3 type PZT ceramics. However, the maximum HFOM value of 3-1 type PZT ceramics achieved 4 755×10^-15 Pa^-1, which makes it suitable for the application of underwater sonar detectors.

Near net size preparation of porous mullite ceramics with controllable shrinkage was achieved via gel casting and pore-forming agent process With Al₂O₃ and kyanite as raw materials, PMMA microspheres as a pore-forming agent, and isobutylene/maleic anhydride copolymer (Isobam104) as a gelling/dispersing agent. The effect of sintering temperature on the phase composition and the effect of solid loading on the microstructure, phase composition, shrinkage, porosity, and compressive strength of samples were investigated. The results show that the shrinkage of samples sintered at 1 500 ℃ firstly decreases and then increases as the solid loading increases. At the solid loading of 30%(in volume fraction) and the content of pore forming agent of 30%(in mass fraction), the total shrinkage of the samples is close to zero, realizing the near net size preparation of porous mullite ceramics. The prepared porous mullite ceramics exhibite a higher porosity (i.e., 60.4%), a smaller average pore size (i.e., 3.75 μm) and a higher compressive strength (i.e., 8.3 MPa). The shrinkage of porous ceramics can be effectively controlled by the volume expansion effect in the preparation process, and the near net size preparation of porous mullite ceramics is of great significance for the preparation of large-size and complex porous ceramic parts and the reduction of processing cost.

Alumina ceramics were joined with Ag-10CuO-10B₂O₃ filler via reactiver air brazing for the formation of whiskers (Cu₂Al₆B₄O₁₇) in the joints. The effects of brazing temperature and holding time on the microstructure, properties of the joints and the whisker structure were investigated. The results show that Al₂O₃ reacts with CuO and B₂O₃ to form Cu₂Al₆B₄O₁₇ whiskers in the brazing process, thus obtaining a reliable joint with the superior performance. At the holding time of 60 min and the brazing temperature of > 950 ℃, an effective connection of Al₂O₃ ceramic joint can be realized. The center area of the brazing joint is mainly Ag, while the interface at the both sides of the joint is composed of CuO、Cu₃B₂O₆ phase and Cu₂Al₆B₄O₁₇ whiskers. The shear strength of the joint increases with the increase of the brazing temperature. The whiskers become thick and the adhesion occurs when the brazing temperature is > 1 050 ℃, thus reducing the strengthening effect on the joint and the shear strength of the joint. The whiskers decompose into some lamellar products, and the shear strength of the joint decreases when the brazing temperature is >1 100 ℃. The shear strength of the joint prepared under optimum process parameters (i.e., at 1 000 ℃ for 60 min) can reach 86.7 MPa, and the fracture occurs in the weld nearby alumina ceramic base material.

Porous ceramics were prepared by a high-temperature melting foaming method with red mud and high-aluminum fly ash as main raw materials. The effects of raw material composition and foaming process on the cellular structure and properties were investigated. The results show that the cellular structure of porous ceramic obtained after melt foaming at 1 200 ℃ for 1 h changes from closed to open, eventually forming a composite cellular structure. Also, the average pore size and porosity increase, while the bulk density and thermal conductivity decrease with increasing the mass ratio of SiO₂/Al₂O₃ from 1.2 to 1.9 in the raw material composition. At the blowing agent of 5% (in mass fraction) SiC and the melting agent of albite and microcline of 35%, a composite cellular structure porous ceramic with the bulk density of 0.63 g/cm³, average pore size of 0.98 mm and bulk porosity of 70% is obtained, and its flexural strength and thermal conductivity are 7.17 MPa and 0.34 W/(m·K), respectively.

Single-phase anorthite porous ceramics were in-situ prepared by a foam gel-casting method with CaCO₃, α-Al₂O₃ and SiO₂ as raw materials and environmental-friendly gelatin as a gel. The influence of gelatin amount on the phase composition, microstructure and properties (i.e., porosity, density, compressive strength, and thermal conductivity) of the samples was investigated. The amount of gelatin has no effect on the phase composition of the prepared material, but it affects the properties. The total porosity decreases from 89.7% to 88.0%, the bulk density increases from 0.28 g/cm³ to 0.33 g/cm³, the compressive strength increases from 1.02 MPa to 2.54 MPa, and the thermal conductivity increases from 0.038 W/(m·K) to 0.059 W/(m·K) as the gelatin amount increases from 6% to 12%. It is indicated that porous anothite ceramics with a low density, a high strength and a low thermal conductivity could be prepared by this promising method.

Porous alumina (Al₂O₃) beads were prepared via gel-dripping and pressureless sintering with α-Al₂O₃ as a raw material, nontoxic sodium alginate as a gelation reagent, and the calcium ions were introduced in the preparation using a calcium chloride solution. The effects of coagulation time and sintering temperature on the porosity, compressive strength and microstructure of porous Al₂O₃ beads were investigated. The results show that calcium ions can transfer and react readily with sodium alginate as the coagulation time increases. Alumina particles are solidified to form a bead green body by a three-dimensional network. The number of meso-pores decreases and the microstructure becomes well-distributed with increasing the sintering temperature. The porosity and compressive strength are 47.7%-78.1% and 1.2-36.0 MPa, respectively. The porosity, pore structure and compressive strength can be tailored by optimizing the coagulation time, sintering temperature and calcium ion concentration. The diameter, sphericity and property of porous alumina beads can be controlled through the simple gel-dripping method. The porous alumina beads can be used in many fields, such as catalyst support, adsorption, separation, purification, etc..

Rapid preparation of dense BiFeO₃-BaTiO₃ lead-free piezoelectric ceramics at low temperatures is beneficial to preventing the Bi volatilization, reducing the energy consumption and developing novel materials with some unique properties. In this paper, dense BiFeO₃-BaTiO₃ ceramics with good ferroelectric and piezoelectric properties were prepared in an electric field of 200 V/cm with a current limit of 280 mA via reactive flash sintering of multiphase precursor powders (i.e., Bi₂O₃, Fe₂O₃, TiO₂ and BaCO₃) at 415 ℃ for 30 s. The solid-state reaction in sintering occurs during the flash event. The current limit has an effect on the phase transformation and densification. It is indicated that the reactive flash sintering technology could prepare the single-phase and dense ceramic material through flash sintering of starting precursors in one step, thus providing an effective approach for the rapid production of ceramic materials.

ZTA composite ceramics were prepared via pressureless sintering, and the influence of Cr₂O₃ doping on the microstructure and mechanical properties of ZTA ceramics was investigated. The results show that Cr₂O₃ can promote the growth of Al₂O₃ grains and obtain elongated Al₂O₃ grains. The density, hardness and fracture toughness of ZTA ceramics firstly increase and then decrease with the increase of Cr₂O₃ addition. ZTA ceramics with 0.6% Cr₂O₃ addition exhibit the optimum properties. However, the properties of ZTA ceramics with the excessive addition of Cr₂O₃ can be impaired due to the increased internal porosities. The mechanical properties of ZTA composite ceramics with Cr₂O₃ nano-sized particles are superior to those with Cr₂O₃ micron-sized particles. The addition of Cr₂O₃ nano-sized particles results in greater density and hardness of ZTA ceramics due to the smaller average grain size of Al₂O₃. Besides, the fracture toughness of ZTA ceramics improves due to the formation of more elongated grains and the consumption of more energy for crack propagation.