High-purity ultrafine alumina powder generally refers to alumina powder with purity of 4N (99.99%) or higher and particle diameter (D₅₀) ≤1.0 µm. Alumina has more than ten crystal structures, such as α, γ, θ, δ, η, κ, which are divided into two categories based on the oxygen ion arrangement structure, i.e., face centered cubic and closely packed hexagonal. Among the above-mentioned crystal forms, α-Al₂O₃ is the only thermodynamically stable crystal form. Due to its high melting point, strong corrosion resistance and high wear resistance, it is widely used in high-tech cutting-edge fields, such as integrated circuit substrates, electrical insulation materials, electronic packaging, aerospace and defense industries. Owing the factors of late start, slow development and lack of experience, the domestic enterprises are not competitive in the production of ultrafine alumina powder, especially those with low sodium, ultrafine and high-purity, thus being at relatively low level of industrialization. Such products mainly rely on imports, while the high-purity ultrafine alumina powders produced in large-scale industrial production are mostly from large international enterprises, such as the United States, Japan and Europe.

The main preparation methods and research progress of high-purity ultrafine α-Al₂O₃ powder, including gas-phase method, liquid-phase method, solid-phase method and new preparation methods, were summarized. One of the gas-phase methods is chemical vapor deposition, in which gas or vaporizes reactants are directly used through different heating methods, through physiological reactions in gas state and finally condensation to form ultrafine powder. This method generally has easy control of reaction conditions, easy control of agglomeration, high particle dispersion, small particle size and narrow distribution. The disadvantages included low yield and high cost. Due to the fact that the raw materials in the gas-phase method must be completely vaporized before the reaction, which is highly energy consuming and requires large amount of inert gas with low humidity. The liquid phase methods included spray pyrolysis, alkyd hydrolysis, modified Bayer method, sol-gel method, alkyd aluminum method and so on. In the liquid phase method, dissolve aluminum salt is dissolved into certain solvents, separating solute and solvent through various ways, such as evaporation, sublimation, addition of precipitator, hydrolysis and so on, to obtain particles with certain shape and size, thus obtaining the precursor of Al₂O₃ powder. Finally, the precursor can be heated and decomposed to obtain α-Al₂O₃. This method inevitably results in high-purity alumina powder by calcining the precursor, leading to powders with agglomeration. Therefore, the powder often needs to be ground to eliminate agglomeration. Among various techniques for preparing nanosized alumina powders, liquid-phase method is the most widely used method. Solid phase methods included new ones, such as mechanical crushing, detonation and solid-phase reaction. The solid-phase method involves mixing two or more powders and directly reacting to them at certain temperatures and atmosphere to produce nanosized alumina. The advantages of solid-phase method included the simple production process, high yield and short production cycle, which can be used in some fields with low requirements for particle size and purity. It does not require drying process, so there is no powder agglomeration, owing to the hydroxyl condensation and dehydration in between particles. The requirement of high temperature is one of the disadvantages, with high energy consumption, low efficiency, easy introduction of impurities, wide particle size distribution and easy oxidation and deformation of particles. Therefore, it is difficult to obtain fine-grained and high-purity α-Al₂O₃ powder using this method.

In summary, in the future, China needs to improve the equipment level and production process for the preparation of α-Al₂O₃ powder, increase development efforts, ensure uniform particle size and reduce agglomeration of high-purity and ultrafine alumina powder, ensure purity of over 4N level, thus gradually narrowing the gap with international advanced production enterprises and hence comprehensively improving the preparation capacity of high-purity and ultrafine alumina powder in China.

Dielectric properties of passive electronic components are highly connected with their capacitive properties. For a parallel plate capacitor, the capacitance increases as dielectric materials are inserted. Giant dielectric ceramics usually have dielectric contant over 10⁴, which are promising capacitive materials. Therefore, higher energy storage capability and higher energy density could be achieved, thus offering more powerful integrated circuit system and maintaining the developing rate defined by Moore's Law. Perovskite structured materials have been most widely and intensively studied, due to their outstanding dielectric properties. However, since the current perovskite structured materials have temperature sensitivity, the limited level of dielectric constant and even toxicity of those containing lead, it is necessary to further develop perovskite-like structure materials.

Three typical modules are introduced with representing materials. The conventional perovskite materials, such as BaTiO₃ (BTO), possess spontaneous polarization. As indicated in the Clausius-Mosotti equation, permittivity of material is directly propertion to its polarizability. However, the polarizability of BTO is highly temperature sensitive as their intrinsic Curie temperature limitation, corresponding to structure transforming. The polarizability is generated due to the offset of the central Ti ion with respect to the O ions. Therefore, it is necessary to extend the temperature window for tetrahedral structure so as to maintain high permittivity. In this article, representive research about achieving the aforementioned task will be introduced. Moreover, various other strategies, such as substitution with donor or acceptor ions and introduction of oxygen vacancy, to increase dielectric properties, will be elaborated. Perovskite-like structured CaCu₃Ti₄O₁₂(CCTO) is introduced due to its giant dielectric constant and achievable temperature stability. In this case, internal barrier layer capacitor (IBLC) will also be summarized. The IBLC model could be simply concluded as a combination of conductive grain and insulating grain boundary. It is started from the escaping of oxygen atom during heat treatment at high temperatures, causing loss of oxygen in grains and hence the reduction of Ti⁴⁺ and Cu²⁺ to to Ti³⁺ and Cu⁺ by obtaining electron. Meanwhile, a copper-rich phase is formed at grain boundary during cooling, since copper atom has strong affinity to oxygen. Therefore, the model to describe the effects of copper-rich phase on dielectric properties will be introduced. Then, the core-shell structure of CCTO family is presented. Lastly, the escape of oxygen from the material is discussed. Other ferroelectric materials with perovskite structure but low performance are also introduced, such as SrTiO₃ (STO), with cubic structure at room temperature. Although they have no spontaneous polarization, they could have high dielectric constant. They can be used to dope perovskite ceramics, thus generating spontaneous polarization and eventually resulting in colossal permittivity. Also, colossal permittivity could be achieved by making difference in conductivity through ion doping.

Perovskite and perovskite-like structures giant dielectric constant materials have drawn strong attention, because of their high dielectric constant and possibility of doping. Theoretical models on spontaneous polarization, IBLCs and mixing effects are introduced, with typical ceramics, as BaTiO₃, CCTO and SrTiO₃. To be summarized, BTO has high element affinity and acceptable dielectric properties, but limited for applications owing to their temperature dependent permittivity. CCTO has shown giant dielectric constant, while the dielectric loss cannot be ignored. A mixture model could be achieved by incorporating with STO, since it could be employed to suppress the dielectric loss of CCTO ceramics.