Electrical and Magnetic Ceramics

Crystal-liquid structure of a forsterite composition (150x). 

Triaxial porcelains contain considerable amounts of alkalis derived from feldspar. Alkalis in feldspar help in the formation of flux during the produc-hon of porcelain. The presence of alkalis leads to high electrical conductivity and high dielectric loss. Electrical conductivity and loss of dielectricity are not desirable in an insulator. The low-loss composihons are made alkali-free by using alkaline earth oxides as fluxing constituents. 

The next type of insulating ceramics used in electrical applications is alumina. In this ceramic, AI2O3 forms the crystalline phase, which is bonded 

Cordierite insulator has the formula 2Mg0-2Al203-5Si02. A variety of fluxes are used in its manufacture. The cordierite phase develops as prismatic habit crystals. These are again bonded with a glassy phase. In addition, some mullite, corundum, spinel, forsterite, or enstatite (Mg0-Si02) phases may also be seen. 

High-alumina porcelain and heavily etched to remove silicate-bonding phase (2300x). 

---

Lakeman C.D.E., Payne D.A. Sol-gel processing of electrical and magnetic ceramics. Mater. Chem. Phys. 1994 38 305-324... 

Maiwa H Advances in ceramics - Electric and magnetic ceramics, bioceramics, ceramics and environment. InTech, 2011 p.3-22. 

The main categories of electrical/optical ceramics are as follows phosphors for TV, radar and oscilloscope screens voltage-dependent and thermally sensitive resistors dielectrics, including ferroelectrics piezoelectric materials, again including ferroelectrics pyroelectric ceramics electro-optic ceramics and magnetic ceramics. 

In its simplest form a DR is a cylinder of ceramic of relative permittivity 8r sufficiently high for a standing electromagnetic wave to be sustained within its volume because of reflection at the dielectric-air interface. The electric and magnetic field components of the fundamental mode of a standing electromagnetic field are illustrated in Fig. 5.33. 

Ceramic powder characteristics are important because the purity of the powder sets the maximum purity level of the final processed ceramic part, and the particle size and size distribution play major roles in defining the microstructure and properties of the final parts. Both the purity and the microstructure of sintered ceramics influence the properties of ceramic materials, including mechanical, thermal, electrical, and magnetic properties and chemical corrosion resistance. 

In contrast to the physical properties, transition metal carbides and nitrides possess electric and magnetic properties that are often similar to metals. For example, electrical resistivities of Ti or W are 39 and 5.39 pft cm at room temperamre, while their respective carbides have only slightiy higher resistivities of 68 and 22 p,fl cm. For comparison the electrical resistivity of the hard SiC ceramics is significantly higher (1000 pfi cm). 

Schoenes, J. (1992). Magneto-optical properties of metals, alloys and compounds. In Electrical and Magnetic Properties of Metals and Ceramics, Part I. Ed. K. H. J. Buschow. VCH Publishers Inc., New York, pp. 147-257. 

According to the matrix, nanocomposites may be classified into three categories i) Ceramic matrix nanocomposite, ii) metal matrix nanocomposites, and iii) polymer matrix nanocomposite. In the first group of composites the matrix is a ceramic material, i.e., a chemical compoxmd from the group of oxides, nitrides, borides, silicides, etc. In most cases of ceramic-matrix nanocomposites the dispersed phase is a metal, and ideally both components, the metallic one and the ceramic one, are finely dispersed in each other in order to elicit the particular nanoscopic properties. Nanocomposites from these combinations were demonstrated to improve their optical, electrical and magnetic properties [5,4], as well as tribological, corrosion-resistance and other protective properties [6,5]. Thus the safest measure is to carefully choose immiscible metal and... 

A wide range of materials can be usefully characterized by IS, namely electrical and structural ceramics, magnetic ferrites, semiconductors, membranes, polymeric materials, and protective paint films. The measurement techniques used to characterize materials are generally simpler than those used for electrode processes. Impedance spectra are usually independent of applied potential (both ac amplitude and dc bias) up to potentials of 1V or more. Consequently, it is unnecessary to fix the potential of electrodes, as is the case with potentiostatic experiments, and two-electrode symmetrical cells are commonly used. 

This class of ceramic is named after the mineral MgAl204 and many members have been fabricated because of the sensitive dependence of their electrical and magnetic properties on composition and temperature. Such a dependence arises from the variations in cation site occupancy that can be engineered. Spinels containing iron are particularly useful because of their magnetically soft properties that led to their application in computer hardware, memory devices, high-frequency transformers, and phase shifters. 

The interest in transition metal-containing polymers stems from the optical, biological, thermal, catalytic, electrical, and magnetic properties of these materials. They are candidates as essential materials for a wide variety of applications for the 21st century because of these properties. They are used in the coatings, colorants, pharmaceutical, aerospace, and communications industries. They can serve as precursors for ceramics. 

Mechanical, Thermal, Electrical, and Magnetic Properties of Structural Materials, Wadd Technical Report Handbock on Materials for Superconducting Machinery Metals and Ceramics, Information Center, Battelle, Columbus Laboratories, 1974 (with 1975 and 1977 Supplements). [Inconel]... 

In the broad range of ceramic materials that are used for electrical and electronic apphcations, each category of material exhibits unique property characteristics which directiy reflect composition, processing, and microstmcture. Detailed treatment is given primarily to those property characteristics relating to insulation behavior and electrical conduction processes. Further details concerning the more specialized electrical behavior in ceramic materials, eg, polarization, dielectric, ferroelectric, piezoelectric, electrooptic, and magnetic phenomena, are covered in References 1—9. 

---