In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

This chapter discusses leading problems linked to energy that the world is now confronting and to propose some ideas concerning possible solutions. Oil deserves special attention among all energy sources. Since the beginning of 1981, it has merely been continuing and enhancing the downward movement in consumption and prices caused by excessive rises, especially for light crudes such as those from Africa, and the slowing down of worldwide economic growth. Densely-populated oil-producing countries need to produce to live, to pay for their food and their equipment. If the economic growth of the industrialized countries were to be 4%, even if investment in the rational use of energy were pushed to the limit and the development of nonpetroleum energy sources were also pursued actively, it would be extremely difficult to prevent a sharp rise in prices. It is evident that it is absolutely necessary to pursue actively the development of coal, natural gas, and nuclear power if a physical shortage of energy is not to block economic growth.

World energy outlook

Interpretation of X-ray Powder Diffraction Patterns . By H. Lipson and H. Steeple. Pp. viii + 335 + 3 plates. (Mac-millan: London; St Martins Press: New York, May 1970.) £4.

X-Ray Diffraction

Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

Über die Adsorption in Lösungen

http://magneticliquid.narod.ru/autority_2/022.pdf

Magnetic properties of nanocrystalline CoFe2O4 powders prepared at room temperature: variation with crystallite size

AWO 4 (where A=Zn 2+ , Mg 2+ , Ni 2+ and Co 2+ ) tungstate ceramics were synthesised from a standard ceramic route, and their sintering behaviour and microwave dielectric properties were investigated. All AWO 4 powders formed single-phase materials, except MgWO 4 , and they all had a wolframite-like monoclinic P2/c (13) structure, not the tetragonal scheelite structure found in CaWO 4 . MgWO 4 had a poor Qf which deteriorated with further sintering, due to discontinuous grain growth at relatively low temperatures of 1000 °C and over. At higher temperatures, ZnWO 4 , NiWO 4 and CoWO 4 have high Qf values at 1200 °C of 62,800, 24,900 and 38,600 GHz, respectively, which will all almost certainly be improved with firing at higher temperatures, particularly in the case of CoWO 4 . Although not at all well sintered, ZnWO 4 and CoWO 4 have high Qf values of 34,000 and 28,900 GHz, respectively, at temperatures below the LTCC limit.

MgWO4, ZnWO4, NiWO4 and CoWO4 microwave dielectric ceramics

Undoped TiO2 exhibited deterioration in microwave (MW) dielectric loss as it reached its maximum density due to the reduction of Ti4+ to Ti3+ causing oxygen vacancies at high sintering temperatures. By adding small amounts of acceptor dopants with ionic radii between 0.5 and 0.95 A, reduction during sintering was suppressed. The upper limit of ionic radius was discrete with almost no observed effect using dopants >0.96 A ionic radius. In addition, the microwave dielectric loss of undoped TiO2 could be improved by annealing at 1500 °C for 10 h in air, presumably as a result of re-oxidation. High loss samples exhibited a dark ‘core’ to the naked eye which was absent in low loss ceramics. Transmission electron microscopy revealed that grains in the dark core contained planar defects attributed to the condensation of O vacancies onto specific crystallographic planes, in a manner similar to that observed in Magnelli phases.

Dielectric loss caused by oxygen vacancies in titania ceramics

We explain the basic mechanism of the low-temperature sintering called reactive liquid-phase sintering. The mechanism involves the presence of a low-temperature liquid phase that must be able to directly or indirectly accelerate a reaction with the matrix phase. The mechanism is explained in details for the case of the low-temperature sintering of BaTiO 3 , which was sintered to more than 95% of relative density in 15 min at 820 °C. We have applied reactive liquid-phase sintering to a number of different compounds with very different crystal-chemistry characteristics, and managed to sinter them as much as 400 °C below their original sintering temperatures. A thorough understanding of this sintering mechanism makes it possible to closely control the sintering behavior.

Robert C. Pullar

Papers

Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics

Magnetic properties of nanocrystalline CoFe2O4 powders prepared at room temperature: variation with crystallite size

MgWO4, ZnWO4, NiWO4 and CoWO4 microwave dielectric ceramics

Dielectric loss caused by oxygen vacancies in titania ceramics

A mechanism for low-temperature sintering