Hafnia

The need to stabilize hafnia makes it difficult to deposit this oxide by CVD and CVD has been used so far mostly on an experimental basis. Its characteristics and properties are summarized in Table 11.3. 

Chemical Resistance. Stabilized hafnia is resistant to oxidation but is particularly susceptible to oxygen diffusion, through oxygen vacancies in the lattice. In other words, it is not a good oxygen barrier. It is not attacked by most chemical reagents at room temperature. 

Hafnia can also be deposited by the pyrolysis of a metallo-organic such as hafnium acetylacetonate, ( 511702)3, at 400-750°C, or hafnium trifluoro-acetylacetonate, Hf(C5H402F3)4, at 500-550°C with helium and oxygen as carrier gases. 

Hafnia good fair good poor good... 

Other refractory oxides that can be deposited by CVD have excellent thermal stability and oxidation resistance. Some, like alumina and yttria, are also good barriers to oxygen diffusion providing that they are free of pores and cracks. Many however are not, such as zirconia, hafnia, thoria, and ceria. These oxides have a fluorite structure, which is a simple open cubic structure and is particularly susceptible to oxygen diffusion through ionic conductivity. The diffusion rate of oxygen in these materials can be considerable. 

Name from Hafnia (Latin = Copenhagen), as both researchers correctly interpreted their results at the institute of Niels Bohr. 

Hafnium (Hf, [Xe]4/ l45 /26.r), name and symbol from the Latin name of Copenhagen (Hafnia). Discovered (1923) in Copenhagen by Dirk Coster (Danish) and Georg Karl von Hevesy (Hungarian). 

ORIGIN OF NAME Named after Hafnia, the Latin name for the city of Copenhagen, Denmark. 

Compounds 146-149 were examined for their iron-transport properties in a series of microorganisms including P. agglomerans, Hafnia and E. coli. All compounds were active however, they behaved as fenioxamine in H. alvi, as coprogen in E. coli and both as ferrioxamine and coprogen in P. agglomerans . In order to increase the differential... 

In addition to requiring high dielectric films for DRAM capacitors (dynamic random access memories) and for the active memory elements in FRAMs, the microelectronics industry has a stated demand for a replacement for Si02 gate oxides very soon. The leading candidate is hafnia (Hf02), and there are significant opportunities for the ferroelectrics community to contribute to the solution of this problem. 

Fig. 3. Top Resolution-enhanced 600 MHz H NMR spectrum of the octasaccharide repeating unit from Hafnia alvei strain 2 O-specific polysaccharide, (a)-(g) 500 MHz ID TOCSY subspectra with selective excitation of the respective anomeric proton resonances, (h) ID TOCSY subspectrum with the H3eq resonance of the neuraminic acid residue selectively excited. (Reproduced (adapted) with permission from Gamian et al. [25J. Copyright 1991...

Hafnium was discovered in 1922 by Coster and deHevesy. They named it for Hafnia, the Latin word for Copenhagen. It is found in aU zirconium ores, such as zircon, (ZrSi04) and baddeleyite (Zr02). It occurs in the earth s crust at about 3 mg/kg. Its average concentration in sea water is 7 ng/L. 

Formula Hf02 MW 210.49 Synonym hafnium(lV) oxide hafnia... 

The causative bacteria in the intestinal tract may be Salmonella typhimurium. Shigella flexneri. Shigella sonnei, Campylobacter jejuni, Campylobacter lari, Chlamidia trachomatis. Yersinia ente-rocolitica. Staphylococcus aureus, Hafnia alvei or the protozoan pathogen Cryptosporidium. 

Professor von Hevesy and Thai Jantzen separated hafnia from zirconia by repeated recrystallization of the double ammonium or potassium fluorides (20, 26). Metallic hafnium has been isolated and found to have the same crystalline structure as zirconium. A small specimen of the first metallic hafnium ever made is on permanent display at the American Museum of Natural History in New York City. Dr. von Hevesy, who prepared it, presented it to the Museum for the collection of chemical elements (29). A. E. van Arkel and J. H. de Boer prepared hafnium by passing the vapor of the tetraiodide over a heated tungsten filament (26, 30). 

W. Jachymek, J. Czaja, T. Niedziela, C. Lugowski and L. Kenne, Structural studies of the -specific polysaccharide of Hafnia alvei strain PCM 1207 lipopolysaccharide, Eur. J. Biochem., 1999, 266, 53-61. 

Whereas zirconium was discovered in 1789 and titanium in 1790, it was not until 1923 that hafnium was positively identified. The Bohr atomic theory was the basis for postulating that element 72 should be tetravalent rather than a trivalent member of the rare-earth series. Moseley s technique of identification was used by means of the x-ray spectra of several zircon concentrates and lines at the positions and with the relative intensities postulated by Bohr were found (1). Hafnium was named after Hafnia, the Latin name for Copenhagen where the discovery was made. 

On heating to 1500 °C/6 h/Ar, the Zr material crystallizes to a mixture of monoclinic and tetragonal zirconia and crystobalite with loss of considerable original surface area (36 m2 g 1). The Hf material behaves similarly, although it partially crystallizes at 1000 °C to produce cubic or tetragonal hafnia. Cristobalite is only observed in materials heated to 1400 °C. Finally, thin films of the Zr and Hf derivatives could be cast from hydrocarbon solutions on quartz and then converted to thin films of the corresponding amorphous or ceramic materials. 

Hafnium Hf 72 Dirk Coster,G.von Hevesy Denmark Latin name "Hafnia" meaning "Copenhagen"... 

Hafnium was discovered in 1923 by Danish chemist Dirk Coster working together with Hungarian physicist Gyorgy K. Hevesy. The electronic structure of hafnium had been predicted by Niels Bohr, and Coster and Hevesy found evidence of a substance whose pattern matched what had been predicted. The element predicted by Bohr was finally identified as being part of the mineral zircon by means of x-ray spectroscopy analysis. Due to its discovery in Copenhagen (whose ancient Latin name was Hafnia), the element was named hafnium. 

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