Alpha structure metastable

In addition to the thermal CVD reactions listed above, tungsten can be deposited by plasma CVD using Reaction(l)at350°C.[ ll P At this temperature, a metastable alpha structure (aW) is formed instead of the stable be.c. Tungsten is also deposited by an excimer laser by Reaction (1) at < 1 Torr to produce stripes on silicon substrate.P l... 

The beta crystalline form of isotactic polypropylene differs from the alpha form by having a lower crystalline density and lower melting point. The beta form is metastable to the alpha form and will rearrange to the alpha structure when heated to approximately 100 °C or placed under strain. Beta lamellae form parallel stacks as shown in Fig. 19.4. 

Crystalline selenium exhibits two monochnic forms an alpha form constituting dark red transparent crystals, density 4.50 g/cm. The alpha form converts to a metastable beta form of hexagonal crystal structure when heated to about 170°C. Both the crystalline forms are insoluble in water soluble in sulfuric and nitric acids very slightly soluble in carbon disulfide. Also, both the crystalline forms convert into gray metallic modification on heating. 

Stratification of asymmetric aqueous films from NaDoS, CTAB and a commercial surfactant (alpha-olefin sulphonate) solutions at C > Ccmc on a decane substrate have been studied by Bergeron and Radke [236], They found three transitions by thickness in the metastable multilayer films. They observed also stratified CTAB aqueous films on glass. The n(/i) isotherms of stratified films were analysed considering also the oscillatory structural component of disjoining pressure. 

Slow cooling from the melt leads to the B-form, which has a melting point of 228°C. Annealing above 228 C converts the fi-form to the orform. The -j-form Is metastable and arises from orientation. In this modification the polymer backbone has a planar zig-zag structure and Is extended 1.6 times with respect to the alpha-form helix. Upon annealing It reverts to the alpha-form with concomitant shrinkage. As would be expected, this reversibility has a strong influence upon the properties of polyplvalolactone. 

TVansition Phases of Metastable Beta Alloys. The decomposition of metastable beta into a Type 1 or Type 2 alpha may be preceded by the formation of an isothermal omega ((Oigo) phase or the splitting of the beta phase into solute-rich and solute-lean regions. Occurrence of these transition products indicates the difficulty of the direct beta-to-alpha decomposition, possibly because the hexagonal-dose-packed structure of the alpha phase... 

Alumina is hexagonal close-packed with a corundum structure. Several metastable structures exist, but they all ultimately irreversibly transform to the hexagonal alpha phase. 

Beta phase alloys. Beta phase alloys are usually metastable, formable as quenched, and can be aged to the highest strengths but then lack ductility. Fully stable beta alloys need large amoimts of beta stabilizers (vanadium, chromium and molybdenum) and are therefore too dense. In addition, the modulus is low (<100 GPa) unless the beta phase structure is decomposed to precipitate the alpha phase. They have poor stability at 200 to 300° C, have low creep resistance, and are difficult to weld without embrittlement. Metastable beta alloys have some application as high-strength fasteners. 

Ruberto C, Yourdshahyan Y, Lundqvist BI (2003) Surface properties of metastable alumina a comparative study of kappa- and alpha-Al203. Phys Rev B Condens Matter 67(19) 195412 Lin Y, Becerra-Toledo AE, Silly F, Poeppelmeier KR, Castell MR, Marks LD (2011) The (2 X 2) reconstructions on the SrTiOa (001) surface a combined scanning tunneling microscopy and density functional theory study. Surf Sci 605(17-18) L51-L55 Becerra-Toledo AE, Marshall MSJ, Castell MR, Marks LD (2012) c(4 x 2) and related structural units on the SrTi03 (001) surface scanning tunneling microscopy, density functional theory, and atomic structure. J Chem Phys 136(21) 214701... 

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