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- d'Heurle, F. M., et al.
(författare)
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Formation of C54TiSi(2) : Effects of niobium additions on the apparent activation energy
- 2001
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 90:12, s. 6409-6415
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Tidskriftsartikel (refereegranskat)abstract
- The formation of C54 TiSi2 using Ti-Nb alloys deposited on polycrystalline Si substrates was studied by means of in situ x-ray diffraction and resistance measurements during temperature ramping. Alloys with Nb contents ranging from 0 to 13.6 at. % were used. The formation temperature of C54 TiSi2 was reduced in the presence of Nb. However, the addition of Nb in Ti did not cause fundamental changes in the evolution of resistance versus temperature. This latter observation suggests that the mechanism for the formation of C54 TiSi2 remained the same in spite of the enhancement effect. For alloys with up to 8 at. % of Nb, the C49 TiSi2 phase formed first, as with pure Ti. When annealing the alloy with 13.6 at. % Nb, neither C49 TiSi2 nor C54 were found in the usual temperature ranges, instead, C40 (Nb,Ti)Si-2 was observed. This phase transformed to C54 (Nb,Ti)Si-2 above 950 degreesC. The apparent activation energy associated with the formation of C54 TiSi2 was obtained by annealing the samples at four different ramp rates from 3 to 27 K/s; it decreased continuously from 3.8 to 2.5 eV with increasing Nb content from 0 to 8 at. %. The apparent activation energy for the formation of C40 (Nb,Ti)Si-2 was found to be 2.6 eV. The possible physical meaning, or lack thereof, of the high activation energies derived from experimental measurements is extensively discussed. A qualitative model is proposed whereby nucleation would be rate controlling in pure TiSi2, and interface motion in samples with 8 at. % Nb.
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- Mergulhao, F. J. M., et al.
(författare)
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Medium and copy number effects on the secretion of human proinsulin in Escherichia coli using the universal stress promoters uspA and uspB
- 2003
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Ingår i: Applied Microbiology and Biotechnology. - : Springer Science and Business Media LLC. - 0175-7598 .- 1432-0614. ; 61:06-maj, s. 495-501
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Tidskriftsartikel (refereegranskat)abstract
- The use of the uspA and uspB promoters (universal stress promoters) for heterologous protein production in Escherichia coli is described. Best results were obtained with a moderate copy number vector (1560 copies) bearing the uspA promoter, reaching 4.6 mg/g dry cell weight (DCW) of ZZ-proinsulin secreted to the periplasm and 1.9 mg/g DCW secreted to the culture medium. These values are about 1.7-fold higher than those previously reported with the same ZZ fusion tag and the SpA leader peptide showing that these stress promoters are potentially valuable for recombinant protein secretion in E. coli. It is further demonstrated that the use of M9 minimal medium is advantageous for protein secretion as compared to LB rich medium.
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