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1.
Abstract Optical reflection spectra are measured for Ba 8Ag xSi 46?x (0 ≤ x ≤ 6) at room temperature. A systematic decrease in the plasmon energy is found with increasing x, indicating that the carrier concentration decreases with increasing x. When x increases, the superconducting transition temperature Tc decreases. An origin of the decrease in Tc is assigned to the decrease in the density of states at the Fermi level due to the decrease in the carrier concentration. 相似文献
2.
Abstract Superconducting critical temperature, Tc and the shielding volume fraction, SVF, of layered nitride superconductor Na xHfNCl have been studied as a function of x, i.e. Na concentration. Although Tc decreases gradually with increasing x from 20.0 K at x=0.11 to 16.5 K at x=0.85, SVF has a sharp peak around x=1/6, where strong coupling between local ordering of Na atoms and Fermi surface instability can be expected. Structural disorder in the samples above x=0.5, observed by powder neutron diffraction, does not affect the superconductivity appreciably. Electronic specific heat coefficient, γ, is estimated to be about 7.7 mJ/mol/K 2 by its difference of magnetic susceptibility between HfNCl and Na 0.5HfNCl. The γ value is relatively small compared with the high Tc value, revealing double honeycomb lattice system as new potential higher Tc superconductor series by intercalation. 相似文献
3.
Abstract Correlations among lattice parameter, external pressure, and critical temperature Tc has been surveyed on fullerene superconductors with a wide range of lattice parameters and various valence states. The observed value of dTc/dP for Na 2Rb 0.5Cs 0.5C 60 and Li 3CsC 60, having small interfullerene separations, is about a few times lager than that of K 3C 60. In contrast, small dTc/dP values are found in fullerides with expanded unit cells, such as (NH 3) xNaAA ?C 60 (A, A ? = K, Rb and Cs) and A 3Ba 3C 60. Interestingly, the chemical and physical pressure effects on Tc are considerably different in these fullerides with large interfullerene spacings. Our results suggest that the pressure dependence is scaled by an interfullerene distance when the interfullerene distances is small, but that this scaling fails when the unit cell is expanded. 相似文献
4.
The optical properties of Si clathrate, which is believed to have potential as a new Si-based material for optical devices, were investigated in this study. Si clathrates with type II structure (Na xSi 136) with low Na content ( x = 1.3, 2.0) were synthesized by thermal decomposition of NaSi. The synthesized samples were purified by centrifugation using a solution of CH 2Br 2–C 2Cl 4 to remove impurities. Using the obtained high purity samples (Na 1.3Si 136, 93 wt.%; Na 2.0Si 136, 90 wt.%), the optical absorption spectra of Na xSi 136 were clarified, for the first time, from diffuse reflection measurements. The inclusion of Na in Si 136 was found to cause free carrier absorption in the infrared region but had little effect on the fundamental absorption edge found in the visible region. 相似文献
5.
A reaction mechanism will be suggested for interpreting Si 1–xGe x CVD layer deposition from a silicon source and germane reaction gas mixtures in order to explain the observed silicon layer growth increase in consequence of the presence of germane. A successive substitution of the hydrogen atoms available in germane molecules by silyl groups forming a silicon containing intermediate of germane will be assumed. It will further be supposed that both the original silicon source and the germane intermediates will independently be decomposed by chemical reactions leading to Si 1–xGe x CVD layer deposition from dichlorosilane-germane-hydrogen reaction gas mixtures at temperatures in the range of 600 ≦ T(°C) ≦ 900 as recently published by KAMINS and MEYER . 相似文献
6.
Abstract Raman spectra have been measured on Ba xC 60 binary systems for x=3, 4 and 6. The pentagonal pinch Ag(2) mode exhibits a softening in an approximately proportional manner to the formal valence of C 60 molecule. This result suggests that the valence of C 60 in Ba doped systems are understood by the naive ionic crystal model based on Ba 2+. The broader Raman peaks in superconducting Ba 4C 60 indicates that the electron-phonon interaction is considerably large comparing to those for Ba 3C 60 and Ba 6C 60. 相似文献
7.
Si crystals and nano-rods were formed in Al-added amorphous Si films (a-Al xSi 1−x; 0.025 x 0.100) by the irradiation of a focused electron-beam; the films were in situ heated to be kept at 400 °C and the current density of the electron-beam was 15.7 pA/cm 2. The size, shape, and concentration of the Si crystallites were varied sensitively with the Al content as well as the irradiation time. Under the electron-beam irradiation, crystallization occurred to produce polycrystalline phases in the a-Al 0.025Si 0.975 film, while rod-shaped Si nanostructures were formed in the a-Al 0.050Si 0.950 and a-Al 0.100Si 0.900 film. It is evident that the removal of Al and as a result the atomic rearrangements and/or local restructuring in the Al/a-Si film are critically affected by the electron-beam irradiation, which lead to the local crystallization and growth of Si nanocrystallites. 相似文献
8.
The previously published “three-partial rates” model of Si 1–xGe x layer growth (Kühne 1992) has been extended for describing Si 1–(x+y)Ge xB y ternary systems by taking additionally into account a partial rate of boron and a partial rate of boron-originated silicon deposition. Silicon deposition from a silane-germane-diborane-helium gas mixture at T = 500°C and Ptotal = 0.2 Torr is clearly enhanced by the presence of germane though to a low degree only, but not by diborane. Germanium partial rate as well as boron partial rate depends linearly on the partial pressure of the respective source gas. For illustration the data published by Murase have been used. 相似文献
9.
It has been found that simultaneous doping of Tl and Bi at Hg site of HgBa 2Ca 2Cu 3O 8+δ HTSC phase induces interesting microstructural variants in the form of long period polytypoid (LPP) like structure embodying native defect substructures. It has been observed that the as synthesized (HgTl 0.2−xBi x)Ba 2Ca 2Cu 3O 8+δ (with x = 0.05, 0.10, 0.15) phases have superconducting onset transition temperature (T c) of about ∼133 K. However, the intragrain critical current density (J c) for the various materials with different compositions varies significantly. It varies from 6.2 × 10 6 A/cm 2 to 2.9 × 10 6 A/cm 2 at 5 K and 1 T for (HgTl 0.10Bi 10)Ba 2Ca 2Cu 3O 8+δ and (HgTl 0.15Bi 0.05)Ba 2Ca 2Cu 3O 8+δ HTSC phase respectively. A correlation between the intragrain J c and the defect substructures has been found to be present. This correlation has been described and disscussed. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
The problem of growth dynamics for crystals of binary solid solutions, obtained by the constitutional supercooling of the
melt with a silicon feeding rod, has been solved within the Pfann approximation. The dependences of the change in the axial
growth rate of Ge 1−x
Si
x
crystals (0 ≤ x ≤ 0.3) are calculated. It is shown that the Ge 1−x
Si
x
crystallization rate significantly changes during growth. The results make it possible to determine the optimal conditions
and technological parameters for growing Ge 1−x
Si
x
single crystals (0 ≤ x ≤ 0.3) with a specified concentration gradient along the crystallization axis. 相似文献
11.
In the system BaO-Al 2O 3-SiO 2 a metastable solid solution Ba 1−0,5xAl 2−xSi 2+xO 8 next two stable and one metastable modification of composition BaAl 2Si 2O 8 was observed. The discrete differences in nucleation behaviour and anisotropy of crystal growth of these crystalline phases favoury the formation of the solid solution. The production of defined crystals of metastable solid solution or of the needle-shaped stable low temperature modification is of relevance for the thermal stability of ceramics and glass ceramics. 相似文献
12.
The problem regarding the distribution of aluminum and indium impurities in bulk crystals of solid solutions with a variable composition Ge1−x
Si
x
(0 ≤ x ≤ 0.3) is solved in order to establish regularities of the changes in the segregation coefficients of impurities with variations in the composition of the host lattice in the germanium-silicon system. Aluminum-and indium-doped crystals of Ge1−x
Si
x
(0 ≤ x ≤ 0.3) solid solutions with a silicon content decreasing along the crystallization axis are grown by a modified Bridgman method with the use of a silicon seed. The concentration distribution of impurities over the length of the crystals is determined from Hall measurements. It is demonstrated that the experimental data on the concentration distribution of impurities in the crystals are in good agreement with the results obtained from the theory according to which the equilibrium segregation coefficients of impurities vary linearly with a change in the composition of Ge-Si solid solution crystals. 相似文献
13.
It is shown by Hall measurements that quenching complexly doped Ge 1 − x
Si
x
〈Cu, Al〉 (0 ≤ x ≤ 0.20) crystals from 1050–1080 K leads to the formation of additional electroactive acceptor centers in them. The activation
energy of these centers increases linearly with an increase in the silicon content in the crystal and is described by the
relation E
k
x
= (52 + 320 x) meV. Annealing these crystals at 550–570 K removes the additional acceptor levels. It is established that the most likely
model for the additional electroactive centers is a pair composed of substituent copper and aluminum atoms (Cu
s
Al
s
) or interstitial copper and substituent aluminum atoms (Cu
i
Al
s
). It is shown that the generation of additional deep acceptor levels must be taken into account when using the method of
precise doping of Ge 1 − x
Si
x
〈Al〉 crystals with copper. 相似文献
14.
With the aid of the methods of the statistical design of experiments for each wafer site ( x) on the graphite susceptor an empiric polynomial was obtained for CVD epitaxial silicon from SiH 4–HCl–H 2 mixtures, linking the growth rate vepi to the process parameters p0, p0HCl, and T( x). By means of the statistical model optimum temperatur characteristics along the susceptor were selected and discussed for specific deposition conditions. 相似文献
15.
We have recently prepared solid solutions of Ba ~6.3Ca ~0.7F 12Cl 2?xBr x with x ranging from 0 to 2. In this work, the synthesis and single crystal X-ray structure of calcium-free crystals of Ba ~6.9Na ~0.2F 12Br 0.6Cl 1.4 (space group P6 3/m, a = 10.6024(10), c = 4.2034(4) Å), Ba ~6.9Na ~0.2F 12Br 1.4 Cl 0.6 (space group P6 3/m, a = 10.6155(9), c = 4.2355(4) Å) and Ba ~6.9Na ~0.2Br 1.32Cl 0.68F 12 (space group P6 3/m, a = 10.6218(9), c = 4.2284(4) Å) are reported.These crystals systematically present additional electron density at the 0 0 0.25 position which is associated with the presence of small, but significant amounts of Na + ions in the crystal. 相似文献
16.
The influence of silicon on the crystallographic phases of the binary niobium-gallium system is studied by X-ray diffraction methods. The samples were prepared by rapid quenching. Silicon substituted gallium in all phases of the binary system. It exists a complete pseudobinary solid solution Nb 5Ga 3-Nb 5Si 3. The maximum degree of substitution in the A 15 phase Nb 3Ga 1–xSi x is x = 0.5. The presence of two A 15 phases with different cell parameters indicates the existence of a miscibility gap. Contrary to the expections the supercondusting transition temperatures decreases with an increasing degree of substitution. 相似文献
17.
Alternating differential scanning calorimetric (ADSC) studies and electrical switching experiments have been undertaken on Al 15Te 85−xSi x (2 ? x ? 12) system of glasses. These glasses are found to exhibit two crystallization reactions ( Tc1 and Tc2), for compositions with x < 8. Above x = 8, a single-stage crystallization is seen. Further, a trough is seen in the composition dependence of non-reversing enthalpy (Δ HNR), based on which it is proposed that there is a thermally reversing window in Al 15Te 85−xSi x glasses, in the composition range 4 ? x ? 8. Electrical switching studies indicate that Al 15Te 85−xSi x glasses exhibit a threshold type electrical switching at ON state currents less than 2 mA. Further, the switching voltages are found to increase with the increase in silicon content. It is interesting to note that the start ( x = 4) and the end ( x = 8) of the thermally reversing window are exemplified by a kink and a saturation in the composition dependence of switching voltages, respectively. 相似文献
18.
The displacements of Ba 2+ cations in the cationic motif of Ba 0.78Tm 0.22F 2.22 crystals, which are representatives of nonstoichiometric fluorite phases Ba 1 ? x R x F 2 + x , are proved for the first time with the use of precision investigations of the fine atomic structure. It is shown that the cation displacements, like the previously revealed displacements in the anionic motif, reflect the formation of {Ba 8[ R 6F 68–69]} superclusters of structural defects with nanometer linear sizes. The Ba 2+ cations are displaced from the fluorite crystallographic positions 4 a (space group Fm $ \bar 3 The displacements of Ba2+ cations in the cationic motif of Ba0.78Tm0.22F2.22 crystals, which are representatives of nonstoichiometric fluorite phases Ba1 − x
R
x
F2 + x
, are proved for the first time with the use of precision investigations of the fine atomic structure. It is shown that the
cation displacements, like the previously revealed displacements in the anionic motif, reflect the formation of {Ba8[R
6F68–69]} superclusters of structural defects with nanometer linear sizes. The Ba 2+ cations are displaced from the fluorite crystallographic positions 4a (space group Fm
m) to the positions 32f. The static nature of the cation displacements is confirmed by the fact that these displacements are retained at a temperature
of 110 K. The correctness of the interpretation of the correlation between the cation displacements and the formation of superclusters
of structural defects is supported by the coincidence of the intercationic distances determined in the disordered phase Ba0.78Tm0.22F2.22 with those found in the previously studied ordered phases Ba4
R
3F17 (R = Y, Yb). The model with splitting of cationic positions is appropriate for testing in structural investigations of crystals
of the fluorite phases M
1 − x
R
x
F2 + x
at room temperature.
Original Russian Text ? A.M. Golubev, L.P. Otroshchenko, V.N. Molchanov, B.P. Sobolev, 2008, published in Kristallografiya,
2008, Vol. 53, No. 6, pp. 1023–1030. 相似文献
19.
Sol-gel processing of ceramic materials for advanced applications involves several steps starting from precursor synthesis and ending up with multicomponent metal oxides. A simple sol-gel synthesis technique has been refined to prepare the precursors for the superconducting (Y 1−xSc x)Ba 2Cu 4O 8 and (Y 1−xGa x)Ba 2Cu 4O 8 compounds. The amorphous gel powders were characterized by powder X-ray diffraction, infrared spectroscopy, thermal analysis and elemental analysis. A systematic characterization of precursor gels led us to predict the approximate composition and the chemical reactions involved during gelation. The stability and high level of homogeneity obtained for the gels make them suitable as processable precursors to substituted (Y 1−xSc x)Ba 2Cu 4O 8 and (Y 1−xGa x)Ba 2Cu 4O 8 superconducting compounds. 相似文献
20.
Results on ZnSe, ZnSe xS 1−x and ZnS crystal growing from the vapour phase up to 7.5 cm 3 in volume are described. Crystals were grown on sapphire, ZnS, ZnSe xS 1−x and quartz glass substrates without a contact of the growing crystal with a growth ampoule and using the molten tin as a heating medium. Large high-purity crystals with a density of etch pits of 10 3 cm −2 were obtained They exhibited an effective exciton luminescence and rather high radiation efficiency (30 ± 10% for ZnSe at T = 77 K). This made it possible to use these crystals for fabricating laser screens for a cathode ray tube. The main laser parameters obtained on a ZnSe screen at T = 80 and 300 K using a 75 keV electron beam excitation are presented. The light power output reached 0.8 W at T = 80 K; this allowed to obtain a 10 cd · m −2 TV image of 1.5 × 2 m 2 in area. 相似文献
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