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1.
Yoshio Ohashi Yuji Masubuchi Keiichiro Murai 《Journal of solid state chemistry》2011,184(8):2061-2065
New niobium oxynitrides containing either magnesium or silicon were prepared at 1000 °C by ammonia nitridation of oxide precursors obtained via the citrate route. The products had rock-salt type crystal structures. Crystallinity was improved by annealing in 0.5 MPa N2 and the final compositions were (Nb0.95Mg0.05)(N0.92O0.08) at 1500 °C and (Nb0.87Si0.09□0.04)(N0.87O0.13) at 1200 °C. The magnesium and oxide ions partially co-substitute the niobium and nitride ions in the octahedral sites of the δ-NbN lattice, respectively. Silicon ions were also successfully doped together with oxide ions into the rock-salt type NbN lattice. The Si doped product exhibited relatively large displacement at the octahedral sites and was accompanied by a small amount of cation vacancies. Superconductivity was improved by annealing to obtain critical temperatures/volume fractions of Tc=17.6 K/100% for Mg- and Tc=16.2 K/95% for the Si-doped niobium oxynitrides. 相似文献
2.
Noboru Kimizuka Motohiko Ishii Isao Kawada Masanobu Saeki Mitsuoki Nakahira 《Journal of solid state chemistry》1974,9(1):69-77
The effects of nonstoichiometry upon the behavior of vanadium dioxide single crystals in the vicinity of the semiconductor/metal transition temperature (Tc) were experimentally investigated. According to the electrical and thermal measurements, more stoichiometric vanadium dioxide exhibited the less electrical conductivity gap, the larger thermal and electrical hysteresis, and the lower transition temperature than the increased nonstoichiometric one near transition. Infrared absorptions and X-ray observations indicated the local and overall lattice distortion in the nonstoichiometric crystal due to the existence of V5+ ions. Furthermore, an intermediate phase between the low-temperature monoclinic and the high-temperature tetragonal phases was found in the nonstoichiometric VO2. On the other hand, no evidence for this intermediate phase was found in the stoichiometric one. Finally, some comparisons and discussions of our present data with the previously published ones were made. 相似文献
3.
The long spacing l of lamellar crystals of linear polyethylene increases with the crystallization temperature Tc. For low degrees of supercooling, the ratio Δl/ΔT is around 0.5 nm K?1 for PE single crystals obtained from solution in xylene. In the restricted situation where only conduction in the crystallization vessel is involved, a heat transfer analysis shows that about 20 s is needed to change by 5 K the crystallization temperature Tc in a cylindrical vessel of 1.5 mm radius. Such rapid change of the crystallization temperature induces a sharp increase or decrease of the thickness of the single crystals. After conventional shadowing with palladium–gold alloy, the steps on the crystals are observed by conventional bright-field electron microscopy. A pioneering work was performed in this way by Bassett and Keller in 1962. Our technique allows one to determine both the shape and the dimensions of single crystals or twinned crystals of polyethylene as a function of the time of crystallization, and therefore give the quasi-instantaneous growth rates at various times. 相似文献
4.
The rates of growth of polyethylene single crystals grown from dilute solution in hexadecane and tetradecanol have been measured over the temperature range Tc = 98–120°C by following the change in turbidity during crystallization of a suspension of crystals of known shape and final size. The rates decrease similarly with Tc in each solvent, but for a given supercooling crystals grow much faster in tetradecanol where the corresponding crystallization temperature is higher. Similarly, the rates are much higher in hexadecane than those previously reported from xylene at equivalent supercoolings but lower Tc. Changes in the corresponding crystal morphologies as Tc is raised are quantified in terms of the axial ratio and the degree of curvature of the nominally {100} faces, both of which increase with Tc. The results can be interpreted as showing a transition from regime I to regime II growth in both solvents, which agrees both qualitatively and quantitatively with the predictions of the nucleation-based kinetic theories. Such a transition has never before been reported for solution crystallization. Using this analysis, reasonable values are obtained for the crystal side-surface energy σ of 7.4–7.5 erg cm?2 and for the regime I substrate length L of 0.14 μm. No correlation is found between crystal morphology and growth rate and there are no discontinuous changes in morphology at the proposed transition points. The occurrence of curved crystal edges raises the fundamental issue of how to reconcile noncrystallographic growth surfaces with nucleation-controlled growth. A new approach to polymer crystal growth based on equilibrium surface roughening, which does not require nucleation, is therefore very pertinent in this respect and this is discussed. 相似文献
5.
Crystal orientation and melting behavior of poly(ε-caprolactone) in a diblock copolymer of poly(ε-caprolactone)-block-poly(2,5-bis[4-methoxyphenyl]oxycarbonyl)styrene) (PCL-b-PMPCS) was investigated. The degrees of polymerization of the PCL and PMPCS block are 200 and 98, respectively. With the PMPCS in a columnar liquid crystalline phase, the diblock is rod-coil one, which exhibits a lamellar phase morphology with the PCL layer thickness of 15.2 nm. Since the glass transition temperature of PMPCS block is much higher than the melting temperature of PCL, the crystallization of PCL is in a one-dimensionally "hard" confinement environment. Mainly on the basis of two-dimensional wide-angle X-ray diffraction experiments, we identified the orientation of PCL isothermally crystallized at various crystallization temperatures (Tcs). At high Tcs (Tc≥10℃), the c-axis of the PCL crystal is along the layer normal of the microphase-separated sturcture. Decreasing Tc can result in the tilting of PCL c-axis with respect to the layer normal. The lower the Tc is, the more the c-axis inclines. Meanwhile, the b-axis of PCL remains perpendicular to the layer normal. At a very low Tc of -78℃, the orientation of the PCL crystals is completely random. For the samples isothermally crystallized at Tc≤10℃, double melting behavior can be observed. While the low temperature endotherm reflects the melting of the crystals originally formed at the Tc applied, the high temperature one is associated with the crystals subjected to the process of recrystallization/reorganization upon heating due to the annealing effect. 相似文献
6.
H. Werheit A. Leithe-Jasper T. Tanaka K.A. Schwetz 《Journal of solid state chemistry》2004,177(2):575-579
Large and high-quality floating zone grown single crystals of boron carbide of the composition B∼4.3C corresponding to the carbon-rich limit B4.3C of the homogeneity range, were presented by Leithe-Jasper and Tanaka at the ISBB’99 in Dinard for the first time. Such crystals now allow determining the physical properties of this refractory semiconductor free from the influence of impurities or coarse structural imperfections. Some solid-state properties of these single crystals like electronic transport properties, IR optical absorption spectrum, IR reflectivity spectrum, FT-Raman spectrum, and Knoop hardness are presented and discussed with respect to the properties of less perfect boron carbide previously determined. Outstanding properties are the hardness exceeding that of technical boron carbide by 14% (∥c) and 24% (⊥c) respectively, and the high optical absorption at energies below the absorption edge and the reflectivity strongly increasing towards low frequencies suggesting a destinctly higher contribution of free carriers. 相似文献
7.
Ping Song Guangyi Chen Zhiyong Wei Wanxi Zhang Jicai Liang 《Journal of Thermal Analysis and Calorimetry》2013,111(2):1507-1514
The polymorphous crystallization and multiple melting behavior of poly(l-lactic acid) (PLLA) with an optical purity of 92 % were investigated after isothermally crystallized from the melt state by wide-angle X-ray diffraction and differential scanning calorimetry. Owing to the low optical purity, it was found that the disordered (α′) and ordered (α) crystalline phases of PLLA were formed in the samples crystallized at lower (<95 °C) and higher (≥95 °C) temperatures, respectively. The melting behavior of PLLA is different in three regions of crystallization temperature (T c) divided into Region I (T c < 95 °C), Region II (95 °C ≤ T c < 120 °C), and Region III (T c ≥ 120 °C). In Region I, an exothermic peak was observed between the low-temperature and high-temperature endothermic peaks, which results from the solid–solid phase transition of α′-form crystal to α one. In Region II, the double-melting peaks can be mainly ascribed to the melting–recrystallization–remelting of less stable α crystals. In Region III, the single endotherm shows that the α crystals formed at higher temperatures are stable enough and melt directly without the recrystallization process during heating. 相似文献
8.
The temperature and pressure dependences of the electrical resistivities in the single crystals of the niobium iodides, NbI5, NbI4 and Nb3I8, were measured. The resistivity in ab plane of Nb3I8 was 50 Ω cm and along the c axis was about 100 Ω cm. The activation energy was 0.26 eV at atmospheric pressure. The electrical resistivity along niobium chain in NbI4 was about 300 Ω cm. The resistivity ratio of the single crystal () is nearly 5. At around 150 kbar, only NbI4 showed the transition from insulator to metal. The relation between electrical properties and the crystal structure is discussed. 相似文献
9.
The influence of crystallization temperature (T c) on the number of spiral growths on poly(butylene succinate) (PBS) single crystals, obtained by self-seeding method, was systematically studied. The studies show that the statistical average number of spiral growths formed on the PBS single crystals decays exponentially with respect to the Tc. Inspired by BCF (Bruton, Cabrera and Frank) theory and L-H (Lauritzen and Hoffman) theory, a thermodynamic model has been proposed, in which the origin of spiral growth was treated as a nucleation process. The model suggests that the nucleation rate of spiral growth depends on the inverse square of super-cooling degree, which predicted the density of spiral growth formed on lamellae, and was consistent with the experiments very well. 相似文献
10.
Decomposition studies on ground samples of anhydrous Ba(N3)2 with defined particle size are described. The kinetic equations derived for single crystals hold for the decomposition of powders too. The far faster decomposition of powders is caused both by the increased numberN 0 of potential nuclei forming sites and the larger specific surfaceF 0/V 0, whereas the rate constantsk 1 andk 2 for nucleation and linear nucleus growth, resp., and their respective activation energies coincide with the data for single crystals. The proportionality between the rate of decomposition and the specific surface is confirmed experimentally and thereby a further proof of the geometric decomposition model is established. Independent of particle size and temperture always 75% of the azide are transformed into nitride during thermal decomposition, this value corresponding exactly to the theoretical one. It is shown experimentally that with decomposition conditions no reaction takes place between metallic Ba and N2 in its electronic ground state and therefore the mechanism proposed for nitride formation is confirmed. 相似文献
11.
G. Levchenko A. Lyashchenko A. Evdokimova Yu. Paderno 《Journal of solid state chemistry》2006,179(9):2949-2953
ScB2 single crystals were grown by inductive floating zone melting. The ScB2 structure was refined on single crystal and powder data, the latter obtained from parts of single crystals which were prepared by controlled crushing. The ScB2 structure corresponds to the AlB2 structure type, sp. gr. P6/mmm, No. 191 (R1=0.0191, wR2=0.0474), lattice parameters are equal to a=0.314820(3) nm, c=0.351483(5) nm, c/a=1.117, X-ray density is 3.670 g/cm3. The measured hydrostatic density is 3.666 g/cm3 which correspond to the Sc0.99B2 composition. The ScB2 Young modulus value is equal to 480 GPa and the Debye characteristic temperature is 1020 K. 相似文献
12.
The subject of this paper is the degree of crystallinity and annealing behavior of solution grown single crystals of isotactic polystyrene (IPS) in relation to the fold length, an enquiry which acquires special significance in view of the fact that previously the fold length had been found to be identical over a wide range of crystallization temperatures (Tc). It was found that both crystallinity and thermal stability increase with Tc even over the range of constant fold length thus invalidating the usual assumption that the fold length and crystal properties are uniquely correlated. Further, the crystallinity figures as measured by conventional calorimetry are very low (<50% throughout) which by conventional ideas would require an unrealistically thick amorphous surface layer. However, the “linear crystallinity” (crystal core thickness as determined from x-ray linewidths) is much larger, commensurate with crystallinities in single crystals from other materials. It is suggested that this is the more relevant figure for the subdivision of the lamellas into crystal core and surface layer. The additional amorphous content being otherwise accommodated. Further, this “linear crystallinity” is broadly unaffected by fold length changes induced by heat annealing. The thermal stability (including annealing ability) of the crystals differs markedly whether Tc is above or below ~60°C, a Tc value which is in the range where the fold length is constant, but corresponds to a maximum in the crystallization rate. Possible connections between crystallization conditions and the stability of the resulting crystals (fold length considerations apart) are pointed out. 相似文献
13.
《Radiation Physics and Chemistry》2006,75(11):1586-1590
Lifetime-broadening-suppressed X-ray absorption near-edge structures (LBS-XANES) spectra, where the resolution is not restricted by core-hole lifetime widths, have been measured at the Cu K-edge on the prototypical high-Tc La2−xSrxCuO4 (x=0, 0.15, and 0.29) single crystals. Thanks to improved resolution, 1s-3d excitation bands have been clearly separated from the main transitions and their concentration as well as polarization dependences are examined. 相似文献
14.
Maria Cristina Righetti Michele Laus Maria Laura Di Lorenzo 《Colloid and polymer science》2014,292(6):1365-1374
The multiple melting behavior of poly(ethylene terephthalate) (PET) is generally attributed to the fusion of original crystals recrystallized during the heating at conventional scanning rate. In the present study, the triple and double melting behavior that is observed after isothermal crystallization at T c lower and higher than 215 °C, respectively, is put in relation with the presence and absence of rigid amorphous fraction around the original primary crystal lamellae. The complex melting behavior is explained by assuming that two different morphologies of primary crystals develop during crystallization at temperatures lower than 215 °C, in a proportion that is a function of the crystallization temperature: chain cluster aggregations with a high percentage of rigid amorphous fraction on the boundaries and small crystals with a high percentage of adjacent reentry folding and reduced constraints at the amorphous/crystal interphase. These distinct morphologies differently transform upon heating at low scanning rate, originating two endotherms. On the contrary, after crystallization at T c ?>?215 °C, all the primary crystalline structure, which probably are characterized by the same morphology made of tightly chain folded lamellae and absence of rigid amorphous fraction, undergo the same reorganization route, originating a single endotherm. 相似文献
15.
A new simple method of evaluating non-isothermal crystallization kinetics is proposed. The procedure based on mathematical treatment of DSC cumulative crystallization curves at their inflection point provides three kinetic parameters: temperature of start of crystallization (Ts), temperature of maximum crystallization rate (Ti) and numerical value of the maximum crystallization rate (si), and also final crystallinity after cooling (CRc). The method is demonstrated on the system poly(ɛ-caprolactone)/poly(lactic acid)/clay C15 and related microfibrillar composite. The method provides the values of Ts and Ti with standard deviation σ = 0.3 and 0.4 °C, respectively. The coefficient of variation v of si and CRc is 5.8 and 1.5%, respectively. The proposed method does not refer to any crystallization model and does not require exact determination of the starting time. It is particularly useful for characterizing a series of samples derived by modification of the neat polymer. 相似文献
16.
Guerman N. Zhizhin 《Mikrochimica acta》1987,93(1-6):297-307
Examples of FT-IR applications for high-T
c
superconductor materials research are presented: determination of energy gap by IR reflection temperature dependence; plasma frequency estimation by SEW propagation experiment. The nuclear hyperfine structure of atomic impurities centers in crystals were studied by high resolution FT-IR. 相似文献
17.
H.W. Zandbergen 《Journal of solid state chemistry》1980,35(3):367-375
Results of neutron powder diffraction and magnetic measurements on single crystals of CsMnI3 are reported. Three-dimensional ordering takes place at Tc = 11.1(3) K. Above Tc very broad peaks occur in the neutron powder diffraction diagram, indicating one-dimensional correlations along the chain. Below Tc the Mn2+ ions are coupled antiferromagnetically along the chain. Interchain exchange leads to a 120° structure, slightly distorted due to anisotropy. One-third of the chains have their magnetic moment parallel to the c axis and the rest of the chains have magnetic moments making an angle of 50(2)° with the c axis. The magnetic moment as found from neutron diffraction extrapolated to 0 K is 3.7(1)μB, indicating a considerable zero-point spin reduction. The intrachain exchange was found to be ?9.1(1)K, whereas the ratio of the inter- to intrachain interaction was determined as . A spin flop occurs at H = 54 kOe on application of a magnetic field parallel to the c axis. When a field perpendicular to the c axis is applied a spin reorientation occurs at 1 kOe. 相似文献
18.
The non-isothermal devitrification of Na2O · 2 CaO · 3 SiO2 glass has been studied by differential thermal analysis in order to evaluate, from DTA curves, the temperature of maximum nucleation rate, Tm, and the activation energy values, Ec, for crystal growth.The temperature, Tm=580°C, is very close to the glass transition temperature, Tg=570°C, and the value of Ec=78 Kcal mole?1 for the surface crystal growth is nearly the same as the value Ec=89 kcal mole?1 for the bulk crystal growth; both are consistent with the activation energy for viscous flow. It is also pointed out that the nucleation rate—temperature curve and the crystallization rate—temperature curve are partially overlapped. 相似文献
19.
A mathematical model is presented to describe nonstoichiometric water‐soluble polyelectrolyte complexes, and the predictions are compared with some experimental results. The theory is a mixture of Madelung's theory for ionic crystals and Manning's counterion condensation theory. The central parameters are the degree of complexation, φ, and the degree of counterion binding, θ. All other quantities are known in principle. It is found that there is a competition between complexation and counterion binding. When φ is large, θ is small, or vice versa. The degree of complexation, φ, depends sensibly on the concentration, cs, of the added low molecular salt, the polyanion chain length, N, and the dielectric constant, ϵ, of the solvent. There exists a critical salt concentration, cs,c, at which the complexes salt out and where for cs > cs,c the complexes dissociate back into their single strands, the polyanions, and polycations. Further, φ is larger the smaller the polyanion length and the smaller the solvent dielectric constant are. To prove these predictions we have formed nonstoichiometric complexes between IONENE and PAA and IONENE and PMAA, respectively. The degree of complexation was determined by ultracentrifugation and checked by viscometry. The accord found between theory and experiment is both qualitatively as well quantitatively quite well. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 335–348, 1999 相似文献
20.
《Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy》1977,33(2):213-220
Raman spectra investigation of SrNO3 single crystals cut in two different orientations allows an unambiguously determination of Raman modes symmetry species. The two hypothesis of the crystal structure are discussed, the factor group Th allowing a better interpretation of experimental data than the factor group T.A systematic temperature study of Raman bands (frequencies, broadness, integrated intensities) conducts to the separation of internal and external modes and shows a singularity in the 173–183° K temperature region. 相似文献