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排序方式: 共有420条查询结果,搜索用时 31 毫秒
61.
用强碱性阴离子交换树脂制备了硼氢阴离子交换树脂还原剂(BER),使其在氯化铜等金属化合物的催化下还原一些芳香硝基、氰基、氮氮双键等含氮功能基化合物.还研究了BER的制备条件:如温度、溶剂、不同金属化合物对还原反应的影响,并初步探讨其反应机理. 相似文献
62.
The fluorescence decay of the 4F3/2 transition in neodymium-doped silica optical fibers is analyzed with respect to neodymium concentration and temperature. New experimental data are compared with theoretical calculations based on (i) Grant's supposition [W.J.C. Grant, Phys. Rev. B 4 (1971) 648] and (ii) the Förster [T. Förster, Ann. Phys. (Paris) 2 (1948) 55] and Dexter [D.L. Dexter, J. Chem. Phys. 21 (1953) 836] model. Grant's predictions are in reasonable agreement with the data. The nonexponential decay predicted by Förster and Dexter is insufficient to explain the experimental data. 相似文献
63.
Y16I19C8B4 – a Yttrium Boride Carbide Halide Containing B2C4 Units The new compound Y16I19C8B4 was prepared from Y, YI3, C and B at 1050–1150 °C. The structure of a twinned crystal was determined by means of X-ray diffraction (space group P 1¯, a = 12.311(2) Å, b = 13.996(3) Å, c = 19.695(3) Å, α = 74.96(2)°, β = 89.51(2)°, γ = 67.03(2)°, Z = 2). Y16I19C8B4 is a semiconductor and contains nearly planar B2C4 units which are located in cages built up by 12 yttrium atoms. Assuming (B2C4)12–, these units can be regarded as isoelectronic with B2F4. The yttrium cages are connected via faces to form rods, which are surrounded by iodine atoms. Bridging iodine atoms connect the rods so that layers are formed. The characteristic twinning observed can be understood from the geometry of the crystal structure. 相似文献
64.
Thomas Jüstel Hans Nikol Cees Ronda 《Angewandte Chemie (International ed. in English)》1998,37(22):3084-3103
Owing to their use in fluorescent lamps and many display applications, luminescent materials affect our daily life. Improvement of already very mature as well as development of new materials demanded by a variety of new applications are the focus of research today. The latest advances in the field of electroluminescence enable new displays and light generation concepts that challenge the classical areas of application of luminescent materials. 相似文献
65.
Zn11Rh18B8 and Zn10MRh18B8 with M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Al, Si, Ge and Sn are obtained by reaction of the elemental components in sealed tantalum tubes at 1500 K. They crystallize tetragonally with Z = 2 in the spacegroup P4/mbm with lattice constants a = 1771.2(2) pm, c = 286.40(4) pm for Zn11Rh18B8 and in the range a = 1767.65(9) pm, c = 285.96(3) pm (Zn10NiRh18B8) to a = 1774.04(9) pm, c = 286.79(2) pm (Zn10SnRh18B8) for the quaternary compounds. According to powder photographs all compounds are isotypic. Struture determinations based on single crystal X-ray data were performed with Zn11Rh18B8, Zn10FeRh18B8 and Zn10NiRh18B8. The structure of Zn11Rh18B8 is related to the Ti3Co5B2 type. Along the short axis planar nets of rhodium atoms composed of triangles, squares, pentagons and elongated hexagons alternate with layers containing the boron and zinc atoms. The rhodium atoms form trigonal prisms centered by boron atoms, two kinds of tetragonal and pentagonal prisms centered by zinc atoms and elongated hexagonal prisms containing pairs of zinc atoms. In the quaternary compounds Zn10MRh18B8 the zinc atoms in one sort of tetragonal prisms are replaced by M atoms. 相似文献
66.
Yuan-Yuan Jin 《中国物理 B》2022,31(11):116104-116104
The recent discovery of the novel boron-framework in boron-rich metal borides with complex structures and intriguing features under high pressure has stimulated the search into the unique boron-network in the metal monoborides or boron-deficient metal borides at high pressure. Herein, based on the particle swarm optimization algorithm combined with first-principles calculations, we thoroughly explored the structural evolution and properties of TiB up to 200 GPa. This material undergoes a pressure-induced phase transition of $Pnma$ $\to $ $Cmcm$ $\to $ $Pmmm$. Besides of two known phases $Pnma$ and $Cmcm$, an unexpected orthorhombic $Pmmm$ structure was predicted to be energetically favored in the pressure range of 110.88-200 GPa. Intriguingly, the B covalent network eventually evolved from a one-dimensional zigzag chain in $Pnma$-TiB and $Cmcm$-TiB to a graphene-like B-sheet in $Pmmm$-TiB. On the basis of the microscopic hardness model, the calculated hardness ($H_{\rm v}$) values of $Pnma$ at 1 atm, $Cmcm$ at 100 GPa, and $Pmmm$ at 140 GPa are 36.81 GPa, 25.17 GPa, and 15.36 GPa, respectively. Remarkably, analyses of the density of states, electron localization function and the crystal orbital Hamilton population (COHP) exhibit that the bonding nature in the three TiB structures can be considered as a combination of the B-B and Ti-B covalent interactions. Moreover, the high hardness and excellent mechanical properties of the three TiB polymorphs can be ascribed to the strong B-B and Ti-B covalent bonds. 相似文献
67.
Gunther Mair Hans Georg von Schnering Michael Wrle Reinhard Nesper 《无机化学与普通化学杂志》1999,625(7):1207-1211
Li2B6 is formed from the elements as transparent red microcrystalline compound (Li : B = 1 : 3; Mo crucible in closed Nb ampoule; 1723 K; 4 h). Single crystals are grown from a lithium silicide melt with large Li excess at 1923 K. Li2B6 is a semiconductor with electron as well as Li+ ionic conductivity which dominates above 600 K. Microcrystalline samples react with H2O liberating gases and forming a brownish amorphous product, but larger crystals are not very sensitive. – Li2B6 crystallizes tetragonally in a new tP16 structure type which is a variant of the CaB6 structure (a = 5.975 Å, c = 4.189 Å; Z = 2; space group P4/mbm). The [B62–] net of the polymeric octahedro-anion is slightly distorted to give space for the insertion of a (32434) net of the Li+ cations in the cavities (d(B–B)endo = 1.766 Å; d(B–B)exo = 1.720 Å; d(Li–B) = 2.363 Å; d(Li–Li) = 3.094 Å). The incomplete occupancy of the Li position (80%) and the electron density at a further position (20%) indicate the mobility of the Li+ cations. 相似文献
68.
A. C. Yanes J. J. Velázquez J. del-Castillo J. Méndez-Ramos V. D. Rodríguez 《Journal of Sol-Gel Science and Technology》2009,51(1):4-9
Nanostructured transparent glass-ceramics with composition of 95SiO2–5LaF3 co-doped with 0.3Yb3+, 0.1Ho3+ and 0.1Tm3+ (mol%) were synthesized by thermal treatment of precursor sol–gel derived glasses. X-ray diffraction and transmission electron
microscopy analysis point out the precipitation of hexagonal LaF3 nanocrystals with diameter ranging from 11 to 20 nm in these nano-glass-ceramics. White light generation by means of efficient
blue, green and red up-conversion luminescence under infrared excitation at 980 nm was observed and involved mechanisms were
analyzed. Colour tuneability is achieved by varying the up-conversion emission ratios as a function of pump power. 相似文献
69.
Bambar Davaasuren 《Journal of solid state chemistry》2009,182(6):1331-5692
The lanthanum iron carbide La3.67[Fe(C2)3] was prepared from the elements by argon arc-melting followed by annealing. The crystal structure of the ternary phase was reported previously (space group P63/m with a=878.7(2) pm, and c=535.1(1) pm) [A.M. Witte, W. Jeitschko, Z. Naturforsch. 51b (1996) 249-255]. In the present work the compound was reinvestigated by X-ray powder and single crystal diffraction, and was further characterized by metallographic methods and chemical analyses. Our diffraction data clearly reveal a superstructure with weak superstructure reflections in the space group P63/m with a=879.26(8) pm and c=1604.59(15) pm, thus tripling the previously reported subcell. The crystal structure (refinement to R1=0.044 and wR2=0.075 for 1387 unique reflections and 60 variables) contains Fe(C2)3 trigonal planar groups with the C2 ligands bonded end-on to the Fe atoms. The C-C distance is typical for a double bond. La atoms as the least electronegative component surround the complex anions and form a framework of face-sharing tricapped trigonal prisms. The resulting hexagonal channels at 0, 0, z of the partial structure with chemical composition La3FeC6 are occupied by four additional La atoms per unit cell. These La atoms are fully ordered within a linear chain and display a Peierls-like distortion pattern. However, no long-range order in the a−b plane has been observed due to the random orientation of the chains. Because of the two different orientations which are possible for each chain the situation is similar to an Ising model on a triangular lattice. 相似文献
70.
Radioactive indicator's (152Eu) method was used to study the behavior of rare-earth element (REE) micro-impurities at fluorination of fuel UO2 with bromine trifluoride. The proposed process is very simple and carried out under a blanket layer of Freon-113 in the glassy carbon (vitreous) crucibles.It was shown that uranium matrix completely removes from reaction sphere in the form of UF6 and REE contaminations quantitatively remain in crucible owing to practical non-volatility of their fluorides. High purity of fuel UO2 causes small amount of the non-volatile rest (0.3-2 wt.% from initial weight) that considerably facilitates a problem of confident diagnostics of the strictly limited REE content in this material. Removal of uranium and concentration of REE from analyzed test samples via fluorination is several hundred times more rapid scheme of pre-sampling to atomic emission spectral determination of REE micro-quantities in UO2 than traditionally used extraction and ion-exchange ways of the preliminary concentration of contaminations and removal of uranium. 相似文献