全文获取类型
收费全文 | 978篇 |
免费 | 23篇 |
国内免费 | 72篇 |
专业分类
化学 | 832篇 |
晶体学 | 3篇 |
力学 | 4篇 |
数学 | 2篇 |
物理学 | 232篇 |
出版年
2024年 | 3篇 |
2023年 | 52篇 |
2022年 | 6篇 |
2021年 | 10篇 |
2020年 | 20篇 |
2019年 | 30篇 |
2018年 | 17篇 |
2017年 | 31篇 |
2016年 | 24篇 |
2015年 | 20篇 |
2014年 | 35篇 |
2013年 | 48篇 |
2012年 | 34篇 |
2011年 | 58篇 |
2010年 | 51篇 |
2009年 | 69篇 |
2008年 | 53篇 |
2007年 | 55篇 |
2006年 | 57篇 |
2005年 | 58篇 |
2004年 | 64篇 |
2003年 | 34篇 |
2002年 | 24篇 |
2001年 | 30篇 |
2000年 | 18篇 |
1999年 | 28篇 |
1998年 | 32篇 |
1997年 | 13篇 |
1996年 | 47篇 |
1995年 | 12篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 2篇 |
1991年 | 7篇 |
1990年 | 4篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1985年 | 2篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1970年 | 1篇 |
1968年 | 1篇 |
排序方式: 共有1073条查询结果,搜索用时 15 毫秒
81.
Alan H. Cowley Zheng Lu Jamie N. Jones Jennifer A. Moore 《Journal of organometallic chemistry》2004,689(15):2562-2564
The salt [HC(CMe)2(NAr)2BPh][Al2Cl7] (3); Ar = 2,6-i-Pr2C6H3) has been synthesized via the in situ preparation of [HC(CMe)2(NAr)2BClPh], followed by treatment with AlCl3. X-ray analysis of 3 reveals that the BN2C3 ring of the boron cation is planar and DFT calculations indicate π-type interactions in the HOMO-6 and HOMO-7 orbitals. 相似文献
82.
The present study evaluates the growth kinetics of the boride layer Fe2B in AISI 1045 steel, by means of neural networks and the least square techniques. The Fe2B phase was formed at the material surface using the paste boriding process. The surface boron potential was modified considering different boron paste thicknesses, with exposure times of 2, 4 and 6 h, and treatment temperatures of 1193, 1223 and 1273 K. The neural network and the least square models were set by the layer thickness of Fe2B phase, and assuming that the growth of the boride layer follows a parabolic law. The reliability of the techniques used is compared with a set of experiments at a temperature of 1223 K with 5 h of treatment time and boron potentials of 2, 3, 4 and 5 mm. The results of the Fe2B layer thicknesses show a mean error of 5.31% for the neural network and 3.42% for the least square method. 相似文献
83.
Dongwook Shin 《Applied Surface Science》2007,253(19):8003-8007
The photosensitivity dynamics in SiO2 glass with a composition similar to that of silica planar lightwave circuit (PLC) devices was investigated as a fundamental study prior to device fabrication. Silica bulk glasses with similar composition to the core layer of PLC devices were prepared with various concentrations of B2O3. The photosensitivity in boron and germanium co-doped amorphous SiO2 yields a refractive index change Δn as high as 10−3 after irradiation with a KrF UV laser beam. The index modulation disappeared after thermal annealing. The result of annealing experiment and UV absorption/Raman spectra revealed that the molar volume change by UV irradiation is responsible for the index variation in the material. 相似文献
84.
By using the radio frequency-magnetron sputtering (RF-MS) method, both pure ZnO and boron doped ZnO (ZnO:B) thin films were deposited on glass substrates at ambient temperature and then annealed at 450 °C for 2 h in air. It is found that both ZnO and ZnO:B thin films have wurtzite structure of ZnO with (0 0 2) preferred orientation and high average optical transmission (≥80%). Compared with the resistivity of 6.3 × 102 Ω cm for ZnO film, both as-deposited and annealed ZnO:B films exhibit much lower resistivity of 9.2 × 10−3 Ω cm and 7.5 × 10−3 Ω cm, respectively, due to increase in the carrier concentration. Furthermore, the optical band gaps of 3.38 eV and 3.42 eV for as-deposited and annealed ZnO:B films are broader than that of 3.35 eV for ZnO film. The first-principles calculations show that in ZnO:B thin films not only the band gap becomes narrower but also the Fermi level shifts up into the conduction band with respect to the pure ZnO film. These are consistent with their lower resistivities and suggest that in the process of annealing some substituted B in the lattice change into interstitial B because of its smaller ion radius and this transformation widens the optical band gap of ZnO:B thin film. 相似文献
85.
《Tetrahedron letters》2014,55(51):7124-7129
Mono and di-heteroaryl-4,4′-difluoro-8-(aryl)-4-bora-3a,4a-diaza-s-indacene (BODIPy) (1–5) were synthesized using Suzuki–Miyaura couplings. Hetero aryl substitution on 3- or 3,5-positions caused large bathochromic shifts (up to ∼150 nm) in absorption (569–652 nm) and fluorescence maxima (586–679 nm) in comparison to classical BODIPy. Quantum yields were found to be as high as 0.65. Singlet oxygen production activities of these compounds were studied by monitoring the absorbance quenching of 1,3-diphenylisobenzofuran, on exposure to light (>600 nm). Cellular uptake of compound 4 was demonstrated using cervical cancer cells and fibroblast cell line and was confirmed by the images obtained using confocal microscope. 相似文献
86.
Toralf Peymann Axel Herzog Carolyn B. Knobler M. Frederick Hawthorne 《Angewandte Chemie (International ed. in English)》1999,38(8):1061-1064
No explosion , but per-B-hydroxylation occurs if the icosahedral boron hydrides [closo-B12H12]2− (see picture), [closo-CB11H12]−, or closo-1,12-(CH2OH)2-1,12-C2B10H10 are refluxed in 30 % hydrogen peroxide. Thus, the three isoelectronic species [closo-B12(OH)12]2−, [closo-1-H-1-CB11(OH)11]−, and closo-1,12-H2-1,12-C2B10(OH)10 were obtained. ○=BH, ○=BOH. 相似文献
87.
Dr. Soukaina Bennaamane Barbara Rialland Lhoussain Khrouz Dr. Marie Fustier-Boutignon Dr. Christophe Bucher Dr. Eric Clot Dr. Nicolas Mézailles 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2023,135(3):e202209102
Ammonia, NH3, is an essential molecule, being part of fertilizers. It is currently synthesized via the Haber–Bosch process, from the very stable dinitrogen molecule, N2 and dihydrogen, H2. This process requires high temperatures and pressures, thereby generating ca 1.6 % of the global CO2 emissions. Alternative strategies are needed to realize the functionalization of N2 to NH3 under mild conditions. Here, we show that boron-centered radicals provide a means of activating N2 at room temperature and atmospheric pressure whilst allowing a radical process to occur, leading to the production of borylamines. Subsequent hydrolysis released NH4+, the acidic form of NH3. EPR spectroscopy supported the intermediacy of radicals in the process, corroborated by DFT calculations, which rationalized the mechanism of the N2 functionalization by R2B radicals. 相似文献
88.
利用密度泛函理论通过计算吸附能量、HOMO/LUMO能隙变化、电荷转移、结构扭曲等研究二氧化氮分子在B12N12纳米笼的吸附.此外,通过计算B12N12的电子结合能、Gibbs自由能、态密度和分子表面的静电势研究其稳定性和其它特性.B12N12纳米笼吸附二氧化氮显示三种构型.B12N12团簇的HOMO/LUMO能隙变化对二氧化氮分子的存在非常敏感,从自由团簇的6.84 eV降为NO2/团簇稳定团簇的3.23 eV.团簇的导电性被极大地提高,表明B12N12纳米簇可能是潜在的二氧化氮气体分子检测传感器. 相似文献
89.
Abstract A synchrotron X-ray diffraction study on hexagonal graphite-like boron nitride (h-BN) was performed under high pressures and temperatures. From the measured P-V-Trelation for h-BN (with a three-dimensional ordering parameter P3 = 0.9) in the temperature range from 298 to 1273 K and up to 6.7 GPa, the thermoelastic parameters are derived by fitting a modified high temperature Birch-Murnaghan equation of state. The results are: bulk modulus B0[GPa] = 27.6-0.0081(T[K]-298) and its pressure derivative B1 = 10.5 + 0.0016(T [K] - 298). These values are for samples with P3 = 0.9 and are quite different for samples with different values of the order parameter. This parameter is shown to have a leading role in the determination of the thermoelastic properties of h-BN, which explains and reconciles the differences between previous results. 相似文献
90.
Isotypic Borophosphates MII(C2H10N2)[B2P3O12(OH)] (MII = Mg, Mn, Fe, Ni, Cu, Zn): Compounds containing Tetrahedral Layers The isotypic compounds MII(C2H10N2) · [B2P3O12(OH)] (MII = Mg, Mn, Fe, Ni, Cu, Zn) were prepared under hydrothermal conditions (T = 170 °C) from mixtures of the metal chloride (chloride hydrate, resp.), Ethylenediamine, H3BO3 and H3PO4. The orthorhombic crystal structures (Pbca, No. 61, Z = 8) were determined by X‐ray single crystal methods (Mg(C2H10N2)[B2P3O12(OH)]: a = 936.81(2) pm, b = 1221.86(3) pm, c = 2089.28(5) pm) and Rietveld‐methods (MII = Mn: a = 931.91(4) pm, b = 1234.26(4) pm, c = 2129.75(7) pm, Fe: a = 935.1(3) pm, b = 1224.8(3) pm, c = 2088.0(6) pm, Ni: a = 939.99(3) pm, b = 1221.29(3) pm, c = 2074.05(7) pm, Cu: a = 941.38(3) pm, b = 1198.02(3) pm, c = 2110.01(6) pm, Zn: a = 935.06(2) pm, b = 1221.33(2) pm, c = 2094.39(4) pm), respectively. The anionic part of the structure contains tetrahedral layers, consisting of three‐ and nine‐membered rings. The MII‐ions are in a distorted octahedral or tetragonal‐bipyramidal [4 + 2] (copper) coordination formed by oxygen functions of the tetrahedral layers. The resulting three‐dimensional structure contains channels running along [010]. Protonated Ethylenediamine ions are fixed within the channels by hydrogen bonds. 相似文献