全文获取类型
收费全文 | 1038篇 |
免费 | 30篇 |
国内免费 | 3篇 |
专业分类
化学 | 712篇 |
晶体学 | 18篇 |
力学 | 5篇 |
数学 | 214篇 |
物理学 | 122篇 |
出版年
2021年 | 18篇 |
2020年 | 11篇 |
2019年 | 13篇 |
2018年 | 17篇 |
2017年 | 21篇 |
2016年 | 36篇 |
2015年 | 47篇 |
2014年 | 28篇 |
2013年 | 62篇 |
2012年 | 54篇 |
2011年 | 54篇 |
2010年 | 41篇 |
2009年 | 36篇 |
2008年 | 43篇 |
2007年 | 50篇 |
2006年 | 37篇 |
2005年 | 33篇 |
2004年 | 21篇 |
2003年 | 16篇 |
2002年 | 22篇 |
2001年 | 7篇 |
2000年 | 14篇 |
1999年 | 13篇 |
1997年 | 14篇 |
1996年 | 11篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 11篇 |
1992年 | 11篇 |
1991年 | 12篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 12篇 |
1982年 | 10篇 |
1979年 | 9篇 |
1978年 | 8篇 |
1977年 | 14篇 |
1976年 | 6篇 |
1975年 | 18篇 |
1974年 | 11篇 |
1973年 | 7篇 |
1967年 | 7篇 |
1966年 | 11篇 |
1958年 | 24篇 |
1957年 | 10篇 |
1956年 | 6篇 |
排序方式: 共有1071条查询结果,搜索用时 46 毫秒
51.
Up to now molecular-orbital calculations of the electronic states of linear mixed crystals have been concerned only with binary systems such as AB, AB2, and A2B2. The purpose of the present treatment is to extend this work to crystals whose elementary cells contain any number of different atoms in any ordered sequence. The formulation of this general theory is based on the resolvent method. Although the approach enables the general properties of delocalized and localized states to be discussed, the more simple cases already solved in the literature can still be easily derived from the general equations. Furthermore, the classification of Tamm and Shockley states retains its significance, even when other kinds of states can occur and various transitions between the clear cut cases can exist. As in earlier investigations, the chemisorption states are related to surface states and have similar properties. The resolvent method is summarized in the Appendix and its connection with the partition technique is shown. 相似文献
52.
53.
A facile method for preparing 2-(2-X-vinyl)-5-nitrothiophene has been established by the reaction of (Z)-2-(2-bromovinyl)-5-nitrothiophene with O, S and N nucleophiles. 相似文献
54.
55.
56.
57.
Jaroslav Cvach 《Czechoslovak Journal of Physics》2005,55(1):A163-A171
We report an observation of a narrow resonance state inD *? andD *+ \(\bar p\) in inelastic electron-proton collisions at centre-of-mass energies of 300 and 320 GeV at HERA. The resonance has a mass of 3099±3 (stat.)±5 (syst). MeV and width of 12±3 (stat.) MeV, compatible with the experimental resolution. The resonance is interpreted as an anti-charmed baryon with a minimal constituent quark composition ofuudd \(\bar c\), together with the charge conjugate. 相似文献
58.
The D/L ratio of aspartic acid enantiomers in proteins of low turnover is generally accepted as a reliable procedure for age determination. In our study, twelve samples of eyetooth dentin were analyzed for age determination. The pure insoluble collagen isolated from eyetooth dentin was obtained by an EDTA demineralization process. Free amino acids obtained after collagen hydrolysis were converted into o-phthaldialdehyde-N-acetyl-L-cysteine (OPA-NAC) derivatives for HPLC analysis under modified conditions and into trifluoroacetic acid isopropyl esters for GC analysis, respectively. The modified HPLC procedure used phosphate buffer and acidified sample matrix prior to injection which resulted in suppression of peak tailing of both diastereomers, thus allowing achievement of both good selectivity and good resolution. To ensure the high accuracy of the developed method the other parameters, i.e. specificity, precision, linearity, LOD, and LOQ, were also determined. Nine collagen samples covering the age range of 18 to 84 years were used for the determination of coefficient of racemization (KR) and calculation of parameters for age estimation. The regression equations for the data set analyzed were as follows: KR= 0.0005 x age + 0.0262 (R2 = 0.9639) for HPLC, and KR= 0.0006 x age + 0.0319 (R2 = 0.9374) for GC, respectively. 相似文献
59.
The B3LYP/6-31G(d) level of theory was used for the optimization of [Pt(NH(3))(4)](2+), [Pt(NH(3))(3)(H(2)O)](2+), cis-[Pt(NH(3))(2)(H(2)O)(2)](2+), and related platinum complexes. In addition, water or ammonium ligands were replaced by DNA purine bases so that finally cis-diammineplatinum with two bases (Pt-bridged complexes) is obtained. Single point calculations using the MP2/6-31+G(d) method were performed on the obtained reference geometries and were utilized for estimating bond dissociation energies (BDEs) and stabilization energies, and for electron density analyses. After reoptimization, IR spectra were determined from HF second derivatives. It was found that replacement of both water and ammonium by the DNA base is an exothermic process (20-50 kcal/mol depending on the ligands present in the complex). Asymmetric structures with one interbase H-bond were obtained for cis-diammine[bond](N(7),N(7)'-diadenine)[bond]platinum and mixed cis-diammine[bond](N(7)-adenine)[bond](N(7)-guanine)[bond]platinum complexes. In the case of the diguanine Pt-bridge, a symmetrical complex with two ammonium...O(6) H-bonds was found. The higher stabilization energy of the di-guanine complex is linked to a larger component of the Coulombic interaction. However, the BDE of Pt[bond]N(7)(G) is smaller in this complex than the BDE of Pt[bond]N(7)(G) from the mixed Pt[bond]AG complex. Also, steric repulsion of the ligands is about 10 kcal/mol smaller for the asymmetrical Pt[bond]AA and Pt[bond]AG bridges. The influence of the trans effect on DBE can be clearly seen. Adenine exhibits the largest trans effect, followed by guanine, ammonium, and water. The strength of the H-bond can be determined from the IR spectra. The strongest H-bond is the interbase H-bridge between adenine and guanine in the mixed Pt[bond]AG complex; otherwise, the H-bonds of adenine complexes are weaker than in guanine complexes. BDE can be traced in the guanine-containing complexes. The nature of the covalent bonding is analyzed in terms of partial charges and MO. A general explanation of the lower affinity of transition metals to oxygen than nitrogen can be partially seen in the less favorable geometrical orientation of lone electron pairs of oxygen. 相似文献
60.
Conventional electrothermal atomic absorption spectrometric (ETAAS) equipment was extensively modified to enable automated in situ electrodeposition. The original autosampler injection Teflon capillary was replaced by a composite Pt/Teflon capillary which served as an anode in the electrodeposition circuit. Incorporation of a peristaltic pump and of a three-way solenoid under computer control into the sample dispenser circuit provided all necessary steps for automated electrodeposition-ETAAS determination. The automated sequence controlled addition of Pd modifier and of the analyte, electrolysis, withdrawal of spent electrolyte, rinsing, drying and atomization. Performance of the system was evaluated by analyzing Pb in 3% m/v NaCl. Optimization using factorial design yielded 3sigma detection limit of 20 pg Pb and reproducibility of 1.0-1.4% (for constant current electrodeposition), these values being superior to the results of conventional ETAAS of Pb in 0.5% m/v NaCl. Sensitivity of Pb determination is not affected by NaCl, NaOH, NaNO3 and NH4H2PO4, up to 4.6% m/v, demonstrating efficient matrix removal in the electrodeposition step. 相似文献