全文获取类型
收费全文 | 4609篇 |
免费 | 245篇 |
国内免费 | 87篇 |
专业分类
化学 | 3146篇 |
晶体学 | 26篇 |
力学 | 150篇 |
综合类 | 10篇 |
数学 | 555篇 |
物理学 | 1054篇 |
出版年
2023年 | 21篇 |
2022年 | 57篇 |
2021年 | 63篇 |
2020年 | 80篇 |
2019年 | 72篇 |
2018年 | 69篇 |
2017年 | 65篇 |
2016年 | 125篇 |
2015年 | 120篇 |
2014年 | 162篇 |
2013年 | 263篇 |
2012年 | 305篇 |
2011年 | 348篇 |
2010年 | 206篇 |
2009年 | 161篇 |
2008年 | 284篇 |
2007年 | 271篇 |
2006年 | 265篇 |
2005年 | 266篇 |
2004年 | 208篇 |
2003年 | 178篇 |
2002年 | 159篇 |
2001年 | 100篇 |
2000年 | 117篇 |
1999年 | 59篇 |
1998年 | 46篇 |
1997年 | 44篇 |
1996年 | 43篇 |
1995年 | 59篇 |
1994年 | 55篇 |
1993年 | 55篇 |
1992年 | 46篇 |
1991年 | 32篇 |
1990年 | 30篇 |
1989年 | 26篇 |
1988年 | 30篇 |
1987年 | 32篇 |
1986年 | 21篇 |
1985年 | 28篇 |
1984年 | 30篇 |
1982年 | 30篇 |
1981年 | 29篇 |
1980年 | 22篇 |
1979年 | 39篇 |
1978年 | 32篇 |
1977年 | 32篇 |
1976年 | 26篇 |
1975年 | 22篇 |
1974年 | 22篇 |
1973年 | 14篇 |
排序方式: 共有4941条查询结果,搜索用时 18 毫秒
991.
磁性聚苯胺纳米微球的合成与表征 总被引:1,自引:0,他引:1
报道了具有核壳结构的Fe3O4-聚苯胺磁性纳米微球的合成方法和表征结果.微球同时具有导电性和磁性能.在优化的实验条件下,可得到饱和磁化强度Ms为55.4 emu/g,矫顽力Hc为62 Oe的磁性微球.微球的导电性随着微球中Fe含量的增加而下降.微球的磁性能则随着Fe含量的增加而增大.Fe3O4磁流体的粒径和磁性聚苯胺微球的粒径均在纳米量级.纳米Fe3O4粒子能够提高复合物的热性能.实验表明,磁流体和聚苯胺之间可能存在着一定的相互作用,但这种相互作用较为复杂,难于研究 相似文献
992.
建立了采用 K2 Cr O4 - K2 Cr2 O7混合指示剂测定 L-对甲苯磺酰乳酸乙酯合成物产率的化学分析方法 ,K2 Cr O4 - K2 Cr2 O7的最佳配比为 K2 Cr O4 ∶ K2 Cr2 O7=6∶ 1 (m/m) ,体系的最佳酸度为 p H5.8,测得 L-乳酸乙酯与甲苯磺酰氯反应制备的 L-对甲苯磺酰乳酸乙酯的产率为 94.0 %。 相似文献
993.
报道了实验室可行、工业生产实用的l-对甲苯磺酸乳酸乙酯产率的分析方法,采用K2CrO4-K2Cr2O7混合指示剂,其最佳配比为K2CrO4:K2Cr2O7=6:1(质量比),体系的最佳pH为5.8,测得l-乳酸乙酯与对甲苯碘酰氯反应制备和的l-对甲苯磺酰乳酸乙酯的产率为94.0%。 相似文献
994.
Yan‐Long Qian Xin‐Rong Qin Ji‐Ling Huang Albert Sun‐Chi Chan Shou‐Shan Chen Hong‐Gen Wang 《中国化学》2001,19(1):97-101
The syntheses of a series of l‐methyl‐3‐aryl‐substituted titanocene and zirconocene dichlorides are reported. These complexes are synthesized by the reaction of 2‐ and 3‐methyl‐6, 6‐dimethylfulvenes (1:4) with aryllithium, followed by the reaction with TiCl4·2THF, ZrCl4 and (CpTiCl2)2O respectively, to give complexes 1–5. The complex [η5‐1‐methyl‐3‐(α, α‐dimethylbenzyl) cyclopentadienyl] titanium dichloride has been studied by X‐ray diffraction. The red crystal of this complex is monoclinic, space group P2t/C with unit cell parameters: a =6.973(6) × 10?1 nm, b =36.91(2) × 10?1 nm, c = 10.063(4) × 10?1 nm, α=β= γ = 93.35(5)°, V = 2584(5) × 10?3 nm3 and Z = 4. Refinement for 1004 observed reflections gives the final R of 0.088. There are four independent molecules per unit cell. 相似文献
995.
Copper complexes of chiral quinolinyl‐oxazoline have been studied as the catalysts for enantioselective allylic oxidation of cycloalkenes with tert‐butyl perbenzoate. Using 5 mol% of these chiral catalysts, optical active allylic benzoates were obtained in moderate enantiomeric excesses. CuOTf prepared in situ, CuClO4 and CuPF6 were found to be good precatalysts in acetone. 相似文献
996.
Chang Kon Kim Jongok Won Hoon Sik Kim Yong Soo Kang Hong Guang Li Chan Kyung Kim 《Journal of computational chemistry》2001,22(8):827-834
The formation and physicochemical properties of polymer electrolytes strongly depend on the lattice energy of metal salts. An indirect but efficient way to estimate the lattice energy through the relationship between the heterolytic bond dissociation and lattice energies is proposed in this work. The heterolytic bond dissociation energies for alkali metal compounds were calculated theoretically using the Density Functional Theory (DFT) of B3LYP level with 6‐311+G(d,p) and 6‐311+G(2df,p) basis sets. For transition metal compounds, the same method was employed except for using the effective core potential (ECP) of LANL2DZ and SDD on transition metals for 6‐311+G(d,p) and 6‐311+G(2df,p) calculations, respectively. The dissociation energies calculated by 6‐311+G(2df,p) basis set combined with SDD basis set were better correlated with the experimental values with average error of ca. ±1.0% than those by 6‐311+G* combined with the LANL2DZ basis set. The relationship between dissociation and lattice energies was found to be fairly linear (r>0.98). Thus, this method can be used to estimate the lattice energy of an unknown ionic compound with reasonably high accuracy. We also found that the dissociation energies of transition metal salts were relatively larger than those of alkaline metal salts for comparable ionic radii. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 827–834, 2001 相似文献
997.
Hui Wen Nies Mohd Saberi Mohamad Zalmiyah Zakaria Weng Howe Chan Muhammad Akmal Remli Yong Hui Nies 《Entropy (Basel, Switzerland)》2021,23(9)
Artificial intelligence in healthcare can potentially identify the probability of contracting a particular disease more accurately. There are five common molecular subtypes of breast cancer: luminal A, luminal B, basal, ERBB2, and normal-like. Previous investigations showed that pathway-based microarray analysis could help in the identification of prognostic markers from gene expressions. For example, directed random walk (DRW) can infer a greater reproducibility power of the pathway activity between two classes of samples with a higher classification accuracy. However, most of the existing methods (including DRW) ignored the characteristics of different cancer subtypes and considered all of the pathways to contribute equally to the analysis. Therefore, an enhanced DRW (eDRW+) is proposed to identify breast cancer prognostic markers from multiclass expression data. An improved weight strategy using one-way ANOVA (F-test) and pathway selection based on the greatest reproducibility power is proposed in eDRW+. The experimental results show that the eDRW+ exceeds other methods in terms of AUC. Besides this, the eDRW+ identifies 294 gene markers and 45 pathway markers from the breast cancer datasets with better AUC. Therefore, the prognostic markers (pathway markers and gene markers) can identify drug targets and look for cancer subtypes with clinically distinct outcomes. 相似文献
998.
在自建的非线性介电测试装置上测得了PT,BT、PZT和VDF-TrFE共聚物形成的0-3型铁电复合物厚片的三阶非线性介电系数ε3.研究发现,三种复合物的ε3都随陶瓷组分含量的上升而增大.测试场强升高,测得的ε3值减小,但对高陶瓷含量(φ>0.4)的BT/VDF-TrFE和PZT/VDF-TrFE复合物则在6MV/m场强下出现极小值.二相都被预极化的复合物小于仅陶瓷相被预极化的ε3值.PZT/VDF-TrFE复合物的温度依赖关系显示了与(-阶)介电系数类似的表观热滞后现象.高PZT含量的复合物在相变区出现较大的ε3值. 相似文献
999.
This paper presents an investigation into the limit cycleoscillation phenomenon for a nonlinear aeroelasticsystem under unsteady aerodynamics. The system consists of a sweptbackwing section carrying a tip mass with one degree of freedom. Thejunction stiffness considered between the wing and the tip mass istrilinear. The method of harmonic balance, which can be very practicalin the study of nonlinear flutter, is used for the theoretical analysisof limit cycle oscillations. Stable, unstable and semi-stable limitcycles are predicted in the system for both cases of hardening andsoftening springs. Results found by numerical simulation provide theamplitudes of limit cycles. The experimental results in wind tunneltests agree well with the predictions obtained both theoretically andnumerically. 相似文献
1000.
X. Ding Z. Sun W. Zhang Y. Peng A. S. C. Chan P. Li 《Colloid and polymer science》2000,278(5):459-463
The average diameter, diameter distribution and surface morphology of Fe3O4/poly(styrene-co-N-isopropylacrylamide)[P(St-NIPAM)] particles were characterized by scanning electron microscopy. The copolymer structure was
confirmed by IR spectroscopy, differential scanning calorimetry and elemental analysis. The content of Fe3O4 entrapped in the particles was determined by atomic absorption spectrometry. A coarse structure was observed on the surface
of the Fe3O4/P(St-NIPAM) particles. The hydrodynamic diameter of the Fe3O4/P(St-NIPAM) particles was found to exhibit about a 15% decrease in diameter on changing the temperature from 25 to 40 °C.
The results also showed that Fe3O4/P(St-NIPAM) an advantage of exploited magnetic separation.
Received: 6 August 1999 Accepted in revised form: 16 November 1999 相似文献