全文获取类型
收费全文 | 686篇 |
免费 | 53篇 |
国内免费 | 21篇 |
专业分类
化学 | 497篇 |
晶体学 | 5篇 |
力学 | 46篇 |
综合类 | 9篇 |
数学 | 79篇 |
物理学 | 124篇 |
出版年
2023年 | 17篇 |
2022年 | 33篇 |
2021年 | 54篇 |
2020年 | 44篇 |
2019年 | 32篇 |
2018年 | 30篇 |
2017年 | 45篇 |
2016年 | 41篇 |
2015年 | 24篇 |
2014年 | 44篇 |
2013年 | 56篇 |
2012年 | 27篇 |
2011年 | 36篇 |
2010年 | 23篇 |
2009年 | 36篇 |
2008年 | 37篇 |
2007年 | 27篇 |
2006年 | 32篇 |
2005年 | 10篇 |
2004年 | 23篇 |
2003年 | 12篇 |
2002年 | 15篇 |
2001年 | 7篇 |
2000年 | 8篇 |
1999年 | 10篇 |
1998年 | 6篇 |
1997年 | 4篇 |
1996年 | 3篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 6篇 |
1992年 | 2篇 |
1990年 | 2篇 |
1988年 | 1篇 |
1987年 | 3篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有760条查询结果,搜索用时 140 毫秒
11.
Elisângela Corradini Luís C. de Morais Nicole R. Demarquette José A. M. Agnelli Luiz H. C. Mattoso 《先进技术聚合物》2007,18(10):861-867
This work involved a study of the effect of processing variables (temperature, water content, rotor speed, and time) on the mechanical properties of starch:gluten:glycerol mixtures in the weight ratio of 40:40:20. The properties of the materials were affected by the processing variables. The torque decreased with water content, indicating that water facilitates the plasticization of mixtures, whereas the increase in temperature accelerated the evaporation of water, thus increasing the torque. Ultimate tensile strength was achieved at the lowest temperature (110°C) and the highest water content (20%), whereas maximum elongation was achieved for the material processed at the highest temperature, 150°C, and the fastest rotor speed, 70 rpm. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
12.
Prakasham RS Rao ChS Rao RS Rajesham S Sarma PN 《Applied biochemistry and biotechnology》2005,120(2):133-144
Optimization of alkaline protease production parameters by Bacillus sp. was investigated using Taguchi methodology. The pH of the medium was observed to be the most significant factor among
all selected optimization parameters at an individual level. The combinatorial influence of least significant factors, inoculum
level and salt solution concentration (at the individual level), resulted in an interacting severity index of 76%, suggesting
their interactive role in the regulation of protease production in this microbial species. Protease production could be improved
more than 100% with Taguchi’s optimized conditions of the medium composition by this microorganism. 相似文献
13.
14.
FT IR spectra of a series of compounds with a general formula (N2H5)2HMF6·2H2O (where M∈{Ga, Al, Fe}) were recorded at variable temperatures (from ∼100 to 300 K, at 10 K intervals). The appearance of the spectral region of ν(N-N) modes due to hydrazinium cations further supports the conclusions regarding the N2H5+?H+?N2H5+ hydrogen bond potential well based on Raman spectroscopic data [J. Raman Spectrosc. 28 (1997) 315]. The appearance of two bands corresponding to the ν(N-N) modes in the low temperature FT IR spectra that merge into one upon heating is a clear evidence of a symmetric potential well through which a phonon-assisted proton transfer (PAPT) occurs at higher temperatures. Ab initio MP2/6-311++G(2d,p) quantum chemical study of the proton transfer potential within the N2H5+?H+?N2H5+ cluster confirmed its double-minimum character. The first-order saddle point found on the MP2/6-311++G(2d,p) potential energy hypersurface corresponds to a centrosymmetric structure (C2h symmetry), with the proton placed at the inversion center. The potential energy curve along the tunnelling coordinate was calculated by the intrinsic reaction coordinate (IRC) methodology, leading to an adiabatic PT barrier height of 3.94 kcal mol−1 and a tunneling rate of 1.98 s−1. The corresponding MP4(SDTQ)/6-311++G(2d,p)//MP2/6-311++G(2d,p) value of the adiabatic PT barrier height is 4.26 kcal mol−1. 相似文献
15.
系统研究了各种多元分辨校正模型、方法及其应用,以模拟体系、病态体系有态体系为对象,考察了它们的多元校正能力与多元分辨效果,给我机多组份分析态体系实例。 相似文献
16.
Three-dimensional quantitative structure-activity relationships of steroid aromatase inhibitors 总被引:1,自引:0,他引:1
Summary Inhibition of aromatase, a cytochrome P450 that converts androgens to estrogens, is relevant in the therapeutic control of breast cancer. We investigate this inhibition using a three-dimensional quantitative structure-activity relationship (3D QSAR) method known as Comparative Molecular Field Analysis, CoMFA [Cramer III, R.D. et al., J. Am. Chem. Soc., 110 (1988) 5959]. We analyzed the data for 50 steroid inhibitors [Numazawa, M. et al., J. Med. Chem., 37 (1994) 2198, and references cited therein] assayed against androstenedione on human placental microsomes. An initial CoMFA resulted in a three-component model for log(1/Ki), with an explained variance r2 of 0.885, and a cross-validated q2 of 0.673. Chemometric studies were performed using GOLPE [Baroni, M. et al., Quant. Struct.-Act. Relatsh., 12 (1993) 9]. The CoMFA/GOLPE model is discussed in terms of robustness, predictivity, explanatory power and simplicity. After randomized exclusion of 25 or 10 compounds (repeated 25 times), the q2 for one component was 0.62 and 0.61, respectively, while r2 was 0.674. We demonstrate that the predictive r2 based on the mean activity (Ym) of the training set is misleading, while the test set Ym-based predictive r2 index gives a more accurate estimate of external predictivity. Using CoMFA, the observed differences in aromatase inhibition among C6-substituted steroids are rationalized at the atomic level. The CoMFA fields are consistent with known, potent inhibitors of aromatase, not included in the model. When positioned in the same alignment, these compounds have distinct features that overlap with the steric and electrostatic fields obtained in the CoMFA model. The presence of two hydrophobic binding pockets near the aromatase active site is discussed: a steric bulk tolerant one, common for C4, C6-alpha and C7-alpha substitutents, and a smaller one at the C6-beta region. 相似文献
17.
《Arabian Journal of Chemistry》2022,15(4):103707
Alsophila spinulosa is a tree-like fern, and many evidences suggested that plant polyphenols had the potential therapeutic for Alzheimer s disease (AD). Herein, polyphenols (ASP) was isolated from A. spinulosa leaves and its major constituent were isoorientin and vitexin. ASP displayed excellent antioxidant activity and obvious anti-lipid peroxidation capacity in vitro. ASP improved the survival rate of C. elegans under high temperature by enhancing the antioxidant enzymes activities and decreasing the lipid peroxidation level. Moreover, ASP alleviated β-amyloid (Aβ) induced paralysis and reduced Aβ deposition, decreased reactive oxygen species (ROS) accumulation and improved the level of skn-1 mRNA. In addition, ASP decreased the levels of pdk-1 and akt-1 mRNA in P13K/AKT signaling pathway. In conclusion, ASP may be a potential ingredient for the alleviation of AD. 相似文献
18.
T.R. KukrejaD. Kumar K. PrasadR.C. Chauhan S. ChoeP.P. Kundu 《European Polymer Journal》2002,38(7):1417-1422
Response surface methodology was used for predicting the optimal composition of vegetable oil and carbon black in rubber compounding. Central composite rotatable design for two variables at five levels was chosen as the experimental design. The data obtained from measurement of properties was fitted as a two variable second order equation and were plotted as contour plots using programme developed in MATLAB v.5. It is observed from the contour plots that the increase in cross-link density caused by the formation of rubber mono-layer from its multi-layer on increasing the carbon black loading upto the central point (50 phr) of experimental region increases 300% modulus and elongation at break and reduces the ultimate properties like tear strength and tensile strength. On the other-hand hardness increases with increase in solid inclusion of carbon black. From the contours it is observed that the addition of vegetable oil upto 2-3 phr, cross-link density increases due to its coupling action leading to increase in hardness and modulus and lowering of ultimate properties like tensile strength and elongation at break. Addition of further amount of vegetable oil shows less coupling and more plasticising effect leading to increase in tear strength, tensile strength and elongation at break and decrease in hardness and 300% modulus. 相似文献
19.
《Electroanalysis》2006,18(3):253-258
The anodic voltammetric behavior of carbaryl on a boron‐doped diamond electrode in aqueous solution is reported. The results, obtained by square‐wave voltammetry at 0.1 mol L?1 Na2SO4 and pH 6.0, allow the development of a method to determine carbaryl, without any previous step of extraction, clean‐up, preconcentration or derivatization, in the range 2.5–30.0×10?6 mol L?1, with a detection limit of 8.2±0.2 μg L?1 in pure water. The analytical sensitivity of this electrochemical method diminished slightly, from 3.07 mA mmol?1 L to 2.90 mA mmol?1L, when the electrolyte was prepared with water samples collected from two polluted points in an urban creek. In these conditions, the recovery efficiencies obtained were around 104%. The effect of other pesticides (fenthion and 4‐nitrophenol) was evaluated and found to exert a negligible influence on carbaryl determination. The square‐wave voltammetric data obtained for carbaryl were typical of an irreversible electrode process with mass transport control. The combination of square‐wave voltammetry and diamond electrodes is an interesting and desirable alternative for analytical determinations. 相似文献
20.
A novel methodology was implemented in the present study to concurrently control power conversion efficiency (η) and durability (D) of co-sensitized dye solar cells. Applying response surface methodology (RSM) and Desirability Function (DF), the main influential assembling (dye volume ratio and anti-aggregation agent concentration) and operational (performance temperature) parameters were systematically changed to probe their main and interactive effects on the η and D responses. Individual optimization based on RSM elucidated that D can be solely controlled by changing the ratio of vat-based organic photosensitizers, whereas η takes both effects of dye volume ratio and anti-aggregation concentration into account. Among the studied factors, the performance temperature played the most vital role in η and D regulation. In particular, however, multi-objective optimization by DF explored the degree to which one should be careful about manipulation of assembling and operational parameters in the way maximization of performance of a co-sensitized dye solar cell. 相似文献