全文获取类型
收费全文 | 1834篇 |
免费 | 196篇 |
国内免费 | 253篇 |
专业分类
化学 | 943篇 |
晶体学 | 6篇 |
力学 | 181篇 |
综合类 | 50篇 |
数学 | 653篇 |
物理学 | 450篇 |
出版年
2024年 | 4篇 |
2023年 | 16篇 |
2022年 | 57篇 |
2021年 | 63篇 |
2020年 | 59篇 |
2019年 | 48篇 |
2018年 | 53篇 |
2017年 | 69篇 |
2016年 | 98篇 |
2015年 | 80篇 |
2014年 | 133篇 |
2013年 | 130篇 |
2012年 | 112篇 |
2011年 | 122篇 |
2010年 | 84篇 |
2009年 | 133篇 |
2008年 | 123篇 |
2007年 | 120篇 |
2006年 | 103篇 |
2005年 | 112篇 |
2004年 | 77篇 |
2003年 | 75篇 |
2002年 | 53篇 |
2001年 | 32篇 |
2000年 | 40篇 |
1999年 | 49篇 |
1998年 | 31篇 |
1997年 | 33篇 |
1996年 | 26篇 |
1995年 | 20篇 |
1994年 | 28篇 |
1993年 | 12篇 |
1992年 | 9篇 |
1991年 | 11篇 |
1990年 | 11篇 |
1989年 | 5篇 |
1988年 | 10篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 7篇 |
1979年 | 6篇 |
1978年 | 1篇 |
排序方式: 共有2283条查询结果,搜索用时 36 毫秒
991.
992.
Metalloderivatives of anionic tetrasulphonated tetraphenylporphyrin (MTPPS, M = Mn(III), Fe(III) and Co(III)) were synthesized
and immobilized on cationically functionalised divinylbenzene(DVB)-crosslinked polystyrene(PS). These supported catalysts
(PS-MTPPS) were found to exhibit peroxidase-like activity. The co-oxidation of 4-aminoantipyrine and phenol by H2O2 was attempted with these catalysts to mimic this enzyme function. The catalytic efficiency of all these immobilized MTPPS
was found to be superior to the corresponding unsupported MTPPS in solution. The effect of the central metal ion of the porphyrin,
pH of the reaction medium and also the temperature effect are investigated. The ideal pH was seen to be in the 8.0–8.5 range,
with maximum effect at 8.2. The efficiency order for the various PS-MTPPS was seen to be Co>Mn>Fe, with CoTPPS showing efficiency
comparable to that of horseradish peroxidase. The catalytic efficiency was found to be increasing with temperature for all
the catalysts. The re-usability of these PS-MTPPS systems for peroxidase-like activity was also studied and it was found that
they exhibited a very high degree of recyclability without much poisoning. 相似文献
993.
994.
995.
陈继光 《数学的实践与认识》2013,43(12)
针对建筑沉降发生的过程,采用支持向量机(SVM)模型对建筑物沉降进行预测.使用前期施工过程中的沉降观测数据作为训练样本集,建立现场动态沉降量预报模型.仿真试验和实践结果表明,模型与BP神经网络预测模型相比能够更准确地反映实际沉降过程,且满足精确性和适用性的要求. 相似文献
996.
《Analytical letters》2012,45(15):2560-2569
A new approach for discrimination of adulterated milk is reported using two-dimensional infrared (IR) correlation spectroscopy by multiway principal component analysis (MPCA) and least squares support vector machines (LS–SVM). First, the synchronous two-dimensional spectra of pure and adulterated milk were calculated. Then, MPCA was used to reduce the dimensions, extract features of two-dimensional correlation data set, and distinguish adulterated milk and pure milk. Finally, a LS-SVM model was developed using the scores of the first thirteen principal components from synchronous two-dimensional correlation spectra computed by MPCA as the input variables. The ratios of correct classification were 100% and 96.3% for calibration set and prediction set, respectively. The area under the receiver operating characteristic curves (ROC) of 0.991 for prediction set was obtained by LS–SVM. The results indicate that two-dimensional correlation infrared spectra combined with MPCA–LS–SVM may be a rapid screening technique for discrimination of adulterated milk with good accuracy. 相似文献
997.
《Analytical letters》2012,45(18):2849-2859
ABSTRACTA novel method was developed for the quality control of Ephedrae herba by near-infrared (NIR) spectroscopy. First, qualitative models established by discriminant analysis and support vector machine were used for the preliminary screening of unqualified samples of E. herba. Then quantitative models of ephedrine and the total alkali (ephedrine and pseudoephedrine) were established by partial least squares regression and particle swarm optimization based least square support vector machine. The contents of test samples were predicted by the established NIR quantitative models. As a result, the accuracies of unqualified identification were 98.9% by discriminant analysis and 100% by support vector machine. The performance of the particle swarm optimization based least square support vector machine models were better than the partial least squares regression models. The correlation coefficients were both more than 0.98 and relative standard errors of calibrations were less than 9% in the calibration sets of particle swarm optimization based least square support vector machine models. As for the test sets, the correlation coefficients were both more than 0.93 and the relative standard errors of prediction were less than 13%, indicating satisfactory predicted results. All of these results demonstrated that NIR spectroscopy may be a powerful tool for the quality control of E. herba. 相似文献
998.
采用等体积浸渍法合成了一系列不同载体负载的Cu基催化剂,并研究了其在甘油氢解反应中的催化性能。借助X射线衍射(XRD)、程序升温还原(H2-TPR)和氨气程序升温脱附(NH3-TPD)等手段对催化剂进行了表征,同时考察了反应温度、反应压力、反应时间和催化剂重复使用次数对其催化性能的影响。结果表明,载体对催化剂反应性能影响较大,且甘油氢解反应需要适当的酸性;当在催化剂8Cu/γ-Al2O3用量为反应原料质量的2.5%、反应温度513 K、反应压力6 MPa、反应时间6 h、反应原料为质量分数为10%的甘油水溶液的条件下,其催化效果最优,甘油转化率最高为88.4%,1,2-丙二醇的选择性可达到86.2%,且催化剂显示了良好的稳定性。 相似文献
999.
以十六烷基三甲基溴化胺为结构导向剂,采用溶胶-凝胶法制备了BaTiO3-BaAl2O4-Al2O3复合载体,采用X射线衍射、红外光谱、N2吸附-脱附、透射电子显微镜和H2程序升温还原等技术对复合载体进行了表征,并以CH4/CO2重整制合成气为探针反应,考察了不同Ni/BaTiO3-BaAl2O4-Al2O3催化剂的性能.结果表明,BaTiO3-BaAl2O4-Al2O3复合载体具有多孔织构特性和较高的比表面积,BaTiO3和BaAl2O4以晶粒状态分布在复合载体的内外表面,晶粒尺寸在20~50nm的范围,复合载体孔径为10~20nm.复合载体上BaTiO3和BaAl2O4的引入,适度削弱了Ni/BaTiO3-BaAl2O4-Al2O3催化剂中Ni物种与γ-Al2O3间的强相互作用,抑止了NiAl2O4尖晶石的生成;当载体中Ba(Ti)含量为17.33%时,其负载的Ni催化剂上CH4/CO2重整制合成气反应的活性和稳定性最高. 相似文献
1000.
Classification of suspect powders, by using laser‐induced breakdown spectroscopy (LIBS) spectra, to determine if they could contain Bacillus anthracis spores is difficult because of the variability in their composition and the variability typically associated with LIBS analysis. A method that builds a support vector machine classification model for such spectra relying on the known elemental composition of the Bacillus spores was developed. A wavelet transformation was incorporated in this method to allow for possible thresholding or standardization, then a linear model technique using the known elemental structure of the spores was incorporated for dimension reduction, and a support vector machine approach was employed for the final classification of the substance. The method was applied to real data produced from an LIBS device. Several methods used to test the predictive performance of the classification model revealed promising results. Published 2012. This article is a US Government work and is in the public domain in the USA. 相似文献