全文获取类型
收费全文 | 7522篇 |
免费 | 1111篇 |
国内免费 | 1040篇 |
专业分类
化学 | 5776篇 |
晶体学 | 159篇 |
力学 | 362篇 |
综合类 | 87篇 |
数学 | 857篇 |
物理学 | 2432篇 |
出版年
2024年 | 20篇 |
2023年 | 183篇 |
2022年 | 267篇 |
2021年 | 360篇 |
2020年 | 411篇 |
2019年 | 378篇 |
2018年 | 296篇 |
2017年 | 320篇 |
2016年 | 396篇 |
2015年 | 431篇 |
2014年 | 513篇 |
2013年 | 645篇 |
2012年 | 706篇 |
2011年 | 688篇 |
2010年 | 490篇 |
2009年 | 479篇 |
2008年 | 529篇 |
2007年 | 447篇 |
2006年 | 394篇 |
2005年 | 279篇 |
2004年 | 265篇 |
2003年 | 167篇 |
2002年 | 209篇 |
2001年 | 167篇 |
2000年 | 143篇 |
1999年 | 103篇 |
1998年 | 70篇 |
1997年 | 47篇 |
1996年 | 38篇 |
1995年 | 25篇 |
1994年 | 38篇 |
1993年 | 20篇 |
1992年 | 11篇 |
1991年 | 22篇 |
1990年 | 21篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1985年 | 15篇 |
1984年 | 4篇 |
1983年 | 8篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1978年 | 3篇 |
1977年 | 5篇 |
1974年 | 4篇 |
1959年 | 2篇 |
1934年 | 2篇 |
1925年 | 2篇 |
1924年 | 2篇 |
排序方式: 共有9673条查询结果,搜索用时 218 毫秒
81.
Elizabeth Nunes Fernandes Mariele Nair de Campos Moura Jos Luis F. Costa Lima Boaventura F. Reis 《Microchemical Journal》2004,77(2):107-112
An automatic flow procedure for the determination of glycerol in wines by employing a flow system based on multicommutation and enzymatic reaction is described. Glycerol dehydrogenase was immobilized on aminopropyl glass beads and packed into a column that was coupled to the flow system. The NADH produced by the enzymatic reaction was monitored by spectrophotometry at 340 nm and its radiation absorption presented a relationship with glycerol concentration. The system manifold comprised a set of three-way solenoid valves controlled by a microcomputer, which was furnished with electronic interfaces and runs a software that was designed to carry out on-line sample dilution, reagent addition, and data acquisition. The procedure allows the determination of glycerol in wine samples without any prior pretreatment. The procedure presented as profitable features a linear response range between 2.0 and 10.0 g l−1 glycerol (R=0.998), a detection limit of 0.006 g l−1 glycerol, a relative standard deviation of 1.8% (n=14) for a typical wine sample presenting 5.3 g l−1 glycerol, a sampling throughput of 33 determinations per hour, and a NAD+ consumption of 0.8 mg per determination. The results were compared with those obtained using a reference method and no significant difference at 90% confidence level was observed. 相似文献
82.
83.
采用高温固相法合成出层状锂离子电池正极材料LiNi1/3Mn1/3Co1/3O2。通过XRD、ICP、SEM和电化学测试手段对产物的结构、组成、形貌及电化学性能进行了研究。XRD结果表明此方法合成的LiNi1/3Mn1/3Co1/3O2具有标准的α-NaFeO2型层状结构,SEM照片显示颗粒粒径大约在500 nm左右,粒径分布较窄。以20 mA·g-1电流密度放电,充放电电压在2.8~4.4 V之间,其首次放电比容量为170 mAh·g-1,40次循环容量保持率为85.3%。进一步加入石墨导电剂后,同样条件下首次放电比容量变为179 mAh·g-1,50次循环容量保持率为89.6%。容量衰减主要发生在前10次循环。XRD和SEM测试表明循环初期容量衰减的原因是由材料本体结构变化和界面反应共同作用的结果。 相似文献
84.
介绍用X-射线荧光光谱仪测定钛铁中Si、Mn、P、Al含量的方法,通过试验确定了合适的研磨时间、压力和保压时间,用压片法制样,建立了各元素的工作曲线,各元素的测定范围分别为Si3.00%~6.00%,Mn1.00%~3.00%,P0.030%~0.070%,Al5.00%~9.00%。通过强度测量得到测定Si、Mn、P、Al的相对标准偏差分别为0.074%、0.308%、0.383%、0.040%,精密度满足测试要求。将该方法测定结果与化学法比对,准确度满足国家标准方法分析误差的要求。 相似文献
85.
We study methods for reducing chemical master equations using the Michaelis-Menten mechanism as an example. The master equation consists of a set of linear ordinary differential equations whose variables are probabilities that the realizable states exist. For a master equation with s(0) initial substrate molecules and e(0) initial enzyme molecules, the manifold can be parametrized by s(0) of the probability variables. Fraser's functional iteration method is found to be difficult to use for master equations of high dimension. Building on the insights gained from Fraser's method, techniques are developed to produce s(0)-dimensional manifolds of larger systems directly from the eigenvectors. We also develop a simple, but surprisingly effective way to generate initial conditions for the reduced models. 相似文献
86.
87.
为减小物资生产与配送不协调造成的成本及生产资源浪费,建立了考虑推动式生产调度的物资配送优化模型,并针对标准模拟退火算法受随机因素影响易陷入局部最优的缺点,设计带有回火与缓冷操作的改进模拟退火算法对模型求解,确定了优化的车辆配送路线以及物资生产计划。对比实验结果表明:相对于单纯的物资配送优化模型,考虑推动式生产调度的配送优化模型,能够有效减小物资滞留时间以及配送延误成本;相较于标准模拟退火算法,改进算法搜索到了更优解,且计算结果的标准差减小了93.42%,稳定性更好;同时,改进模拟退火算法具有较低的偏差率,在中小规模算例中求解质量较高,平均偏差率在0.5%以内。 相似文献
88.
近年来传统电商正加速从经销商转变为连接消费者与供应商的在线市场中间商, 电商平台与供应商形成三种渠道结构:经销商模式、混合模式和在线市场模式。本文通过构建Stackelberg博弈模型讨论产品质量与价格双重竞争情境下电商平台与供应商销售合作模式最新选择问题。研究表明,当价格竞争强度适中而佣金比例较大时,混合模式是电商平台最优选择,此时产品质量差异最大;当价格竞争强度较小而佣金比例较大时,在线市场模式则是最优选择;在其他条件下,经销商模式是其最优选择。本文结论对电商平台与供应商合作模式选择、产品质量与定价决策具有参考价值。 相似文献
89.
L. Fernandes C. Pagano Gonçalves da Silva 《Journal of Radioanalytical and Nuclear Chemistry》1993,172(2):313-318
The radiopharmaceutical201TlCl(thallium-201 chloride) is used in nuclear medicine for myocardial visualization. The solution of201TlCl was prepared using201Tl obtained by irradiating a natural mercury target with protons from the CV-28 cyclotron installed at IPEN-CNEN/SP. This solution was subjected to different quality control processes required for its use in medicine. Some of these controls concerned the determination of the radionuclidic impurities:200Tl,202Tl and203Hg; the chemical identification of201Tl+; the hydrazine concentration, mercury contamination and the presence of phosphate. Furthermore. the biological distribution in Wistar rats and tests for sterility, pyrogenicity and toxicity were carried out. It was verified that the solution obtained was in the form of thallous chloride. This radiopharmaceutical gave good heart images in animals but due to the high levels of200Tl and202Tl its use in humans is not possible unless enriched202Hg is used as target in the irradiation. 相似文献
90.
Adam W Alsters PL Neumann R Saha-Möller CR Seebach D Beck AK Zhang R 《The Journal of organic chemistry》2003,68(21):8222-8231
The epoxidation of allylic alcohols is shown to be efficiently and selectively catalyzed by the oxidatively resistant sandwich-type polyoxometalates, POMs, namely [WZnM(2)(ZnW(9)O(34))(2)](q)(-) [M = OV(IV), Mn(II), Ru(III), Fe(III), Pd(II), Pt(II), Zn(II); q = 10-12], with organic hydroperoxides as oxygen source. Conspicuous is the fact that the nature of the transition metal M in the central ring of polyoxometalate affects significantly the reactivity, chemoselectivity, regioselectivity, and stereoselectivity of the allylic alcohol epoxidation. For the first time, it is demonstrated that the oxovanadium(IV)-substituted POM, namely [ZnW(VO)(2)(ZnW(9)O(34))(2)](12-), is a highly chemoselective, regioselective, and also stereoselective catalyst for the clean epoxidation of allylic alcohols. A high enantioselectivity (er values up to 95:5) has been achieved with [ZnW(VO)(2)(ZnW(9)O(34))(2)](12)(-) and the sterically demanding TADOOL-derived hydroperoxide TADOOH as regenerative chiral oxygen source. Thus, a POM-catalyzed asymmetric epoxidation of excellent catalytic efficiency (up to 42 000 TON) has been made available for the development of sustainable oxidation processes. The high reactivity and selectivity of this unprecedented oxygen-transfer process are mechanistically rationalized in terms of a peroxy-type vanadium(V) template. 相似文献