全文获取类型
收费全文 | 143474篇 |
免费 | 14408篇 |
国内免费 | 9033篇 |
专业分类
电工技术 | 11596篇 |
技术理论 | 2篇 |
综合类 | 17919篇 |
化学工业 | 15807篇 |
金属工艺 | 6243篇 |
机械仪表 | 10439篇 |
建筑科学 | 18185篇 |
矿业工程 | 6568篇 |
能源动力 | 4786篇 |
轻工业 | 7848篇 |
水利工程 | 6490篇 |
石油天然气 | 6127篇 |
武器工业 | 1981篇 |
无线电 | 9696篇 |
一般工业技术 | 18469篇 |
冶金工业 | 5919篇 |
原子能技术 | 2297篇 |
自动化技术 | 16543篇 |
出版年
2024年 | 596篇 |
2023年 | 1683篇 |
2022年 | 3239篇 |
2021年 | 3786篇 |
2020年 | 4324篇 |
2019年 | 3727篇 |
2018年 | 3612篇 |
2017年 | 4565篇 |
2016年 | 5087篇 |
2015年 | 5291篇 |
2014年 | 8667篇 |
2013年 | 8245篇 |
2012年 | 10997篇 |
2011年 | 11120篇 |
2010年 | 8665篇 |
2009年 | 8936篇 |
2008年 | 8251篇 |
2007年 | 10233篇 |
2006年 | 8999篇 |
2005年 | 7593篇 |
2004年 | 6250篇 |
2003年 | 5380篇 |
2002年 | 4499篇 |
2001年 | 3750篇 |
2000年 | 3321篇 |
1999年 | 2876篇 |
1998年 | 2261篇 |
1997年 | 1982篇 |
1996年 | 1658篇 |
1995年 | 1492篇 |
1994年 | 1271篇 |
1993年 | 931篇 |
1992年 | 824篇 |
1991年 | 581篇 |
1990年 | 474篇 |
1989年 | 484篇 |
1988年 | 346篇 |
1987年 | 197篇 |
1986年 | 166篇 |
1985年 | 68篇 |
1984年 | 68篇 |
1983年 | 37篇 |
1982年 | 47篇 |
1981年 | 55篇 |
1980年 | 29篇 |
1979年 | 39篇 |
1978年 | 18篇 |
1975年 | 19篇 |
1959年 | 35篇 |
1957年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
双季戊四醇收率与反应条件依赖关系研究 总被引:3,自引:0,他引:3
利用均匀设计法,通过少量实验,使用计算机回归出双季戊四醇收率与反应条件间的依赖关系:Y=4.1418+0.089x_1-10.08x_2+83.018x_3-0.129x_4-29.638x,其表明增加反应温度和降低甲醛与乙醛配比都可以增加双季戊四醇的收率,结果与实验相吻合。用回归方程得到的优化条件进行实验得到的双季戊四醇收率为25.4%,与预测值很接近。 相似文献
992.
993.
The critical properties of five compounds,including propanal,butanal,1-pentanal,2-methel butanal and trimer of ethanal,were determined for the first time by a new capillary quick-flow method. The apparatus was improved with a capillary tube and checked with hexane and 1-heptene as standard reagents. The experimental results proved that the determination of critical properties by quick-flow method with a capillary tube apparatus was successful. 相似文献
994.
995.
In the previous paper, we reported the stand growth of slash pine (Pinus elliottii) during the first 11.5 years of plantation in response to (1) initial fertilization at plantation establishment with P rates of 11, 22, 45 and 90 kg P ha–1 which were either banded or broadcast in the presence or absence of basal fertilizers containing 50 kg N ha–1, 50 kg K ha–1 and 5 kg Cu ha–1 and (2) application of additional 40 kg P ha–1 at age 10 years. Here we present the responses in foliar nutrient concentrations of slash pine in the first 11.5 years to the initial fertilization and the additional P applied at age 10 years.Foliar N and K concentrations in the first 9.6 years of plantation decreased with the initial P rate. Application of the basal fertilizers improved foliar Cu concentration. Foliar Ca and Mg concentrations increased linearly with the initial P rate. The initial fertilization did not affect foliar Mn concentration in the first 9.6 years. Foliar P concentration increased quadratically with the initial P rate, which accounted for 77–86% of the variation in foliar P concentration. Most of the explained variation in foliar nutrient concentrations was attributable to the plantation age except for foliar P concentration. In the case of foliar P concentration, 53% was explained by the initial P rate, 31% by the plantation age and 2% by the positive interaction between the initial P rate and the plantation age. Foliar P concentration of slash pine at age 11.5 years increased quadratically with the initial P rate and linearly with the additional 40 kg P ha–1 applied at age 10 years, accounting for 81% of the variation in the foliar P concentration. Foliar nutrient analysis indicated that P was the major limiting nutrient affecting the stand growth of slash pine in the first 11.5 years. 相似文献
996.
甲醛法测定铵盐含氮量 总被引:6,自引:0,他引:6
对甲醛法测定铵盐含氮量进行了研究,提出采用百里酚酞和酚酞混和指示剂进行测定。该法简单、快速、准确,能满足生产实际和样品检验需要。 相似文献
997.
《Ceramics International》2022,48(4):4821-4828
We have successfully synthesized multi-layer graphene oxide and graphene oxide based magnetic nanocomposite (M/GO) by microwave-assisted modified Hummers’ method for removal of toxic lead (Pb2+) and cadmium (Cd2+) ions from aqueous solution. The X-ray diffraction spectra of synthesized graphene oxide and M/GO confirm increased interlayer spacing along c-axis. Raman spectra revealed the good quality of synthesized GO and M/GO. The wrinkles were seen in the SEM images of synthesized graphene oxide. The presence of conjugated double bond (CC) and carbonyl (CO) were confirmed by using the UV–Vis spectroscopic spectra. Brunauer–Emmett–Teller (BET) analysis showed high (126 m2/g) surface area M/GO composite which accounts for large number of active binding sites for the adsorption of heavy metal ions. The adsorption studies revealed that Pb2+ ions were efficiently adsorbed on GO sheets. Interestingly, M/GO showed better adsorption for cadmium ions. 相似文献
998.
Doe Gyoon Koo Ho Cheol Park Jin Young Choi Jietae Lee 《Chemical Engineering Communications》2004,191(5):611-624
Each controller in multiloop control systems for multivariable processes can be tuned sequentially with the ultimate information for the paired input and output while former loops have been closed, and hence, single-input single-output autotuning methods can be applied. In this sequential autotuning for multiloop control systems, several iterations are usually required for better control performances. Especially when pairings are undesirable, the autotuning sequences should be repeated with correct pairings, which result in long field experiments. Here, to avoid this drawback, a simple method to identify process models while loops are being sequentially tuned is proposed. The identified models can be used to correct pairings of multiloop control systems and to improve tuning performances without several field iterations. In addition, they can be used to obtain model-based control systems such as decoupling control systems. 相似文献
999.
由二氧化碳出发合成有机碳酸酯 总被引:4,自引:0,他引:4
对以二氧化碳为原料合成有机碳酸酯的环氧路线、酯交换和直接合成方法作了综合评述 ,认为直接合成是最佳的合成路线。碳酸酯的直接合成可在均相及多相催化体系中进行 ,负载金属、金属甲氧基化合物及碱均可作为催化剂。直接法研究的关键在于高性能催化剂的开发、由二氧化碳对金属 -氧键插入反应导致的催化循环的构筑及耦合反应的应用。 相似文献
1000.
采用水热加工技术研究(Mn,Zn)Fe2O4晶体粉末的合成条件,对水热体系的碱度、温度、时间进行了系统实验.结果表明在pH=7~12.5范围内可有效合成(Mn,Zn)Fe2O4.当温度在190~220℃时,完成合成所需的时间为1~2h,提高碱度有利于合成反应.本文还初步探讨了(Mn,Zn)Fe2O4的合成机理,为扩大实验提供依据. 相似文献