全文获取类型
收费全文 | 7070篇 |
免费 | 653篇 |
国内免费 | 134篇 |
专业分类
电工技术 | 15篇 |
综合类 | 483篇 |
化学工业 | 998篇 |
金属工艺 | 4篇 |
机械仪表 | 25篇 |
建筑科学 | 48篇 |
矿业工程 | 8篇 |
能源动力 | 48篇 |
轻工业 | 5797篇 |
水利工程 | 80篇 |
石油天然气 | 30篇 |
无线电 | 34篇 |
一般工业技术 | 135篇 |
冶金工业 | 8篇 |
原子能技术 | 12篇 |
自动化技术 | 132篇 |
出版年
2024年 | 69篇 |
2023年 | 121篇 |
2022年 | 265篇 |
2021年 | 301篇 |
2020年 | 261篇 |
2019年 | 271篇 |
2018年 | 218篇 |
2017年 | 259篇 |
2016年 | 267篇 |
2015年 | 313篇 |
2014年 | 339篇 |
2013年 | 381篇 |
2012年 | 466篇 |
2011年 | 500篇 |
2010年 | 349篇 |
2009年 | 317篇 |
2008年 | 265篇 |
2007年 | 385篇 |
2006年 | 361篇 |
2005年 | 342篇 |
2004年 | 254篇 |
2003年 | 226篇 |
2002年 | 202篇 |
2001年 | 137篇 |
2000年 | 125篇 |
1999年 | 140篇 |
1998年 | 116篇 |
1997年 | 64篇 |
1996年 | 55篇 |
1995年 | 64篇 |
1994年 | 61篇 |
1993年 | 57篇 |
1992年 | 67篇 |
1991年 | 51篇 |
1990年 | 41篇 |
1989年 | 29篇 |
1988年 | 24篇 |
1987年 | 36篇 |
1986年 | 15篇 |
1985年 | 20篇 |
1984年 | 6篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1978年 | 1篇 |
1973年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有7857条查询结果,搜索用时 15 毫秒
21.
22.
23.
Alkylated ureas: mineralization and evaluation as N sources 总被引:2,自引:0,他引:2
An incubation experiment was conducted for 11 weeks to study the mineralization of ten alkylated ureas and urea in soil. Six of the alkylated ureas viz. methylurea(MU), 1,3-dimethylurea(1,3DMU), 1,1-dimethylurea(1,1DMU), ethylurea(EU), 1,3-diethylurea(1,3DEU) and butylurea(BU) and urea mineralized during the experiment. Urea mineralized immediately, while alkylated ureas mineralized after an incubation period ranging from less than a week to four weeks (delay period of mineralization). The delay period increased in the following sequence MU < 1,3DMU < EU < BU < 1,1DMU < 1,3DEU, but after the delay period was over the compounds mineralized almost as rapidly as urea. The delay period varied according to the number of carbon atoms in the alkyl group and their position with respect to each other on the molecule. It appeared to be specific for each compound and was apparently not influenced by the presence of urea or other alkylated ureas. This character can be used to develop mixture of various alkylated ureas to obtain N mineralization at the desired time. Rapid evolution of CO2 and N2O was observed during the mineralization of urea as well as alkylated ureas. Increase in soil pH was also observed during this period. The simultaneous ocurrence of these events suggested the formation of urea as an intermediate during the mieralization of alkylated ureas. None of the alkylated ureas showed adverse affect on emergence of wheat seedlings and except DEU and BU at high concentration no other alkylated urea showed any adverse effect on initial growth of wheat seedlings. 相似文献
24.
Simulation of nitrogen dynamics and biomass production in winter wheat using the Danish simulation model DAISY 总被引:14,自引:0,他引:14
S. Hansen H. E. Jensen N. E. Nielsen H. Svendsen 《Nutrient Cycling in Agroecosystems》1991,27(2-3):245-259
A dynamic simulation model for the soil plant system is described. The model includes a number of main modules, viz., a hydrological model including a submodel for soil water dynamics, a soil temperature model, a soil nitrogen model including a submodel for soil organic matter dynamics, and a crop model including a submodel for nitrogen uptake. The soil part of the model has a one-dimensional vertical structure. The soil profile is divided into layers on the basis of physical and chemical soil characteristics. The simulation model was used to simulate soil nitrogen dynamics and biomass production in winter wheat grown at two locations at various levels of nitrogen fertilization. The simulated results were compared to experimental data including concentration of inorganic nitrogen in soil, crop yield, and nitrogen accumulated in the aboveground part of the crop. Based on this validation it is concluded that the overall performance of the model is satisfactory although some minor adjustments of the model may prove to be necessary. 相似文献
25.
预处理方式对小麦秸秆制备高吸水性树脂的影响 总被引:5,自引:2,他引:5
以小麦秸秆为原料合成高吸水性树脂需首先对秸秆进行预处理,通过实验分析,比较了预处理方式如酸处理、碱浸泡、氨水浸泡、碱蒸煮及其联合处理对秸秆及高吸水性树脂的影响;用晶相显微镜、IR等表征了处理前后秸秆的微观形貌、化学结构及树脂的化学结构等;结果表明,比较理想的预处理方式是碱蒸煮处理〔w(NaOH)=14%的水溶液、150℃、0.6 MPa、30 m in〕结合浓度为1 mol/L的硝酸在100℃下处理30 m in,及w(NH3.H2O)=10%的氨水室温浸泡48 h结合浓度为1 mol/L的硝酸在100℃下处理45 m in;两种方式所得高吸水性树脂吸收纯水的质量倍率分别为405 g/g和293 g/g,吸收w(复混肥)=0.1%的水溶液的质量倍率分别为124 g/g和82g/g〔复混肥中w(N)=w(P)=w(K)=10%,N、P、K分别以尿素、过磷酸钙、氯化钾存在〕。 相似文献
26.
27.
Mussoorrie rock phosphate (MRP), MRP + pyrite (25% by weight), diammonium phosphate (DAP), ammonium polyphosphate (APP) and nitrophosphate (NP) were compared in a field experiment as fertilizers for wheat. At 20 kg P ha–1, MRP was only 6 per cent as effective as DAP. However, when it was mixed with pyrite, the efficiency of MRP increased to 64 per cent at 20 kg P ha–1 compared with 97 per cent at 40 kg P ha–1. The P requirement for a targeted yield for 4.5 t ha–1 decreased from 39.4 kg P ha–1 as MRP to 23.7 kg P ha–1 as MRP + pyrite. Of the other P fertilizers studied, NP was as effective as DAP, whereas APP was 9 to 37 per cent more effective than DAP. However, the P requirement as DAP, NP and APP for a targeted yield of 4.5 t ha–1 was similar (11 ± 0.5 kg P ha–1). 相似文献
28.
Biodegradability and mechanical properties of agro‐flour–filled polybutylene succinate biocomposites
The objective of this study was the production of rice husk flour (RHF) and wood flour (WF) filled polybutylene succinate (PBS) biocomposites as alternatives to cellulosic material filled conventional plastic (polyolefins) composites. PBS is one of the biodegradable polymers, made from the condensation reaction of 1,4‐butanediol and succinic acid that can be naturally degraded in the natural environment. We compared the mechanical properties between conventional plastics and agro‐flour–filled PBS biocomposites. We evaluated the biodegradability and mechanical properties of agro‐flour–filled PBS biocomposites according to the content and filler particle size of agro‐flour. As the agro‐flour loading was increased, the tensile and impact strength of the biocomposites decreased. As the filler particle size decreased, the tensile strength of the biocomposites increased but the impact strength decreased. The addition of agro‐flour to PBS produced a more rapid decrease in the tensile strength, notched Izod impact strength, and percentage weight loss of the biocomposites during the natural soil burial test. These results support the application of biocomposites as environmentally friendly materials. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1513–1521, 2005 相似文献
29.
高产小麦磷钾效应和适宜氮磷钾配比研究 总被引:2,自引:0,他引:2
黄泛冲积石灰性土壤在适宜施氮水平下磷钾不同用量及配比对小麦生长的影响表现为:(1)磷肥以中量施用水平(每公顷施用P2O5120kg)增产效果最好,分别比未施磷、低量施用水平(每公顷用P2O548kg)增产30.82%、20.27%,高量施用水平(每公顷施用P2O5192kg),虽然比中量施用水平表现出增产,但增产幅度很小,只有2.85%;(2)钾肥对小麦的增产效应小于磷肥,只有在施用磷肥的基础上配施钾肥,才能使钾的效应得到充分发挥;(3)小麦全生育期氮磷钾施用比例1∶0.5∶0.5为最佳,每公顷施用N240kg、P2O5120kg、K2O120kg效益最高。 相似文献
30.
Investigations were carried out in a long term field experiment from 1976 to 1982, on a loamy sand soil to find out the fertilizer requirements of groundnut and wheat grown in fixed rotation. Application of 26 kgP/ha to wheat alone was found to be sufficient for both wheat and succeeding groundnut. Application of phosphorus to both wheat and groundnut did not result in extra beneficial effect over P application to wheat alone. However, application of 26 kg P/ha to groundnut alone was not sufficient for succeeding wheat. There was no response from K application (25 kg K/ha) in either of these two crops. Increasing the dose of N from 50–150 kg/ha to wheat significantly increased the grain yield of wheat but the pod yield of succeeding groundnut were markedly lowered. Response of wheat to 150 kg N/ha was more marked when P was also applied to wheat and response was less when P was applied to preceding groundnut alone. Zinc application at 6.25 kg/ha to wheat alone resulted in significant increase in grain yield of wheat and pod yield of succeeding groundnut. 相似文献