全文获取类型
收费全文 | 24512篇 |
免费 | 3752篇 |
国内免费 | 1206篇 |
专业分类
电工技术 | 1206篇 |
综合类 | 1379篇 |
化学工业 | 5255篇 |
金属工艺 | 1394篇 |
机械仪表 | 242篇 |
建筑科学 | 1176篇 |
矿业工程 | 5279篇 |
能源动力 | 770篇 |
轻工业 | 2068篇 |
水利工程 | 824篇 |
石油天然气 | 1260篇 |
武器工业 | 78篇 |
无线电 | 860篇 |
一般工业技术 | 2212篇 |
冶金工业 | 4805篇 |
原子能技术 | 367篇 |
自动化技术 | 295篇 |
出版年
2024年 | 95篇 |
2023年 | 520篇 |
2022年 | 761篇 |
2021年 | 1048篇 |
2020年 | 1145篇 |
2019年 | 978篇 |
2018年 | 874篇 |
2017年 | 1048篇 |
2016年 | 1302篇 |
2015年 | 1126篇 |
2014年 | 1640篇 |
2013年 | 1572篇 |
2012年 | 1940篇 |
2011年 | 1831篇 |
2010年 | 1316篇 |
2009年 | 1293篇 |
2008年 | 901篇 |
2007年 | 1343篇 |
2006年 | 1185篇 |
2005年 | 1033篇 |
2004年 | 971篇 |
2003年 | 914篇 |
2002年 | 745篇 |
2001年 | 645篇 |
2000年 | 607篇 |
1999年 | 487篇 |
1998年 | 372篇 |
1997年 | 299篇 |
1996年 | 281篇 |
1995年 | 255篇 |
1994年 | 184篇 |
1993年 | 152篇 |
1992年 | 132篇 |
1991年 | 91篇 |
1990年 | 85篇 |
1989年 | 83篇 |
1988年 | 36篇 |
1987年 | 41篇 |
1986年 | 24篇 |
1985年 | 23篇 |
1984年 | 19篇 |
1983年 | 12篇 |
1982年 | 25篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1975年 | 3篇 |
1974年 | 1篇 |
1959年 | 4篇 |
1951年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 578 毫秒
91.
John D. Fitzsimons Bill Williston Georgina Williston Lisa Brown Abdel El-Shaarawi Lenore Vandenbyllaardt Dale Honeyfeld Don Tillitt Martha Wolgamood Scott B. Brown 《Journal of Great Lakes research》2007,33(1):93-103
Alewives (Alosa pseudoharengus), the major prey fish for Lake Ontario, contain thiaminase. They are associated with development of a thiamine deficiency in salmonines which greatly increases the potential for developing an early mortality syndrome (EMS). To assess the possible effects of thiamine deficiency on salmonine reproduction we measured egg thiamine concentrations for five species of Lake Ontario salmonines. From this we estimated the proportion of families susceptible to EMS based on whether they were below the ED20, the egg thiamine concentration associated with 20% mortality due to EMS. The ED20s were 1.52, 2.63, and 2.99 nmol/g egg for Chinook salmon (Oncorhynchus tshawytscha), lake trout (Salvelinus namaycush), and coho salmon (Oncorhynchus kisutch), respectively. Based on the proportion of fish having egg thiamine concentrations falling below the ED20, the risk of developing EMS in Lake Ontario was highest for lake trout, followed by coho (O. kisutch), and Chinook salmon, with the least risk for rainbow trout (O. mykiss). For lake trout from western Lake Ontario, mean egg thiamine concentration showed significant annual variability during 1994 to 2003, when the proportion of lake trout at risk of developing EMS based on ED20 ranged between 77 and 100%. Variation in the annual mean egg thiamine concentration for western Lake Ontario lake trout was positively related (p < 0.001, r2 = 0.94) with indices of annual adult alewife biomass. While suggesting the possible involvement of density-dependent changes in alewives, the changes are small relative to egg thiamine concentrations when alewife are not part of the diet and are of insufficient magnitude to allow for natural reproduction by lake trout. 相似文献
92.
醋酸甲酯在Cs_(1.5)PW/SiO_2催化剂上的水解反应 总被引:1,自引:0,他引:1
通过改变阳离子的种类与配比,制备了一系列SiO2负载的磷钨酸盐催化剂,考察了制备方法对催化剂性能的影响,采用傅里叶变换红外光谱、X射线衍射、X射线荧光光谱、环境扫描电子显微镜等方法对催化剂进行了表征,并考察了该系列催化剂对醋酸甲酯水解反应的活性。实验结果表明,Cs1.5PW/SiO2催化剂的活性最好,当Cs1.5PW负载量(质量分数)为30%时,Cs1.5PW的Keggin结构保持完好,没有硅钨酸阴离子形成,Cs1.5PW在SiO2载体上高度分散;在反应温度55℃、反应时间2h的条件下,醋酸甲酯的转化率为35.79%,约为均相磷钨酸催化剂的1.3倍。该催化剂重复使用4次后,醋酸甲酯的转化率稳定在25%;80℃下反应2h,与NKC-9磺酸树脂催化剂相比,醋酸甲酯的转化率提高了5.17%。 相似文献
93.
Tek Bahadur Gurung 《Lakes & Reservoirs: Research and Management》2007,12(4):237-246
Lake Rupa is a small, subtropical, shallow lake with a surface area of 100 ha situated 600 m a.s.l. in Central Himalaya, Nepal. This degraded lake was studied between 2000 and 2006, with the goal of determining whether or not it could be restored by a community‐based cooperative of local people living in its catchment. Main threats to the lake, its aquatic life and its very existence include encroachment by excessive aquatic vegetation, sedimentation, and low in‐lake concentrations of dissolved oxygen (DO). Small lakes (≤ 500 ha) are relatively prone to the process of ‘succession and climax’, although they also can be of social, environmental and ecological importance. Thus, their disappearance could spark social chaos and disorder in areas already having to cope with other rapid environmental changes. This means that credible mechanisms for revitalizing or protecting small lakes are an important goal. Accordingly, a lake cooperative of 329 households living in close vicinity to Lake Rupa was formed in 2002, by prioritizing traditional fishers, women and other deprived community members with the goals of respecting the citizenry and equity. Following the cooperative's campaigns directed at weed removal and fish stocking, encroachment of vegetation on the lake margins was halted, its aquatic weeds became under control, and its fisheries improved. In addition to the cooperative's restoration activities, several water quality parameters, including water transparency and DO and chlorophyll‐a concentrations, were monitored on a monthly basis. The measured water transparency was inconsistent, exhibiting large variations between 2000 and 2003. Relatively low, but consistent, values, however, were measured after 2004. The results imply that the removal of weeds, and sufficient nutrients and solar radiation are subsequently available to the lake's phytoplankton communities. Supporting this notion is that the chlorophyll‐a concentration spiked to 205 µg L−1 in November 2006, the water transparency became consistent, and the DO concentration increased to >3.8 mg L−1 during the critical months (March–May) after 2004. These water quality indicators indicated improvement in the degraded Lake Rupa, suggesting that the establishment of cooperatives such as that highlighted in this study could be a powerful and sustainable mechanism for restoring degraded lakes in similar socioeconomic settings by maintaining equity, by connecting communities with their resources, and by facilitating integrity, equity, citizenry and social justice. 相似文献
94.
郑州市郑东新区龙湖规划水面面积为6.08km^2,为我国最大的城市人工湖。因其成湖地层主要为细砂,其建设存在开挖施工防渗、成湖后的有限防渗、地下水交换,以及防止周边地区浸没、盐渍化等问题。应用三维渗流有限元法数值模型对龙湖工程施工和运行期防渗方案进行了模拟仿真计算,给出了工程区地下水位、渗漏量、渗透坡降、渗流场的变化以及优化防渗方案。 相似文献
95.
Design for a high power-density Astron reactor 总被引:1,自引:0,他引:1
N. C. Christofilos 《Journal of Fusion Energy》1989,8(1-2):97-105
A liquid lithium blanket surrounding the plasma volume is described. The liquid lithium flows along magnetic flux tubes at a high speed. There is no vacuum wall between the blanket and the plasma. The E-layer of relativistic particles within which the plasma is confined serves as a vacuum wall protecting the plasma from the lithium vapor, which is continuously produced at the surface of the blanket, by ionizing the lithium atoms and ejecting the same along open magnetic lines. The heat load at the surface of the blanket generated by 14 MeV neutrons can be several hundred MW per square meter.Work performed under the auspices of the U.S. Atomic Energy Commission.Deceased September 24, 1972. 相似文献
96.
97.
复杂区块井筒结盐综合治理技术 总被引:3,自引:1,他引:2
针对濮城油田部分低渗透、高矿化度复杂断块油藏生产油井结盐严重的特点,通过对油井盐样室内试验,剖析了结盐的原因及影响因素,提出了相应的井筒清、防盐配套新技术,在现场应用取得了良好效果。 相似文献
98.
高钙盐钻井液体系的研究与应用 总被引:4,自引:0,他引:4
中原油田的白庙、河岸、文东和卫城等地区存在着大段易分散、膨胀的泥页岩,造成井下事故。为此,研制了高钙盐钻井液体系。室内实验结果表明,高钙盐钻井液体系能抗150℃以上高温,具有良好的流变性和较低的滤失量,滤饼韧性好;具有和饱和盐水相当的强抑制性,抗盐膏;具有较好的保护油气层作用。现场应用表明,高钙盐钻井液达到了:①固相容量限高。抑制性强,在密度为1.4g/cm^3、膨润土含量为43g/L的情况下仍保持低粘度和切力;②机械钻速比邻井提高30%左右;③维护处理简单,处理成本低;④使用密度比邻井低0.05g/cm^3.保护油气层效果好,静态和动态岩心渗透率恢复值都大干80%。高钙盐钻井液较好地保护了油气层.成功地解决了泥页岩膨胀分散和垮塌问题。 相似文献
99.
K-S. Park S-H. Park Y-K. Sun K-S. Nahm Y-S. Lee M. Yoshio 《Journal of Applied Electrochemistry》2002,32(11):1229-1233
The structural and electrochemical properties of LiNiO2 powders were investigated as a function of the oxygen flow rate employed in the preparation of lithium nickel oxide. It was found that oxygen played an important role in the synthesis of highly crystallized LiNiO2(R3¯m). In the crystallization process of LiNiO2, a deficiency of oxygen in the calcination reactor induced the formation of impurities and cubic rock-salt structure (Fm3m) in LiNiO2 powders. For LiNiO2 prepared at higher oxygen flow rates, the electrode delivered high discharge capacities with relatively good retention rates. But very low electrode capacity was obtained from LiNiO2 prepared at lower oxygen flow rates. 相似文献
100.
云南锰矿资源丰富,具有品位高、低磷低铁、易选等特点。文章概述了斗南、鹤庆、建水等锰矿的选矿科研进展情况,三个锰矿均可采用强磁选或磁-重选流程选别,可提高品位8—12%;精矿含锰可达33—35%,锰回收率在80%以上的好结果。 相似文献