全文获取类型
收费全文 | 5696篇 |
免费 | 522篇 |
国内免费 | 341篇 |
专业分类
电工技术 | 266篇 |
技术理论 | 1篇 |
综合类 | 707篇 |
化学工业 | 246篇 |
金属工艺 | 429篇 |
机械仪表 | 866篇 |
建筑科学 | 416篇 |
矿业工程 | 158篇 |
能源动力 | 155篇 |
轻工业 | 353篇 |
水利工程 | 1709篇 |
石油天然气 | 78篇 |
武器工业 | 48篇 |
无线电 | 102篇 |
一般工业技术 | 348篇 |
冶金工业 | 347篇 |
原子能技术 | 18篇 |
自动化技术 | 312篇 |
出版年
2024年 | 10篇 |
2023年 | 54篇 |
2022年 | 97篇 |
2021年 | 119篇 |
2020年 | 153篇 |
2019年 | 106篇 |
2018年 | 119篇 |
2017年 | 150篇 |
2016年 | 178篇 |
2015年 | 247篇 |
2014年 | 391篇 |
2013年 | 355篇 |
2012年 | 410篇 |
2011年 | 450篇 |
2010年 | 348篇 |
2009年 | 378篇 |
2008年 | 280篇 |
2007年 | 380篇 |
2006年 | 346篇 |
2005年 | 343篇 |
2004年 | 274篇 |
2003年 | 230篇 |
2002年 | 206篇 |
2001年 | 199篇 |
2000年 | 162篇 |
1999年 | 110篇 |
1998年 | 81篇 |
1997年 | 77篇 |
1996年 | 74篇 |
1995年 | 57篇 |
1994年 | 38篇 |
1993年 | 23篇 |
1992年 | 22篇 |
1991年 | 20篇 |
1990年 | 15篇 |
1989年 | 10篇 |
1988年 | 21篇 |
1987年 | 8篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1964年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有6559条查询结果,搜索用时 31 毫秒
51.
D. D. Palmer D. K. Rehbein J. F. Smith O. Buck 《Journal of Nondestructive Evaluation》1988,7(3-4):167-174
It has been shown that the acoustic response of imperfect interfaces may be described by a quasi-static spring model. In the present paper, experimental data on the geometry of the contacts between two diffusion-bonded blocks have been used to determine the spring stiffness of such interfaces which have been correlated with experimentally determined ultrasonic reflection coefficients. The correlation between the theoretical reflection coefficient and the spring stiffness was found to be in excellent agreement with experimentally-observed values, if the disbonds are of infinitesimally small thickness. For disbonds of finite thickness, the agreement is less satisfactory. Reasons for the discrepancy in the latter case are unknown at the present time. 相似文献
52.
53.
对于一些采用硅锰脱氧冶炼工艺的特殊钢,为保证钢水洁净度,常会选择较长时间的LF软吹处理,导致过程能耗增加。通过工业试验,借助FEI Explorer 4自动扫描电镜检测,研究不同LF精炼软吹时间对硅脱氧弹簧钢55SiCr铸坯氧化物夹杂成分、数量的影响;并采用夹杂物极值统计法,对比评价不同LF精炼软吹时间对应成品盘条横截面最大夹杂物尺寸控制情况。结果表明,在LF软吹10 min与软吹40 min 两种工艺条件下,铸坯中尺寸大于5 μm的氧化物夹杂成分接近,均在CaO-SiO2-Al2O3相图中假硅灰石、钙长石和钙铝黄长石共晶低熔点区,其中软吹10 min工艺铸坯氧化物夹杂组成落入低熔点区的数量所占比例更大。LF软吹10 min与软吹40 min铸坯中尺寸大于5 μm的氧化物夹杂数量密度分别为11.70个/100 mm2和14.59个/100 mm2,尺寸大于15 μm 的氧化物夹杂数量密度分别为0.53个/100 mm2和1.65个/100 mm2,LF软吹10 min工艺铸坯大尺寸氧化物夹杂数量密度略低于LF软吹40 min工艺。当预测面积为30 000 mm2时,两种LF软吹时间对应成品盘条横截面最大夹杂物尺寸分别为27.1 μm和28.1 μm,盘条最大夹杂物尺寸控制无显著差别。结合硅锰脱氧钢中大尺寸低熔点CaO-SiO2-Al2O3系夹杂物主要源自钢包渣乳化卷入,具有与钢水和氩气泡界面接触角很小、难以通过吹氩上浮去除的特点,建议硅锰脱氧钢LF软吹过程按短时间快节奏进行控制。 相似文献
54.
讨论了渐变刚度钢板弹簧的载荷、变形量、刚度和应力的计算,建立了少片渐变刚度钢板弹簧的优化数学模型,利用MATLAB优化工具箱进行优化,并给出了计算实例。 相似文献
55.
56.
S. Multsch A. S. Alquwaizany O. A. Alharbi M. Pahlow H.-G. Frede L. Breuer 《国际水资源开发杂志》2017,33(2):292-309
Agriculture policy changes in Saudi Arabia are investigated by water footprint (WF) assessment. WF is calculated with the model SPARE:WATER for 3758 irrigated sites. The WF of agriculture areas (WFarea, km3 yr?1) has decreased (–17%) since the year 2000 to 13.84 km3 yr?1 (2011), which is mainly caused by the reduction of cropland by –33%. Nevertheless, water consumption per field has increased about 16%, which can be attributed to the cultivation of fodder crops (+12%). A scenario analysis revealed that a shifting cropping pattern towards less fodder crops reduces WFarea by –15%, and implementing improved irrigation technology leads to a combined reduction of up to 32% 相似文献
57.
This study performed a quantitative evaluation of the impact of water-saving irrigation on the groundwater regime in the Hebei Province plains area. In this work, the change in groundwater regime and the contributions of precipitation and water-saving irrigation development were investigated. The results indicate that the groundwater overdraft has been mitigated to some extent, mainly due to changes in precipitation and the implementation of water-saving irrigation, with contributions of 64.3% and 35.7%, respectively, when considering only these two factors. Water-saving irrigation is accepted as an important means for reducing groundwater depletion, but should be used in conjunction with other measures. 相似文献
58.
The performance of three different models, which simulate changes in the inorganic N content of the soil, was evaluated in respect of their ability to predict Nmin content in the spring under cereal crops. The models of British, Dutch and German origin, were tested using data from farmers' fields supplied by 70 farmers over two growing seasons in FRG. The models were run between harvest of the previous crop and spring of the following year, and predictions of Nmin in the spring compared to soil measurements. The performance of the models was assessed by counting the number of cases in which predictions agreed within 10 or 20 kg (N) ha–1 of the measurements. Predictions were less than ± 10 kg (N) ha–1 of measured values in only 30–44% and 28–55% of cases in 1988 and 1989, respectively. Predictions were less than ± 20 kg (N) ha–1 of measured values in 62–70% and 68–82% of cases in 1988 and 1989, respectively. Predictions in 1989 were better because the initial Nmin content in the autumn was included in the model input. None of the models tested had been designed to use input data of the type available to farmers. It is concluded that, at present, the results are too variable for any of the models to be used with confidence as tools to aid in N fertilizer recommendations. 相似文献
59.
北方渠道大多使用保温板防止混凝土衬砌受冻胀破坏。在计算保温板厚度时为了降低成本,容易忽略保温板使用过程中受各种因素的影响导致保温效果变差的情况,使混凝土衬砌达不到设计使用年限便提前被破坏。内蒙古河套灌区试验结果表明,由于受酸雨、冻融作用、含水量等因素的影响,保温板保温效果平均每年降低0.9℃,其中受含水量变化的影响最大。经试验测得:聚苯乙烯保温板(EPS)含水量每增加1%,会导致导热系数增大5%。基于多层材料串联热阻叠加原理和热阻等效原理,对保温板厚度计算公式进行完善,并提出相应的铺设方法和施工技术要求。该设计对保温板在季节性冻土区的推广应用具有一定参考价值。 相似文献
60.
André F. van Rooyen Peter Ramshaw Martin Moyo Richard Stirzaker Henning Bjornlund 《国际水资源开发杂志》2017,33(5):804-823
AbstractMany small-scale irrigation systems are characterized by low yields and deteriorating infrastructure. Interventions often erroneously focus on increasing yields and rehabilitating infrastructure. Small-scale irrigation systems have many of the characteristics of complex socio-ecological systems, with many different actors and numerous interconnected subsystems. However, the limited interaction between the different subsystems and their agents prevents learning and the emergence of more beneficial outcomes. This article reports on using Agricultural Innovation Platforms to create an environment in which irrigation scheme actors can engage, experiment, learn and build adaptive capacity to increase market-related offtake and move out of poverty. 相似文献