全文获取类型
收费全文 | 38584篇 |
免费 | 4638篇 |
国内免费 | 3719篇 |
专业分类
电工技术 | 3307篇 |
技术理论 | 2篇 |
综合类 | 6807篇 |
化学工业 | 1677篇 |
金属工艺 | 816篇 |
机械仪表 | 2690篇 |
建筑科学 | 2652篇 |
矿业工程 | 808篇 |
能源动力 | 782篇 |
轻工业 | 2310篇 |
水利工程 | 1059篇 |
石油天然气 | 854篇 |
武器工业 | 499篇 |
无线电 | 5118篇 |
一般工业技术 | 3677篇 |
冶金工业 | 854篇 |
原子能技术 | 488篇 |
自动化技术 | 12541篇 |
出版年
2024年 | 130篇 |
2023年 | 482篇 |
2022年 | 878篇 |
2021年 | 990篇 |
2020年 | 1016篇 |
2019年 | 945篇 |
2018年 | 950篇 |
2017年 | 1162篇 |
2016年 | 1274篇 |
2015年 | 1368篇 |
2014年 | 2056篇 |
2013年 | 2311篇 |
2012年 | 2724篇 |
2011年 | 2936篇 |
2010年 | 2327篇 |
2009年 | 2519篇 |
2008年 | 2625篇 |
2007年 | 3005篇 |
2006年 | 2658篇 |
2005年 | 2363篇 |
2004年 | 1975篇 |
2003年 | 1638篇 |
2002年 | 1355篇 |
2001年 | 1147篇 |
2000年 | 1000篇 |
1999年 | 837篇 |
1998年 | 681篇 |
1997年 | 588篇 |
1996年 | 534篇 |
1995年 | 474篇 |
1994年 | 409篇 |
1993年 | 290篇 |
1992年 | 260篇 |
1991年 | 218篇 |
1990年 | 192篇 |
1989年 | 173篇 |
1988年 | 116篇 |
1987年 | 66篇 |
1986年 | 34篇 |
1985年 | 35篇 |
1984年 | 29篇 |
1983年 | 36篇 |
1982年 | 24篇 |
1981年 | 14篇 |
1980年 | 14篇 |
1979年 | 12篇 |
1978年 | 6篇 |
1977年 | 6篇 |
1976年 | 7篇 |
1964年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
141.
142.
In this paper we propose a new method to extract the vortices, sources, and sinks from the dense motion field preliminary estimated between two images of a fluid video. This problem is essential in meteorology for instance to identify and track depressions or convective clouds in satellite images. The knowledge of such points allows in addition a compact representation of the flow which is very useful in both experimental and theoretical fluid mechanics. The method we propose here is based on an analytic representation of the flow. This approach has the advantage of being robust, simple, fast and requires few parameters. 相似文献
143.
Large woody debris (LWD) can increase stream habitat heterogeneity by providing structure, altering flow patterns, enhancing sediment deposition, forming pools and retaining organic matter. In North America, the role of LWD has been studied extensively in streams of mature forests (e.g. Pacific Northwest), but few studies have assessed LWD in streams of younger forests (e.g. Midwestern USA). Our objectives were to: (1) quantify the volume and abundance of LWD in a set of Midwestern streams; (2) evaluate possible factors influencing LWD quantity; (3) identify the functional roles of LWD; and (4) compare LWD levels in the upper Midwest to those elsewhere in North America. In 2002 and 2003, we measured LWD and geomorphological variables in 15 low‐gradient streams draining previously logged watersheds in the Upper Peninsula of Michigan. Mean (±SE) LWD volume (0.77 ± 0.12 m3 100 m−2) and abundance (33 ± 3 pieces 100 m−1) were 71% and 10% lesser, respectively, than in streams of similar gradient elsewhere in North America. Channel shape (width:depth ratio) explained 30% of the variation in LWD volume (multiple stepwise regression, P = 0.015) while LWD length and length:channel width combined, explained 72% of the variation in LWD density (multiple stepwise regression, P < 0.0001). About 50% of the LWD either stored sediment or stabilized banks and 14% of the LWD formed pools, although pool density was not significantly related to LWD volume or density. LWD levels, overall, were low in upper Midwestern streams, but the relative importance of that LWD to ecosystem function may be magnified in these wood‐poor systems. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
144.
145.
E. Stenitzer H. Diestel Th. Zenker R. Schwartengräber 《Water Resources Management》2007,21(9):1567-1584
The performance of the water balance and crop growth model SIMWASER to estimate the water supply of a lawn by capillary rise
from shallow ground water was investigated by using lysimeter measurements with a sandy and a clayey soil. Moreover the robustness
of the model was evaluated by running it either with measured hydraulic soil parameters or with estimated pedotransfer functions
(PTFs) derived from texture and bulk density. Simulations were performed for the years 1996–1998 with input data from the
lysimeter station Berlin-Dahlem (Germany) for lysimeters containing undisturbed sandy or clayey soil monoliths, with groundwater
at 135 cm depth. Simulated evapotranspiration and percolation/capillary rise were in good agreement with the measured data
for all variants, while simulated soil water storage was acceptable only for the variants using hydraulic soil data based
on laboratory measurements or using PTFs derived from known soil class and bulk density. PTFs based on mean total pore volume
of the respective soil classes yielded soil water storages which were evidently too high; therefore they should be used with
care and must be avoided at all in simulating the soil water balance of arid sites with shallow groundwater. 相似文献
146.
On‐Line Detection Of State‐Of‐Charge In Lead Acid Battery Using Radial Basis Function Neural Network
To realize a stable supply of electric power in an automobile, an accurate and reliable detection method of SOC (state‐of‐charge) in a lead acid battery is required. However the dynamics of the battery is very complicated. The characteristics of the battery greatly change due to its degradation. Moreover a automobile has many driving patterns, which are unknown beforehand. Thus it is not easy to detect the SOC analytically. In this paper, to overcome this problem, a new on‐line SOC detection method with a radial basis function neural network is proposed. In order to increase the detection accuracy of degraded batteries, physical values related to the degradation degree are used as input signal in the neural network. The detection accuracies for different sized batteries and various degradation states are investigated. 相似文献
147.
148.
提出了一种计算积分方程的新方法——改进的逐次逼近解法 (MSAM) ,并用该方法计算了轴对称二维井间电磁场。与传统的逐次逼近解法 (SAM)相比 ,该方法收敛性强 ,应用范围广 ,可适用于高电导率对比地层。由于不必进行直接的大型矩阵求逆运算 ,因此与积分方程的直接解法 (IE)相比 ,该方法计算速度更快 ,所需内存更少。采用矩阵求逆方法计算了成层介质中的二维 Green函数 ,并对含 Green函数的积分进行了简化 ,从而加快了计算速度。数值计算结果显示 ,在地层电导率对比度达到 2个数量级时 MSAM仍收敛 ,且计算结果与直接求解积分方程的结果一致 ,因此 MSAM是一种有效的计算轴对称二维井间电磁场的方法 相似文献
149.
变异函数在进间砂体预测中的应用 总被引:1,自引:0,他引:1
应用变异函数探讨了砂体的定量预测问题。通过对北二东密井网试验区不同类型砂体,不同井网密和岩厚度的预测,检验了此方法的精度(为70%左右),在原方法的基础上,通过对预报精度的分析并结合精度地质研究进一步完善了该方法,提出二次建模法并应用于北二东西块聚驱井的砂岩厚度预测中,预测精度可提高5%以上,从而说明应用变异函数研究砂体的定量预测问题具有可借鉴性,可以为今后新井调整厚度的预测及砂体描述提供依据。 相似文献
150.
Keh-Shin Lii 《时间序列分析杂志》1985,6(3):153-169
Abstract. A distributed lag model with a rational transfer function is considered. We have demonstrated that the C -table can be used to determine the orders of the rational transfer function and the Padé table can be used to estimate the parameters of an identifiable transfer function. In case the model is not uniquely identified, the C -table reduces possible competing models to only a few for further examination. Padé approximant gives consistent initial values for possibly more efficient iterative procedures to estimate the coefficients of the identified model. Some asymptotic results on the estimation of the C -table and the Padé table are given. A few examples are presented to demonstrate the effectiveness of the method. 相似文献