全文获取类型
收费全文 | 62715篇 |
免费 | 6765篇 |
国内免费 | 3837篇 |
专业分类
电工技术 | 5553篇 |
技术理论 | 17篇 |
综合类 | 7363篇 |
化学工业 | 5383篇 |
金属工艺 | 1435篇 |
机械仪表 | 2647篇 |
建筑科学 | 13637篇 |
矿业工程 | 1701篇 |
能源动力 | 1621篇 |
轻工业 | 2947篇 |
水利工程 | 2034篇 |
石油天然气 | 2061篇 |
武器工业 | 693篇 |
无线电 | 6123篇 |
一般工业技术 | 4669篇 |
冶金工业 | 2517篇 |
原子能技术 | 616篇 |
自动化技术 | 12300篇 |
出版年
2024年 | 336篇 |
2023年 | 871篇 |
2022年 | 1690篇 |
2021年 | 1931篇 |
2020年 | 2267篇 |
2019年 | 1985篇 |
2018年 | 1641篇 |
2017年 | 2212篇 |
2016年 | 2408篇 |
2015年 | 2477篇 |
2014年 | 4267篇 |
2013年 | 4280篇 |
2012年 | 4309篇 |
2011年 | 4783篇 |
2010年 | 3739篇 |
2009年 | 3768篇 |
2008年 | 3554篇 |
2007年 | 4096篇 |
2006年 | 3804篇 |
2005年 | 3154篇 |
2004年 | 2563篇 |
2003年 | 2353篇 |
2002年 | 2090篇 |
2001年 | 1835篇 |
2000年 | 1519篇 |
1999年 | 1088篇 |
1998年 | 815篇 |
1997年 | 680篇 |
1996年 | 487篇 |
1995年 | 415篇 |
1994年 | 358篇 |
1993年 | 275篇 |
1992年 | 216篇 |
1991年 | 177篇 |
1990年 | 135篇 |
1989年 | 120篇 |
1988年 | 81篇 |
1987年 | 78篇 |
1986年 | 53篇 |
1985年 | 60篇 |
1984年 | 42篇 |
1983年 | 33篇 |
1982年 | 34篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 24篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1964年 | 14篇 |
1955年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
This article is the second of two papers that review the field of spatially sensitive social scientific research into the links between social status and transport disadvantage. The first paper undertook a comprehensive review of the social scientific and transport planning literature to mark the level of development in the field and identify conceptual and methodological issues and constraints in this field of inquiry. The present article supports the advancement of socially and geographically sensitive transport research by opportunities for the development of more sophisticated spatial analytical methodologies. The approach we present is able to account for factors not previously addressed in either social or transport planning research, in particular the temporal dimensions of transport service accessibility. The article articulates the methodology through an empirical case study of socio-spatial transport disadvantage within the Gold Coast City. The article demonstrates that there are important theoretical and practical lessons to be gained for researchers and policy makers in addressing the social dimensions of transport and infrastructure provision. Further, the article argues that an attentiveness to new ways of combining and representing social and transport data-sets can promote policy relevant empirical social inquiry. The article also contributes in a productive way to the empirical knowledge of Australia's sixth-largest metropolitan area, which is often overlooked by urban scholars. 相似文献
42.
潜油电动机滑行时间是评定电动机装配质量的重要指标。鉴于手工测量电动机滑行时间存在效率低和误差较大的不足,设计了潜油电动机滑行时间自动测量系统。它通过分压电路和计算机自动测量系统测量潜油电动机定子上感应的电压变化状况,即可得到电动机滑行时间。用手工测量、波形分析和自动测量3种方法测量6种型号电动机的滑行时间,其中5种滑行时间长于3s,判定为合格;1种滑行时间短于3s,判定为不合格。3种方式测量的滑行时间具有较好的一致性,仅波形分析法得出的滑行时间比自动测量的滑行时间稍长,表明自动测量方法的精度和重复性可满足要求。 相似文献
43.
The paper considers a generalized discrete‐time order‐replacement model for a single unit system, which is subject to random failure when in operation. Two types of discrete randomized lead times are considered for a spare unit; one is for regular (preventive) order and another is for expedited (emergency) order. The model is formulated based on the discounted cost criterion. The underlying two‐dimensional optimization problem is reduced to a simple one‐dimensional one and then the optimal ordering policy for the spare unit is characterized under two extreme conditions: (i) unlimited inventory time and (ii) zero inventory time for the spare unit. A numerical example is used to determine the optimal spare‐ordering policy numerically and to examine the sensitivity of the model parameters. 相似文献
44.
本文从梯形水力最优断面的基本思路出发,通过理论分析找到了梯形断面水力计算的一种新方法——即通过特征参数进行梯形断面的水力计算。 相似文献
45.
飞行时间二次离子质谱--强有力的表面、界面和薄膜分析手段 总被引:8,自引:0,他引:8
二次离子质谱(Secondary ion mass spectrometry,简称SIMS)是一种对表面灵敏的质谱技术,建立在表面各种类型带正、负电荷原子或分子发射的基础上。用飞行时间(Time of flight,简称TOF)仪器对这些二次离子进行质量分析,能确保并行质量登录、高质量范围、高流通率下的高分辨和精确质量测定这些优异性能。配合细聚焦扫描一次离子束,可在优于1nm的高深度分辨和优于50nm的横向分辨本领下,实现对表面优于单层ppm(百万分之一)量级的极高检测灵敏度。当今TOF-SIMS已发展为一种成熟且完善的表面分析技术。极高的灵敏度,再加上即使对大分子及不易挥发性分子都独具的敏感性,使它成为很多高技术领域不可缺少的分析手段,这些领域包括微电子学、化学和材料科学以至纳米技术和生命科学等。本文简述了TOF-SIMS的原理、仪器及其多方面的应用和展望。 相似文献
46.
S. Vaidyanathan K. M. Kavadia L. P. Borkar S. P. Mahajan 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1985,35(2):121-128
With a view to evaluate the effects of hydraulic retention time and volatile solids loading rate on methane production using water hyacinth, experiments were conducted in a 160 dm3 capacity, well-mixed continuous digestion unit at ambient temperature. The optimal design of biogas plant using water hyacinth was elucidated, to provide a unit giving a maximum methane yield of 0.16 nm3 kg?1 volatile solids added, which was 60% greater than the conventional plant using cowdung. The calculations of optimal design factors were based on reactor size, hydraulic retention time and volatile solids loading rate; considerations were made of capital cost, running costs and operational revenue. 相似文献
47.
揭示了城市环境的基本性质的基本特征,研究了城市环境对人的心理影响和生理影响。指出城市环境建设不公要满足人的物质需要,还要满足人的精神需要。 相似文献
48.
49.
50.
Transport problems typically involve at least two types of constraints, on income and on time. Therefore, the indirect utility
function depends either on the income available after having subtracted the cost of the discrete alternative and on the free
time left after having worked and travelled by each competing option. In the typical linear-in-the-attributes and in-the-parameters
specification, that represents the first grade approximation of the indirect utility function, the effect of income and time
constraints cancel out and only the cost and time of the alternatives matter in the comparison between them. From a microeconomic
point of view this is equivalent to assume that income and time effects could be disregarded; which is not always the case.
To account for these effects the utility function should include second order attributes; however, in non-linear utility functions
it may not be easy to distinguish among several effects that could be relevant: direct preferences for good and leisure, and
simple interactions between attributes other than income and time effects. This paper analyses these effects from a theoretical
point of view focusing on the possible confounding problem in detecting income and time effects. We use a dataset collected
for a modal choice context and containing both revealed and stated preference data, and estimate several NL models examining
the effect of the different second-order terms on detecting income and time effects. We compared specifications including
square cost and time attributes, interactions between time and cost, cost divided by the income available to be spent on free
time, and time multiplied by free time. Our results confirm the strong effect of direct preferences for goods and leisure
time on choice, and the potential confounding effect between quadratic attributes and other non-linear omitted terms. Finally,
we also found that care should be taken in highlighting income and time effects using mixed data sources, since confounding
effects can occur when non-linearities are accounted for in both data sets. 相似文献