全文获取类型
收费全文 | 180455篇 |
免费 | 10335篇 |
国内免费 | 5311篇 |
专业分类
电工技术 | 6663篇 |
技术理论 | 5篇 |
综合类 | 9358篇 |
化学工业 | 27404篇 |
金属工艺 | 10453篇 |
机械仪表 | 9328篇 |
建筑科学 | 9448篇 |
矿业工程 | 3956篇 |
能源动力 | 3867篇 |
轻工业 | 10193篇 |
水利工程 | 3047篇 |
石油天然气 | 5726篇 |
武器工业 | 819篇 |
无线电 | 20762篇 |
一般工业技术 | 27816篇 |
冶金工业 | 7222篇 |
原子能技术 | 1327篇 |
自动化技术 | 38707篇 |
出版年
2024年 | 410篇 |
2023年 | 1602篇 |
2022年 | 2672篇 |
2021年 | 3951篇 |
2020年 | 2981篇 |
2019年 | 2684篇 |
2018年 | 16952篇 |
2017年 | 16242篇 |
2016年 | 12653篇 |
2015年 | 4498篇 |
2014年 | 5033篇 |
2013年 | 6049篇 |
2012年 | 9519篇 |
2011年 | 16048篇 |
2010年 | 14062篇 |
2009年 | 11006篇 |
2008年 | 12064篇 |
2007年 | 12848篇 |
2006年 | 5644篇 |
2005年 | 5852篇 |
2004年 | 4474篇 |
2003年 | 4431篇 |
2002年 | 4092篇 |
2001年 | 3256篇 |
2000年 | 2801篇 |
1999年 | 2583篇 |
1998年 | 2091篇 |
1997年 | 1724篇 |
1996年 | 1548篇 |
1995年 | 1294篇 |
1994年 | 1102篇 |
1993年 | 740篇 |
1992年 | 592篇 |
1991年 | 521篇 |
1990年 | 340篇 |
1989年 | 283篇 |
1988年 | 245篇 |
1987年 | 142篇 |
1986年 | 118篇 |
1985年 | 76篇 |
1984年 | 38篇 |
1982年 | 42篇 |
1980年 | 43篇 |
1968年 | 45篇 |
1966年 | 43篇 |
1965年 | 45篇 |
1959年 | 41篇 |
1958年 | 38篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
991.
Chih-Chun Hsieh Tao-Chih Chang Dong-Yih Lin Ming-Che Chen Weite Wu 《Metals and Materials International》2007,13(5):411-416
The purpose of this study is to investigate the precipitation characteristics of σ phase in the fusion zone of stainless steel
welds at various welding passes during a tungsten are welding (GTAW) process. The morphology, quantity, and chemical composition
of the δ-ferrite and σ phase were analyzed using optical microscopy (OM), a ferritscope (FS), a X-ray diffractometer (XRD),
scanning electron microscopy (SEM), an electron probe micro-analyzer (EPMA), and a wavelength dispersive spectrometer (WDS),
respectively. Massive δ-ferrite was observed in the fusion zone of the first pass welds during welding of dissimilar stainless
steels. The σ phase precipitated at the inner δ-ferrite particles and decreased δ-ferrite content during the third pass welding.
The σ and δ phases can be stabilized by Si element, which promoted the phase transformation of σ→ϱ+λ2 in the fusion zone of the third pass welds. It was found that the σ phase was a Fe−Cr−Si intermetallic compound found in
the fusion zone of the third pass welds during multi-pass welding. 相似文献
992.
Marielle Montginoul 《Water Resources Management》2007,21(5):861-871
This paper presents the results of a 2003 national survey of urban water pricing structure implemented by 429 district level
water utilities. After providing some background elements on the diversity of existing water pricing structures, the paper
shows how each structure can be used to achieve different management objectives (water allocation efficiency, costs recovery,
and equity). It then describes the structures adopted by French water utilities, showing that flat rate are rarely adopted;
declining blocks frequently used; and increasing blocks pricing which should be used to promote water use efficiency remain
extremely rare. A statistical clustering is then conducted and a typology of situations elaborated. The paper concludes with
highlighting that current pricing structures are influenced by past practices and that the dominant objective of water utilities
is to cover costs. 相似文献
993.
Bong-Un Lee Hwa-Suk Oh Ja-Young Kang 《Journal of Mechanical Science and Technology》2007,21(3):436-447
The geometric singularity problem is one of principal difficulty when using single-gimbal control moment gyros as spacecraft
attitude control devices. To overcome singularity, new steering logics are suggested in this paper which results in a reduction
in the difficulty of generating gimbal rates around a singular state. One of the suggested steering laws presented is the
reduced dimensional singular value decomposition steering law, which adopts the singular value decomposition in reduced dimensional
forms. Two other steering laws make use of the least square method in reduced dimensional forms. All of the suggested steering
laws have been generated for the compensation of the torque insufficiency. These logics are verified mathematically and simulations
at a singular condition and non-singular condition are performed to see how well they work. 相似文献
994.
深水钻井液体系中气体水合物的形成研究 总被引:1,自引:0,他引:1
近年来海洋石油勘探逐步向深水区域发展,由于水深问题带来的各种复杂情况给钻、完井带来了更为严峻的挑战。目前国内深水钻井液领域的研究尚属起步阶段,实验手段缺乏。结合相关的模拟实验对海洋所使用的钻井液体系气体水合物的生成情况进行了详细地研究,得出了抑制水合物形成的方法,即通过现场调节钻井液的密度来控制井筒中的压力;改变钻井液体系的冰点,使其自身的冰点尽可能低,这样就可以有效地避免深海钻进过程中气体水合物的生成。通过室内实验及分析,对海洋深水钻井液的发展提供一些借鉴。 相似文献
995.
996.
Use of climate scenarios to aid in decision analysis for interannual water supply planning 总被引:1,自引:0,他引:1
This work addresses the issue of climate change in the context of water resource planning on the time scale of a few years.
Planning on this time scale generally ignores the role of climate change. However, where the climate of a region has already
shifted, the use of historical data for planning purposes may be misleading. In order to test this, a case study is conducted
for a region, the Australian Capital Territory, where long term drought is raising concerns of a possible climate shift. The
issue is cast in terms of a particular planning decision; the option to augment water supply in the next few years to hedge
against the drought persisting. A set of climate scenarios are constructed for the region corresponding to the historical
climate regime and to regimes where progressively greater levels of change are assumed to have already taken place (5%, 10%,
20% reductions in mean rainfall). Probabilities of the drought persisting are calculated for each of the scenarios. The results
show substantial increases in the probability of the drought persisting for even moderate reductions in mean rainfall. The
sensitivity of the decision to augment supply to the scenario results depends ultimately on the planners tolerable thresholds
for the probability of the drought persisting. The use of different scenarios enables planners to explore the sensitivity
of the decision in terms of their risk tolerance to ongoing drought and to their degree of belief in each of the scenarios
tested. 相似文献
997.
李雯 《浙江工业大学学报》2007,(2)
新闻专业主义在西方拥有很大的理论市场,但这一理念所具有的理想主义色彩也不容忽视。多年来,新闻专业主义在其发展过程中不断受到严峻的现实挑战。本文分析了新闻媒体新闻专业主义缺失的深层原因,认为政治权力和市场机制等制度性框缚是新闻专业主义难以发挥其预期效果的根源所在。同时指出,新闻专业主义并非新闻改革的唯一出路。 相似文献
998.
Ram Devanathan William J. Weber 《JOM Journal of the Minerals, Metals and Materials Society》2007,59(4):32-35
The safe immobilization of nuclear waste in geological repositories is one of the major scientific challenges facing humanity
today. Crystalline ceramics hold the promise of locking up actinides from nuclear fuel and excess weapons plutonium in their
structure thereby isolating them from the environment. This paper presents the atomistic details of radiation damage in a
model ceramic, zircon. 相似文献
999.
难选氧化锰矿的选别工艺特征 总被引:2,自引:1,他引:1
氧化锰矿的泥化现象一般较严重,有的矿石很容易在机械作用下碎裂泥化,并且锰在粗、细粒级中的含量差别较大,因此在选别过程中避免过碎、强搅拌、强擦洗,并利用合理的分级工序,往往能获得十分理想的选别指标。 相似文献
1000.
利用不可逆循环图代替传统的半不可逆循环推导出Clausius积分式,并用熵方程证明之,得出可逆过程膨胀功大于不可逆过程膨胀功。 相似文献