首页 | 官方网站   微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1627篇
  免费   190篇
  国内免费   129篇
工业技术   1946篇
  2024年   5篇
  2023年   28篇
  2022年   52篇
  2021年   71篇
  2020年   63篇
  2019年   44篇
  2018年   57篇
  2017年   52篇
  2016年   88篇
  2015年   92篇
  2014年   114篇
  2013年   117篇
  2012年   102篇
  2011年   140篇
  2010年   94篇
  2009年   106篇
  2008年   91篇
  2007年   87篇
  2006年   95篇
  2005年   82篇
  2004年   61篇
  2003年   58篇
  2002年   42篇
  2001年   29篇
  2000年   40篇
  1999年   23篇
  1998年   18篇
  1997年   18篇
  1996年   12篇
  1995年   15篇
  1994年   12篇
  1993年   4篇
  1992年   4篇
  1991年   4篇
  1990年   3篇
  1989年   5篇
  1988年   1篇
  1986年   5篇
  1985年   3篇
  1984年   1篇
  1983年   2篇
  1982年   2篇
  1980年   1篇
  1979年   1篇
  1976年   1篇
  1959年   1篇
排序方式: 共有1946条查询结果,搜索用时 62 毫秒
1.
In our quest to understand wine industries and their location and development worldwide, much emphasis has been given to key environmental, cultural and economic factors. The concept of terroir is used widely to describe, delimit and promote favoured wine regions. In this, we see constructions of rurality that tend to render invisible important connections between wine regions and urban places. However, many if not most wine regions owe much to their urban neighbours. Cities are sources of labour, capital and customers. Indeed, many wine regions are, in effect, metropolitan wine regions that owe as much to their proximity to cities than they do to any putative environmental distinctiveness and advantages. Wineries in such places develop strategies to survive and prosper not by producing bottles of wine that are widely sold on global markets – and compete on the basis on price or global reputation – but rather because they develop strong and loyal local markets tied to urban economies and because they appeal to the rural idylls held by urban consumers. This paper explores the way urban places have been critical in the development of certain wine regions in New Zealand.  相似文献   
2.
3.
Greenhouses in northern climates require a large amount of supplemental heating for growing crops in winter seasons, so energy‐efficient design of greenhouses based on local climate is important to minimize the heating demand. In this study, greenhouse design parameters including shape, orientation, the angle of the roof, and width of the span have been studied for the conceptual design of conventional greenhouses for Canadian Prairies using a heating simulation model. Five different shapes of greenhouses including even‐span, uneven‐span, modified arch, vinery, and quonset shape have been selected for the study. The simulation results proved that the uneven‐span gable roof shape receives the highest solar radiation, whereas the quonset shape receives the lowest solar radiation. However, the quonset shape greenhouse requires about 7.6% less annual heating as compared to the gable roof greenhouse, but the quonset would not be adopted as multispan greenhouses. Therefore, the gable roof greenhouse is considered as energy efficient for the multispan gutter connected greenhouses whereas quonset shape as a free‐standing single‐span greenhouses. In high northern latitudes, the greenhouse with east‐west orientation is more energy efficient from heating and cooling point of view when the length‐width ratio of the greenhouse is more than 1. The heating energy saving potential of the large span width in single‐span greenhouses is relatively higher as compared to the multispan greenhouses.  相似文献   
4.
5-Hydroxymethylcytosine (5hmC) is a functionally active epigenetic modification. We analyzed whether changes in DNA 5-hydroxymethylation are an element of age-related epigenetic drift. We tested primary fibroblast cultures originating from individuals aged 22–35 years and 74–94 years. Global quantities of methylation-related DNA modifications were estimated by the dot blot and colorimetric methods. Regions of the genome differentially hydroxymethylated with age (DHMRs) were identified by hMeDIP-seq and the MEDIPS and DiffBind algorithms. Global levels of DNA modifications were not associated with age. We identified numerous DHMRs that were enriched within introns and intergenic regions and most commonly associated with the H3K4me1 histone mark, promoter-flanking regions, and CCCTC-binding factor (CTCF) binding sites. However, only seven DHMRs were identified by both algorithms and all of their settings. Among them, hypo-hydroxymethylated DHMR in the intron of Rab Escort Protein 1 (CHM) coexisted with increased expression in old cells, while increased 5-hydroxymethylation in the bodies of Arginine and Serine Rich Protein 1 (RSRP1) and Mitochondrial Poly(A) Polymerase (MTPAP) did not change their expression. These age-related differences were not associated with changes in the expression of any of the ten-eleven translocation (TET) enzymes or their activity. In conclusion, the distribution of 5hmC in DNA of in vivo aged human fibroblasts underwent age-associated modifications. The identified DHMRs are, likely, marker changes.  相似文献   
5.
This study considers an efficient method for the estimation of quantiles associated to very small levels of probability (up to O(10−9)), where the scalar performance function J is complex (eg, output of an expensive-to-run finite element model), under a probability measure that can be recast as a multivariate standard Gaussian law using an isoprobabilistic transformation. A surrogate-based approach (Gaussian Processes) combined with adaptive experimental designs allows to iteratively increase the accuracy of the surrogate while keeping the overall number of J evaluations low. Direct use of Monte-Carlo simulation even on the surrogate model being too expensive, the key idea consists in using an importance sampling method based on an isotropic-centered Gaussian with large standard deviation permitting a cheap estimation of small quantiles based on the surrogate model. Similar to AK-MCS as presented in the work of Schöbi et al., (2016), the surrogate is adaptively refined using a parallel infill criterion of an algorithm suitable for very small failure probability estimation. Additionally, a multi-quantile selection approach is developed, allowing to further exploit high-performance computing architectures. We illustrate the performances of the proposed method on several two to eight-dimensional cases. Accurate results are obtained with less than 100 evaluations of J on the considered benchmark cases.  相似文献   
6.
An improved multi-recessed 4H–SiC metal semiconductor field effect transistor (MRD-MESFET) with double-recessed p-buffer layer (DRB-MESFET) is proposed in this paper. By introducing a double-recessed p-buffer layer, the gate depletion layer is further modulated, and higher drain saturation current and DC transconductance are obtained compared with the MRD-MESFET. The simulations show that the drain saturation current of the DRB-MESFET is about 42.4% larger than that of the MRD-MESFET. The DC transconductance of the DRB-MESFET is almost 15% higher than that of the MRD-MESFET and very close to that of double-recessed structure (DR-MESFET) at the bias conditions of Vgs=0 V and Vds=40 V. The proposed structure has an improvement of 26.1% and 74.2% in the output maximum power density compared with that of the MRD-MESFET and DR-MESFET, respectively. In the meanwhile, the proposed structure possesses smaller gate-source capacitance, which results in better RF characteristics.  相似文献   
7.
杨程  李夕兵 《矿冶工程》2020,40(2):147-153
以我国矿业对世界主要经济体直接投资为例, 基于2009~2018年投资存量数据, 在多区域结构分析框架下, 运用投资多样性指数、动态时空模型、灰色系统预测模型分别对投资区位分布特征、空间经济效应以及未来分布趋势进行了研究。结果显示: ① 2009~2018年, 我国矿业对世界主要经济体直接投资区位分布呈现出“由高度集中转向渐进扩散”“高者仍高、低者仍低”的集团俱乐部式时空分布特征;② 投资区位分布受到了来自于其他国家(地区)全局效应和相邻效应的影响;③ 2019~2020年, 在空间经济效应作用下, 投资渐进扩散趋势仍将持续, 集团俱乐部式分布有所减弱;我国矿业对欧盟、俄罗斯联邦、东盟、美国的投资机会及热度将有所增加。依据研究结果, 提出了我国矿业海外直接投资的启示及建议。  相似文献   
8.
9.
在高原多年冻土地区,气候恶劣,生态环境脆弱,钻孔灌注桩施工中又不可避免地会遇到土层处于不同的冻结状态,施工中必须考虑并设法预防和消除由于冻土的变形、强度弱化及冻胀、融沉所引起的各种病害,因此钻孔灌注桩施工在质量、安全和环保等方面难度很大。介绍了布曲河特大桥钻孔灌注桩施工工艺,该工艺明显不同于内地通常的钻孔灌注桩施工工艺,在高原多年冻土地区是行之有效的,对高原多年冻土区钻孔灌注桩施工具有重要的参考意义。  相似文献   
10.
针对青藏铁路特有的"高原"和"冻土"问题,阐述了多年冻土区路基设计原则和对路堤、路堑、支挡工程、取土、弃土、环境保护等方面的施工工艺和注意问题。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司    京ICP备09084417号-23

京公网安备 11010802026262号