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   1449774篇
  免费   27202篇
  国内免费   6892篇
工业技术   1483868篇
  2021年   14585篇
  2020年   11763篇
  2019年   14544篇
  2018年   15391篇
  2017年   14773篇
  2016年   21183篇
  2015年   17278篇
  2014年   28572篇
  2013年   87650篇
  2012年   34730篇
  2011年   46774篇
  2010年   41945篇
  2009年   50620篇
  2008年   43861篇
  2007年   41028篇
  2006年   43968篇
  2005年   38716篇
  2004年   40986篇
  2003年   40901篇
  2002年   37870篇
  2001年   34297篇
  2000年   32325篇
  1999年   31563篇
  1998年   39208篇
  1997年   35176篇
  1996年   31634篇
  1995年   29556篇
  1994年   27752篇
  1993年   27608篇
  1992年   25460篇
  1991年   22605篇
  1990年   23001篇
  1989年   21990篇
  1988年   20426篇
  1987年   18792篇
  1986年   18261篇
  1985年   21618篇
  1984年   21850篇
  1983年   19800篇
  1982年   18878篇
  1981年   18945篇
  1980年   17633篇
  1979年   18170篇
  1978年   17412篇
  1977年   17114篇
  1976年   17792篇
  1975年   15705篇
  1974年   15238篇
  1973年   15330篇
  1972年   12875篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The objective of this study was to evaluate the effects of varying the ratio of dietary palmitic (C16:0), stearic (C18:0), and oleic (cis-9 C18:1) acids in basal diets containing soyhulls or whole cottonseed on nutrient digestibility, energy partitioning, and production response of lactating dairy cows. Twenty-four mid-lactation multiparous Holstein cows were used in a split-plot Latin square design. Cows were allocated to a main plot receiving either a basal diet with soyhulls (SH, n = 12) or a basal diet with whole cottonseed (CS, n = 12) that was fed throughout the experiment. Within each plot a 4 × 4 Latin square arrangement of treatments was used in 4 consecutive 21-d periods. Treatments were (1) control (CON; no supplemental fat), (2) high C16:0 supplement [PA; fatty acid (FA) supplement blend provided ~80% C16:0], (3) C16:0 and C18:0 supplement (PA+SA; FA supplement blend provided ~40% C16:0 + ~40% C18:0), and (4) C16:0 and cis-9 C18:1 supplement (PA+OA; FA supplement blend provided ~45% C16:0 + ~35% cis-9 C18:1). Interactions between basal diets and FA treatments were observed for dry matter intake (DMI) and milk yield. Among the SH diets, PA and PA+SA increased DMI compared with CON and PA+OA treatments, whereas in the CS diets PA+OA decreased DMI compared with CON. The PA, PA+SA, and PA+OA treatments increased milk yield compared with CON in the SH diets. The CS diets increased milk fat yield compared with the SH diets due to the greater yield of de novo and preformed milk FA. The PA treatment increased milk fat yield compared with CON, PA+SA, and PA+OA due to the greater yield of mixed-source (16-carbon) milk FA. The PA treatment increased 3.5% fat-corrected milk compared with CON and tended to increase it compared with PA+SA and PA+OA. The CS diets increased body weight (BW) change compared with the SH diets. Additionally, PA+OA tended to increase BW change compared with CON and PA and increased it in comparison with PA+SA. The PA and PA+OA treatments increased dry matter and neutral detergent fiber digestibility compared with PA+SA and tended to increase them compared with CON. The PA+SA treatment reduced 16-carbon, 18-carbon, and total FA digestibility compared with the other treatments. The CS diets increased energy partitioning toward body reserves compared with the SH diets. The PA treatment increased energy partitioning toward milk compared with CON and PA+OA and tended to increase it compared with PA+SA. In contrast, PA+OA increased energy partitioned to body reserves compared with PA and PA+SA and tended to increase it compared with CON. In conclusion, milk yield responses to different combinations of FA were affected by the addition of whole cottonseed in the diet. Among the combinations of C16:0, C18:0, and cis-9 C18:1 evaluated, fat supplements with more C16:0 increased energy output in milk, whereas fat supplements with more cis-9 C18:1 increased energy storage in BW. The combination of C16:0 and C18:0 reduced nutrient digestibility, which most likely explains the lower performance observed compared with other treatments.  相似文献   
992.
Since the 1990s, some urban universities have served as neighborhood anchors with an interest in revitalization. Current theory suggests anchors adopt ‘shared value’ approaches, leveraging resources for mutually beneficial improvement in the community. This study explores assumptions in contemporary anchor frameworks and uses a survey to examine how 22 U.S. universities approach their roles as anchor institutions. The study finds that the universities tend to prioritize place-based initiatives, while contemporary frameworks are more normative and highlight socioeconomic practices. Based on reported strategies, the author proposes an alternate typology that accounts for the ways universities most commonly describe anchor approaches, complementing contemporary theory.  相似文献   
993.
The effects of local body cooling on thermal comfort and sleep quality in a hot environment were investigated in an experiment with 16 male subjects. Sleep quality was evaluated subjectively, using questionnaires completed in the morning, and objectively, by analysis of electroencephalogram (EEG) signals that were continuously monitored during the sleeping period. Compared with no cooling, the largest improvement in thermal comfort and sleep quality was observed when the back and head (neck) were both cooled at a room temperature of 32°C. Back cooling alone also improved thermal comfort and sleep quality, although the effects were less than when cooling both back and head (neck). Mean sleep efficiency was improved from 84.6% in the no cooling condition to 95.3% and 92.8%, respectively, in these conditions, indicating good sleep quality. Head (neck) cooling alone slightly improved thermal comfort and subjective sleep quality and increased Stage N3 sleep, but did not otherwise improve sleep quality. The results show that local cooling applied to large body sections (back and head) could effectively maintain good sleep and improve thermal comfort in a hot environment.  相似文献   
994.
Currently, there is little information pertaining to the airborne bacterial communities of green buildings. In this case study, the air bacterial community of a zero carbon building (ZCB) in Hong Kong was characterized by targeting the bacterial 16S rRNA gene. Bacteria associated with the outdoor environment dominated the indoor airborne bacterial assemblage, with a modest contribution from bacteria associated with human skin. Differences in overall community diversity, membership, and composition associated with short (day‐to‐day) and long‐term temporal properties were detected, which may have been driven by specific environmental genera and taxa. Furthermore, time‐decay relationships in community membership (based on unweighted UniFrac distances) and composition (based on weighted UniFrac distances) differed depending on the season and sampling location. A Bayesian source‐tracking approach further supported the importance of adjacent outdoor air bacterial assemblage in sourcing the ZCB indoor bioaerosol. Despite the unique building attributes, the ZCB microbial assemblage detected and its temporal characteristics were not dissimilar to that of conventional built environments investigated previously. Future controlled experiments and microbial assemblage investigations of other ZCBs will undoubtedly uncover additional knowledge related to how airborne bacteria in green buildings may be influenced by their distinctive architectural attributes.  相似文献   
995.
Smart cities are built upon information and communication technologies (ICTs) to enable a broad range of advanced services. Through a comprehensive literature review, this study identified four pitfalls brought by the pervasive application of ICT, including information insecurity, privacy leakage, information islands, and digital divide. Therefore, a questionnaire survey together with 27 interviews was conducted in Hong Kong to investigate how the public perceived these pitfalls within the context of mobile apps providing real-time parking information which form a major part of smart mobility. System insecurity and privacy leakage were found to arouse worries among the app-users while their awareness of protecting personal data was found to have room for improvement. Islands of real-time parking information occur as a result of the lack of collaboration among private carpark operators. Digital divide existed widely among the disadvantaged groups and the problem cannot be solved by mere provision of ICT facilities. Overall, technologies alone cannot make a city smart or smarter. It is the suitable way in which ICTs are used to serve all citizens that matters.  相似文献   
996.
Carrier aggregation (CA) is a promising bandwidth extension technique which in turn increases the data rate. A scalable and flexible Orthogonal frequency-division multiplexing (OFDM) can be considered as a key element in 5G wireless communications. The combination of CA and OFDM is a prominent factor in LTE-A system followed by strict synchronicity. So, carrier frequency offset (CFO) is an important parameter to be considered and corrected. In carrier-aggregated OFDM (CA-OFDM) systems, multiple carrier frequency offsets (MCFOs) are a non linear synchronization error that induces inter carrier interference (ICI) to the system. To solve this problem, our self proposed Diversity Enhanced Particle Filter (DE-PF) with a novel resampling algorithm is used in downlink CA-OFDM receiver structure with Hadamard sequence based prominent matching algorithm to match the estimated MCFO to its corresponding Component Carrier (CC). Simulations prove that with DE-PF algorithm and proposed Hadamard matching algorithm improves MCFO estimation accuracy to a greater extent than the existing linear and non linear algorithms.  相似文献   
997.
Wireless Personal Communications - The advancements of technology in the field of communication made WSN based IoT attractive and applicable to various areas. It is comprised IoT nodes that work on...  相似文献   
998.
999.
Damage in a thin nickel film irradiated by subpicosecond pulses of terahertz (THz) radiation in the range of 1–3 THz at electric-field strengths up to 20 MV/cm at the center of a focal spot is observed. The damage threshold fluence is evaluated for single-pulse experiments. The damage pattern induced by multiple THz pulses has the appearance of a complex periodic structure in the form of elongated channels of metal film discontinuity that are perpendicular to the in-plane electric field direction of THz radiation.  相似文献   
1000.
Ordered arrays of TiO2 nanotubes with smooth and rippled morphologies were prepared by one-step titanium oxidation in NH4F and ethylene glycol solution. The samples were then decorated with ZnS using a microwave-assisted solvothermal method. The experiments under constant or pulsed applied voltage resulted in smooth and rippled TiO2material morphologies, respectively. Field emission scanning electron microscopy, incident photon-to-current efficiency, linear sweep voltammetry and electrochemical impedance spectroscopy were used to investigate the structure and morphology of the TiO2 nanotubes, along with their photoelectrochemical activity in the water splitting reaction. An envelope function was proposed to correlate the anisotropic morphologies and broad distribution of mobility due to the random nature of charge carrier transport. The smooth and rippled morphologies were evaluated using the transmission line model. First-principles quantum mechanical calculations based on the density functional theory at the B3LYP level are conducted to obtain a better understanding of optical properties of TiO2.  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