全文获取类型
收费全文 | 6150篇 |
免费 | 896篇 |
国内免费 | 276篇 |
专业分类
电工技术 | 76篇 |
综合类 | 536篇 |
化学工业 | 1108篇 |
金属工艺 | 59篇 |
机械仪表 | 40篇 |
建筑科学 | 453篇 |
矿业工程 | 61篇 |
能源动力 | 190篇 |
轻工业 | 2067篇 |
水利工程 | 184篇 |
石油天然气 | 254篇 |
武器工业 | 17篇 |
无线电 | 1032篇 |
一般工业技术 | 225篇 |
冶金工业 | 235篇 |
原子能技术 | 18篇 |
自动化技术 | 767篇 |
出版年
2024年 | 37篇 |
2023年 | 162篇 |
2022年 | 256篇 |
2021年 | 364篇 |
2020年 | 326篇 |
2019年 | 338篇 |
2018年 | 261篇 |
2017年 | 268篇 |
2016年 | 297篇 |
2015年 | 282篇 |
2014年 | 378篇 |
2013年 | 392篇 |
2012年 | 451篇 |
2011年 | 472篇 |
2010年 | 390篇 |
2009年 | 375篇 |
2008年 | 329篇 |
2007年 | 388篇 |
2006年 | 295篇 |
2005年 | 239篇 |
2004年 | 205篇 |
2003年 | 159篇 |
2002年 | 186篇 |
2001年 | 85篇 |
2000年 | 59篇 |
1999年 | 48篇 |
1998年 | 21篇 |
1997年 | 33篇 |
1996年 | 28篇 |
1995年 | 36篇 |
1994年 | 16篇 |
1993年 | 25篇 |
1992年 | 14篇 |
1991年 | 19篇 |
1990年 | 12篇 |
1989年 | 8篇 |
1988年 | 11篇 |
1987年 | 9篇 |
1986年 | 2篇 |
1985年 | 11篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1981年 | 3篇 |
1980年 | 11篇 |
1977年 | 1篇 |
1972年 | 1篇 |
1963年 | 1篇 |
1954年 | 1篇 |
1951年 | 4篇 |
排序方式: 共有7322条查询结果,搜索用时 62 毫秒
991.
曹叶文 《山东大学学报(工学版)》1999,(5)
多载波频率分集扩展频谱码分多址通信(FD/SSMA,Frequency DiversitySpread Spectrum Multiple Access )系统是一种基于多载波传输、利用与直接序列扩展频谱码分多址系统(DS/SSMA)的时间- 频率对偶关系而提出的新的扩展频谱多址方案. 本文分析在AGWN 信道下FD/SSMA 的抗窄带干扰能力,并利用高斯近似及仿真得到了系统的平均误比特率. 相似文献
992.
通过对红油味、姜汁味、蒜泥味3种川味冷菜复合调味汁抑菌作用的研究,发现各调味汁中所用的香辛料不同,对原料自身所携带的微生物及试验中选用的2种肠道病原菌的抑制程度各异。其中蒜泥味的抑菌作用最强,姜汁味次之,红油味几乎无抑菌能力。并揭示了配制冷莱复合调味汁时的用油量与调味汁的抑菌程度呈反比关系,为烹饪工艺科学化提供了一定的理论指导。 相似文献
993.
污泥自然通风静态堆肥试验研究表明,堆肥过程中温度的变化引起微生物数量和种群的交替变化,从而影响对有机质的分解。而污泥堆肥过程有机质的降解主要是在中温阶段完成的,其中中,高温菌群起重要的作用。 相似文献
994.
We consider the use of distributed antennas to increase the capacity and peak data rate achievable in a microcellular CDMA system with limited bandwidth. In additon to the diversity against Rayleigh fading achievable by use of microdiversity among nearly co-located transmit or receive antennas, we exploit macrodiversity against shadow fading that more widely separated antennas permit. We report on antenna configurations for both directional and omni-directional antennas that provide the most uniform signal-to-interference ratio coverage, averaged over a large number of position vectors drawn from a spatially uniform distribution of mobiles. Call capacities and peak transmission rates are determined for an integrated system carrying traffic at different constant rates, where processing gain and the transmission rate are selected to satisfy a common chip rate. For the downlink a 5.5 dB capacity gain can be achieved for 64 kb/s calls using four antennas located on the diagonals of each square cell. A bandwidth of 5 MHz allows two or more calls to be simultaneously supported at data rates up to 512 kb/s, as opposed to only 128 kb/s for three co-located antennas. On the uplink we distinguish between the computationally simpler equal-gain combining of the antenna signals and the possibly more complex maximum-ratio combining. With equal gain combining we achieve a peak data rate of 128 kb/s and a capacity gain of 2.5 dB relative to equal gain combining of three nearly co-located antenna signals. With maximum ratio combining the peak uplink rate can be as high as 512 kb/s and the capacity is increased by 2.0 dB relative to the maximum-ratio combining of three co-located antennas. 相似文献
995.
A comprehensive analytical bit-error-rate (BER) model is presented to analyse the performance of antenna-microdiversity for wideband BPSK modulated signals in the frequency selective fading multipath channel, specified by its complex impulse response. The model includes the disturbance by intersymbol interference (ISI) and co-channel interference (CCI), as well as the channels' impact on the carrier phase- and clock recovery in the receiver. The channel impulse responses at the antenna elements are determined by taking into account the direction of arrival of the individual paths. Computational BER- and SNIR-gain results (SNIR = signal-to-noise+interference-ratio) show that a substantial performance improvement is achieved with antenna combining for wideband signals which suffer ISI and/or CCI. For the indoor multipath channel with exponentially decaying power delay profile, the performance enhancement is compared for several antenna combining schemes. Quasi-coherent equal gain combining (QCEGC) is proposed as an novel EGC scheme based on a less accurate phase estimation technique. For wideband signals, QCEGC shows a slight performance degradation when compared to maximal ratio combining or minimum mean square error combining (MMSEC), but has a much lower implementation complexity. In the channel with CCI, where the best performance is achieved with MMSEC, QCEGC performs very poor. 相似文献
996.
Microbial fuel cells (MFCs) have attracted considerable interest due to their potential in renewable electrical power generation using the broad diversity of biomass and organic substrates. However, the difficulties in achieving high power densities and commercially affordable electrode materials have limited their industrial applications to date. Carbon materials, which can exhibit a wide range of different morphologies and structures, usually possess physiological activity to interact with microorganisms and are therefore fast‐emerging electrode materials. As the anode, carbon materials can significantly promote interfacial microbial colonization and accelerate the formation of extracellular biofilms, which eventually promotes the electrical power density by providing a conductive microenvironment for extracellular electron transfer. As the cathode, carbon‐based materials can function as catalysts for the oxygen‐reduction reaction, showing satisfying activities and efficiencies nowadays even reaching the performance of Pt catalysts. Here, first, recent advancements on the design of carbon materials for anodes in MFCs are summarized, and the influence of structure and surface functionalization of different types of carbon materials on microorganism immobilization and electrochemical performance is elucidated. Then, synthetic strategies and structures of typical carbon‐based cathodes in MFCs are briefly presented. Furthermore, future applications of carbon‐electrode‐based MFC devices in the energy, environmental, and biological fields are discussed, and the emerging challenges in transferring them from laboratory to industrial scale are described. 相似文献
997.
为分析添加PI染料时乳制品中不同的荧光成分如酪蛋白、黄油、维生素B1、维生素B2、维生素C等对荧光图像观察的影响,探究了不同乳制品及其成分在PI染色观察条件下的荧光性质,并以酿酒酵母为目标菌,研究了10种乳制品中酿酒酵母PI荧光图像计数结果。结果表明,酪蛋白、黄油、维生素B1、维生素B2、维生素C等成分及10种乳制品在发射光615 nm均可产生荧光,但不影响PI染色的酿酒酵母荧光计数的观察结果。使用荧光显微镜对添加105~107 CFU/mL酿酒酵母菌液的10种乳制品进行PI染色计数,并将荧光图像计数结果与平板计数结果进行比较。其中经荧光图像计数后得到的菌液浓度对数值分别在5.69~5.93、6.18~6.28、7.13~7.21之间,对应平板计数结果的对数值分别5.49~5.63、6.02~6.06、7.02~7.06之间,二者结果一致。使用 PI 进行荧光图像计数时,乳制品荧光虽然存在但不会对酿酒酵母荧光图像观察与计数造成影响。 相似文献
998.
Ying-Xia Du Qiao Yang Wang-Ting Lu Qing-Yu Guan Fei-Fei Cao Geng Zhang 《Advanced functional materials》2023,33(27):2300895
Single metal atom isolated in nitrogen-doped carbon materials (M N C) are effective electrocatalysts for oxygen reduction reaction (ORR), which produces H2O2 or H2O via 2-electron or 4-electron process. However, most of M N C catalysts can only present high selectivity for one product, and the selectivity is usually regulated by complicated structure design. Herein, a carbon black-supported Co N C catalyst (CB@Co N C) is synthesized. Tunable 2-electron/4-electron behavior is realized on CB@Co-N-C by utilizing its H2O2 yield dependence on electrolyte pH and catalyst loading. In acidic media with low catalyst loading, CB@Co N C presents excellent mass activity and high selectivity for H2O2 production. In flow cell with gas diffusion electrode, a H2O2 production rate of 5.04 mol h−1 g−1 is achieved by CB@Co N C on electrolyte circulation mode, and a long-term H2O2 production of 200 h is demonstrated on electrolyte non-circulation mode. Meanwhile, CB@Co N C exhibits a dominant 4-electron ORR pathway with high activity and durability in pH neutral media with high catalyst loading. The microbial fuel cell using CB@Co N C as the cathode catalyst shows a peak power density close to that of benchmark Pt/C catalyst. 相似文献
999.
Vidit Kumar Hemant Petwal Ajay Krishan Gairola Pareshwar Prasad Barmola 《计算机系统科学与工程》2023,46(3):2711-2724
Fine-grained image search is one of the most challenging tasks
in computer vision that aims to retrieve similar images at the fine-grained
level for a given query image. The key objective is to learn discriminative
fine-grained features by training deep models such that similar images are
clustered, and dissimilar images are separated in the low embedding space.
Previous works primarily focused on defining local structure loss functions
like triplet loss, pairwise loss, etc. However, training via these approaches
takes a long training time, and they have poor accuracy. Additionally, representations learned through it tend to tighten up in the embedded space and
lose generalizability to unseen classes. This paper proposes a noise-assisted
representation learning method for fine-grained image retrieval to mitigate
these issues. In the proposed work, class manifold learning is performed
in which positive pairs are created with noise insertion operation instead
of tightening class clusters. And other instances are treated as negatives
within the same cluster. Then a loss function is defined to penalize when
the distance between instances of the same class becomes too small relative
to the noise pair in that class in embedded space. The proposed approach is
validated on CARS-196 and CUB-200 datasets and achieved better retrieval
results (85.38% recall@1 for CARS-196% and 70.13% recall@1 for CUB-200)
compared to other existing methods. 相似文献
1000.
介绍了近年来微生物絮凝剂发展的概况及其物理化学性质(属性,结构组成,相对分子量和活性基团)和絮凝机理,并重点讨论了其物理化学性质与絮凝机理的关系,提出了微生物絮凝剂絮凝机理的研究方向。 相似文献