首页 | 本学科首页   官方微博 | 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   112157篇
  免费   8156篇
  国内免费   4226篇
电工技术   5853篇
技术理论   11篇
综合类   6632篇
化学工业   18990篇
金属工艺   5844篇
机械仪表   6687篇
建筑科学   8683篇
矿业工程   3110篇
能源动力   3105篇
轻工业   6626篇
水利工程   1722篇
石油天然气   6246篇
武器工业   693篇
无线电   13997篇
一般工业技术   14275篇
冶金工业   5917篇
原子能技术   1238篇
自动化技术   14910篇
  2024年   433篇
  2023年   1754篇
  2022年   2894篇
  2021年   4004篇
  2020年   3054篇
  2019年   2599篇
  2018年   2989篇
  2017年   3382篇
  2016年   2939篇
  2015年   3866篇
  2014年   4907篇
  2013年   6277篇
  2012年   6566篇
  2011年   7193篇
  2010年   6257篇
  2009年   6178篇
  2008年   6188篇
  2007年   5773篇
  2006年   6061篇
  2005年   5434篇
  2004年   3708篇
  2003年   3206篇
  2002年   2859篇
  2001年   2768篇
  2000年   2883篇
  1999年   3404篇
  1998年   2925篇
  1997年   2563篇
  1996年   2299篇
  1995年   1929篇
  1994年   1557篇
  1993年   1220篇
  1992年   971篇
  1991年   741篇
  1990年   603篇
  1989年   502篇
  1988年   392篇
  1987年   295篇
  1986年   207篇
  1985年   176篇
  1984年   108篇
  1983年   81篇
  1982年   87篇
  1981年   57篇
  1980年   47篇
  1979年   32篇
  1978年   25篇
  1977年   25篇
  1976年   40篇
  1973年   11篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A soybean oil derived biodiesel was prepared and blended with a conventional No. 0 petrodiesel. The pour points (PP) and the cold filter plugging points (CFPP) of biodiesel blends were evaluated on a low-temperature flow tester. Dynamic viscosities of the blends at different temperatures and different shear rates were measured on a rotary rheometer. The crystal morphologies of biodiesel blends at low temperatures were analyzed using a polarizing microscope. The results indicated that blended fuels demonstrated slight decrease in PPs and CFPPs as compared with those of neat soybean oil derived biodiesel and pure petrodiesel. Below the temperatures of PPs or CFPPs, the dynamic viscosity of biodiesel blends dramatically increased with a decreasing temperature, but decreased with an increasing shear rate, so that biodiesel blends exhibited non-Newtonian behavior. At temperatures higher than PPs or CFPPs, a linear relationship appeared between the dynamic viscosity and shear rate and biodiesel blends became Newtonian fluids. At low temperatures, wax crystals of biodiesel blends grew and agglomerated rapidly. Loss of fluidity for biodiesel blends at low temperatures could therefore be attributed on one hand to the sharp increase of viscosity and on the other hand to the rapid growth and agglomeration of wax crystals.  相似文献   
992.
Product distributions from the pyrolysis of a common sample of pine-wood have been determined for two reactors with different configurations. The ablative pyrolysis reactor operates on the principle of “scraping” a continuous stream of biomass particles onto a heated surface under conditions of high relative motion and high applied pressure. In the wire-mesh reactor configuration, fine dispersion of a small quantity [4–6 mg] of sample and the rapid removal of volatiles from the reaction zone ensures that volatiles released during pyrolysis are captured under conditions minimising extra particle secondary reactions.

Comparison of liquid yields determined for the two reactors has been undertaken in order to assess the effect of secondary reactions on yields during ablative pyrolysis. Structural characterisations and comparison of liquids produced in the two reactors have been carried out by size exclusion chromatography, UV-fluorescence spectroscopy and FTIR spectroscopy. Slight differences in structures were apparent either due to cracking of lignin-derived macromolecules on the heated reactor surface or low molecular weight components formed during slow pyrolysis reactions of a small proportion of the feed material. Comparison of the ablative liquids with those from other ablative pyrolysis reactors show similar trends in molecular mass distributions and structures suggesting that the ablative pyrolysis process inherently cracks some liquids during volatilisation. Dry organic liquid yields from the ablative pyrolysis reactor were between 2.5 and 5.3% lower than the wire-mesh reactor between 55° and 600°C. This is believed to be a result of non-optimised reactor operation of the ablative pyrolysis reactor.  相似文献   

993.
A service-life evaluation program on the electricity generation units of Taipower Company (TPC) has been inaugurated since 1985. Based on a survey of five different types of boiler units generated by fossil fuel, a generic procedure for the component evaluation and service-life assessment was established. The major degradation mechanisms of each of the key components/systems of the fossil fuel power units are discussed. The elements of the database software are described which are relevant in carrying out life assessment work. The limitations of applying these methods are addressed based on the experiences in performing the life assessment work for the fossil power plants of Taipower Company in the ROC.  相似文献   
994.
制备了Ti-25V-15Cr-0.2Si和Ti-25V-15Cr-2Al-0.2Si2种阻燃钛合金,分别测试其在不同热暴露和蠕变工艺条件下的热稳定性能和蠕变性能,并观察了组织。结果表明,Al元素提高了Ti—V-Cr系阻燃钛合金的室温力学性能,降低了热稳定性能和抗蠕变性能;在实验的热稳定和蠕变工艺参数下,2种合金中只存在a相和Ti5Si3相,没发现其他过渡相和中间化合物;在热暴露和蠕变过程中,组织稳定性降低,Al元素的添加促进了a相和Ti5si,相的析出,降低了Ti-V-Cr系阻燃钛合金抗蠕变性能和热稳定性能。  相似文献   
995.
This paper presents a novel filtering technique based on sample adaptive offset (SAO) in H.265/high-efficiency video coding (HEVC) for reduction in the temporal flickering artifacts and improving the coding performance. SAO is a newly introduced technique for in-loop filtering in H.265/HEVC, which derives the offsets independently for each frame in the spatial domain without considering temporal frame correlation. As a result, the temporal distortion artifacts which will have a negative effect on the subjective quality, such as flickering artifacts, cannot be effectively addressed. In this paper, the rate-distortion optimization of the newly developed SAO method, referred to as Inter-SAO, is performed on the residual samples between adjacent frames. Inter-SAO and SAO in the reference software of H.265/HEVC (i.e., the test model HM) are then combined to form the novel in-loop filter-based method, denoted as 3D-SAO filtering method, where both spatial information and temporal information are effectively utilized to reduce the overall distortion in reconstructed videos. Compared with the SAO in HM, 3D-SAO has demonstrated its advanced performance for flickering artifacts suppression. Furthermore, 3D-SAO improves the coding efficiency compared with the SAO in HM with a performance gain of up to 0.91 dB in \(\Delta PSNR\), 1.74 dB in \(\Delta PSPNR\) and 7.33 % in BD-rate reduction.  相似文献   
996.
997.
The microstructure and magnetic properties of Mn-doped ZnO films with various Mn contents, synthesized by magnetron sputtering at room temperature, are investigated in detail. X-ray diffraction (XRD) measurement results suggest that the doped Mn ions occupy the Zn sites successfully and do not change the crystal structure of the ZnO films. However, the microstructure of the Mn-doped ZnO films apparently changes with increasing the Mn concentration. Arrays of well-aligned nanoscale rods are found in the Mn-doped ZnO films with moderate Mn concentrations. Magnetic measurement results indicate that the ZnO films doped with moderate Mn concentration are ferromagnetic at room temperature. The possible origin of the ferromagnetism in our samples is also explored in detail.  相似文献   
998.
Cartilage loss is a leading cause of disability among adults, and effective therapy remains elusive. Human mesenchymal stem cells (hMSCs), which have demonstrated self‐renewal and multipotential differentiation, are a promising cell source for cartilage repair. However, the hypertrophic differentiation of the chondrogenically induced MSCs and resulting tissue calcification hinders the clinical translation of MSCs for cartilage repair. Here, a multifunctional nanocarrier based on quantum dots (QDs) is developed to enhance chondrogenic differentiation and suppress hypertrophy of hMSCs simultaneously. Briefly, the QDs are modified with β‐cyclodextrin (β‐CD) and RGD peptide. The resulting nanocarrier is capable of carrying hydrophobic small molecules such as kartogenin in the hydrophobic pockets of conjugated β‐CD to induce chondrogenic differentiation of hMSCs. Meanwhile, via electrostatic interaction the conjugated RGD peptides bind the cargo siRNA targeting Runx2, which is a key regulator of hMSC hypertrophy. Furthermore, due to the excellent photostability of QDs, hMSCs labeled with the nanocarrier can be tracked for up to 14 d after implantation in nude mice. Overall, this work demonstrates the potential of our nanocarrier for inducing and maintaining the chondrogenic phenotype and tracking hMSCs in vivo.  相似文献   
999.
The emergence of semiconducting transition metal dichalcogenide (TMD) atomic layers has opened up unprecedented opportunities in atomically thin electronics. Yet the scalable growth of TMD layers with large grain sizes and uniformity has remained very challenging. Here is reported a simple, scalable chemical vapor deposition approach for the growth of MoSe2 layers is reported, in which the nucleation density can be reduced from 105 to 25 nuclei cm?2, leading to millimeter‐scale MoSe2 single crystals as well as continuous macrocrystalline films with millimeter size grains. The selective growth of monolayers and multilayered MoSe2 films with well‐defined stacking orientation can also be controlled via tuning the growth temperature. In addition, periodic defects, such as nanoscale triangular holes, can be engineered into these layers by controlling the growth conditions. The low density of grain boundaries in the films results in high average mobilities, around ≈42 cm2 V?1 s?1, for back‐gated MoSe2 transistors. This generic synthesis approach is also demonstrated for other TMD layers such as millimeter‐scale WSe2 single crystals.  相似文献   
1000.
Low-rate turbo-Hadamard codes   总被引:2,自引:0,他引:2  
This paper is concerned with a class of low-rate codes constructed from Hadamard code arrays. A recursive encoding principle is employed to introduce an interleaving gain. Very simple trellis codes with only two or four states are sufficient for this purpose, and the decoding cost involved in the trellis part is typically negligible. Both simulation and analytical results are provided to demonstrate the advantages of the proposed scheme. The proposed scheme is of theoretical interest as it can achieve performance of BER=10/sup -5/ at E/sub b//N/sub 0//spl ap/-1.2dB (only about 0.4 dB away from the ultimate low-rate Shannon limit) with an information block size of 65534. To the authors' knowledge, this is the best result achieved to date with respect to the ultimate Shannon limit. With regard to practical issues, the decoding complexity of the proposed code is considerably lower than that of existing low-rate turbo-type codes with comparable performance.  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