全文获取类型
收费全文 | 5819篇 |
免费 | 177篇 |
国内免费 | 18篇 |
专业分类
电工技术 | 135篇 |
综合类 | 6篇 |
化学工业 | 1165篇 |
金属工艺 | 157篇 |
机械仪表 | 106篇 |
建筑科学 | 141篇 |
矿业工程 | 27篇 |
能源动力 | 255篇 |
轻工业 | 313篇 |
水利工程 | 20篇 |
石油天然气 | 18篇 |
无线电 | 758篇 |
一般工业技术 | 1167篇 |
冶金工业 | 879篇 |
原子能技术 | 40篇 |
自动化技术 | 827篇 |
出版年
2023年 | 52篇 |
2022年 | 87篇 |
2021年 | 131篇 |
2020年 | 121篇 |
2019年 | 111篇 |
2018年 | 146篇 |
2017年 | 150篇 |
2016年 | 128篇 |
2015年 | 101篇 |
2014年 | 155篇 |
2013年 | 397篇 |
2012年 | 228篇 |
2011年 | 288篇 |
2010年 | 224篇 |
2009年 | 228篇 |
2008年 | 251篇 |
2007年 | 199篇 |
2006年 | 197篇 |
2005年 | 194篇 |
2004年 | 151篇 |
2003年 | 137篇 |
2002年 | 118篇 |
2001年 | 117篇 |
2000年 | 90篇 |
1999年 | 97篇 |
1998年 | 221篇 |
1997年 | 164篇 |
1996年 | 112篇 |
1995年 | 83篇 |
1994年 | 113篇 |
1993年 | 106篇 |
1992年 | 88篇 |
1991年 | 68篇 |
1990年 | 61篇 |
1989年 | 48篇 |
1988年 | 62篇 |
1987年 | 54篇 |
1986年 | 52篇 |
1985年 | 73篇 |
1984年 | 58篇 |
1983年 | 48篇 |
1982年 | 39篇 |
1981年 | 49篇 |
1980年 | 41篇 |
1979年 | 38篇 |
1978年 | 36篇 |
1977年 | 45篇 |
1976年 | 46篇 |
1974年 | 33篇 |
1973年 | 35篇 |
排序方式: 共有6014条查询结果,搜索用时 31 毫秒
991.
Bandyopadhyay Sumita; Deb Sunanda; Bose Sudeep; Roy Siddhartha 《Protein engineering, design & selection : PEDS》2002,15(5):393-401
A site-directed mutation, F235C, was created at the penultimateresidue of the -repressor. Measurement of dimermonomerdissociation constant suggested that dimermonomer dissociationof the mutant repressor is similar to that of the wild-type.Affinity towards a single operator OR1 is also similar to thatof the wild-type repressor. The mutant repressor gene in a multi-copyplasmid confers immunity towards infection by a cI- lambda phage,suggesting preservation of functional integrity. Far-UV circulardichroism spectra show no major change in the secondary structure.Fluorescence quenching experiments, however, suggest increasedexposure of some tryptophan residues. The urea denaturationprofile indicates decreased stability of a part of the C-terminaldomain. Under non-denaturing conditions, cysteine-235 showshalf-of-the-sites reactivity, i.e. on average only one out oftwo cysteine-235 residues in the dimer shows reactivity towardssulfhydryl reagents. Fluorescence energy transfer between randomlylabeled donor and acceptor fluorescent probes indicates thatonly one sulfhydryl per dimer is reactive, suggesting true half-of-the-sitesreactivity. The structural role of the C-terminal tail in thewhole repressor dimer is discussed. 相似文献
992.
L. L. Hammond S. H. Chien A. H. Roy A. U. Mokwunye 《Nutrient Cycling in Agroecosystems》1989,19(2):93-98
Partially acidulated phosphate rock (PAPR) has been shown to be an effective source of P for plants grown on acid soils. Less information in available, however, regarding the effect of the phosphate rock (PR) source on the solubility and agronomic effectiveness of PAPR.The effect of Fe2O3 + Al2O3 content in PR on the quality of PAPR produced was investigated in this study. Nine sources of PR from Africa, Latin America, and the United States, representing a range of Fe2O3 + Al2O3 from 0.7% to 12.4%, were used. In a single-step process, the finely ground PRs were partially acidulated with H2SO4 at the 30% or 50% acidulation level and granulated (–3.35 + 1.18 mm or –6 + 14 mesh). It was found that the water-soluble P content in PAPR decreased with increasing Fe2O3 + Al2O content in the PR used. Apparently, the presence of Fe2O3 + Al2O3 resulted in a reversion of some of the water-soluble P to citrate-soluble P and sometimes even to citrate-insoluble P.A short-term (6 weeks) greenhouse study was conducted to evaluate crop response to PAPRs and single superphosphate (SSP); maize, the test crop, was grown on an acid soil (pH 4.5)—Hartsells silt loam (Typic Hapludults). The agronomic effectiveness of PAPRs with respect to SSP (in terms of dry-matter yield of maize) decreased with increasing Fe2O3 + Al2O3 content in PRs. Phosphorus uptake by maize from PAPRs was found to correlate well with water solubility but not with citrate solubility. The results obtained in this study show that the detrimental effect of Fe2O3 + Al2O3 content on the solubility and P availability of PAPR should be considered when selecting a PR for PAPR production. 相似文献
993.
Lionel K. Arnold R. Basu Roy Choudhury 《Journal of the American Oil Chemists' Society》1962,39(8):379-380
Extraction of soybean flakes with 90, 95, 98 and 100% ethanol resulted in more rapid lipid and less rapid non-lipid removal
with the increasing ethanol concentrations. There was little difference in the quality of the oil produced by the different
solvents. Protein content of the residual meal averaged 52.1%. 相似文献
994.
Unusually high strengths have been generated in materials produced by employing “hot-pressing” techniques, and intermediate ranges of strengths have been achieved by applying high pressures at room temperature, to portland cement pastes. By pressing at ca. 250°C and 50, 000 psi strengths are as high as 95, 000 psi (compressive), and 9250 psi (indirect tensile). The hot-pressed materials are volume stable when immersed in water and subsequently evacuated. The microstructures of such materials are very compact, consisting of an intergrowth of dense hydrated cement “gel” surrounding residual unhydrated cement grain cores. The lowest porosity of the materials measured was approximately 1.8%, by far the closest approach to zero porosity or theoretical density yet achieved in cement pastes. The effect of microstructure and porosity are discussed, and high pressure techniques are compared with other methods of strength generation. 相似文献
995.
Roy W. Rice 《Journal of the American Ceramic Society》1990,73(10):3116-3118
Calculation of apparent fracture energies (γa ) using the applied failure stress and fractographically determined flaw sizes (C) in B4 C shows γa decreasing with C , once C < 100 μm. This is attributed to increasing contributions of microstructural stresses (σi ) due to thermal expansion anisotropy ( 1 × 10−6 °C−1 ). Extrapolation of this σi contribution to C ∼ G (the grain size) and calculation from maximum thermal expansion mismatch are in reasonable agreement. e.g., giving σ∼ 1000 MPa. Strength-mirror size data also show deviations at higher strengths-smaller mirror sizes consistent with both the occurrence and estimated level of such microstructural stress contributions to failure. 相似文献
996.
Jute stick was bleached with sodium chlorite and alkaline hydrogen peroxide solutions. The infrared (IR) spectra of bleached samples and jute stick were analyzed and compared. The bleached samples were characterized by higher absorbance intensity ratios (Av/A2900) of the bands attributed to hemicellulose. Among the bleached samples the chlorite-bleached sample showed higher intensity for the bands mentioned. The bands attributed to lignin are either absent or very weak in the chlorite-bleached jute stick as compared to that of peroxide-bleached jute stick, although some residual lignin was present in the substrate. The 1635 cm?1 band attribute to the vibration of adsorbed water molecules in the noncrystalline regions in cellulose appears as a sharp peak in chlorite-bleached jute stick and as a shoulder in peroxide-bleached jute stick. This can be attributed to the difference in the nature of the two bleaching processes. 相似文献
997.
Hydration products of fly ash-portland cements were studied with x-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) as part of a continuing research effort to understand the pozzolanic activity of fly ashes. It was found that the amount of calcium hydroxide crystals in the cement pastes is diminished due to the addition of fly ash to the cement. Ettringite was produced in the early age, and the consumption of sulfate by the formation of ettringite was accelerated by the addition of fly ash. A partial conversion of ettringite to monosulfate within the first 7 days of hydration in the fly ash-portland cement pastes, but the formation of ettringite continued to form up to at least 28 days of hydration in the pastes without fly ash. Examination of the fly ash bearing pastes showed, in all cases, varying amounts of calcium hydroxide and unreacted portland cement, with minor quartz and gehlenite hydrate. It appears that hydration reactions actually occur in the fly ash cement pastes more or less on a particle-by-particle basis. 相似文献
998.
999.
The importance of concrete permeability is discussed relative to its effect on durability. Experimental studies were made of the effect of water/cement ratio and curing temperature on the porosity, pore size distribution and permeability of cement pastes. Although total porosities of samples cured at 60°C are smaller than in those cured at 27°C, the pore volume larger than 750Å radius is greater in the 60°C samples and is related to higher permeabilities also in the latter. 相似文献
1000.
Paul J. Schilling Leslie G. Butler Amitava Roy Harvill C. Eaton 《Journal of the American Ceramic Society》1994,77(9):2363-2368
b29 Si and 27 Al MAS-NMR were performed on NaOH-activated blast-furnace slag to better characterize the amorphous and poorly crystalline phases which occur in this system. The unreacted glass has a mainly dimeric silicate structure represented by a broad 29 Si peak (FWHM = 15 ppm) centered at –74.5 ppm [ Q 1 ], with aluminum present exclusively in tetrahedral coordination. Upon reaction with 5M NaOH ( w/s = 0.4), three new 29 Si peaks with widths of ca. 2 ppm are formed at -78.5 Q 1 , –81.4 [ Q 2 (1Al)J, and -84.3 [ Q 2 ]. Relative peak areas indicate a mostly dimeric silicate structure for the tobermorite-like C─S─H layers, with roughly a third of the bridging sites occupied by aluminum, and less than 10% by silicon. In addition to the tetrahedrally coordinated aluminum substituted in the C─S─H structure, 27 Al MAS-NMR reveals the presence of aluminum in octahedral sites, which is attributed to the aluminate phase (C,M)4 AH13 . 29 Si results indicate rapid initial consumption of the glass, with roughly a third of the glass reacting within the first day and another third consumed over the following 27 days. 相似文献