全文获取类型
收费全文 | 3624篇 |
免费 | 58篇 |
国内免费 | 4篇 |
专业分类
电工技术 | 30篇 |
化学工业 | 438篇 |
金属工艺 | 35篇 |
机械仪表 | 77篇 |
建筑科学 | 119篇 |
矿业工程 | 5篇 |
能源动力 | 66篇 |
轻工业 | 184篇 |
水利工程 | 19篇 |
石油天然气 | 11篇 |
无线电 | 236篇 |
一般工业技术 | 409篇 |
冶金工业 | 1726篇 |
原子能技术 | 46篇 |
自动化技术 | 285篇 |
出版年
2021年 | 23篇 |
2020年 | 19篇 |
2019年 | 22篇 |
2018年 | 31篇 |
2017年 | 25篇 |
2016年 | 28篇 |
2015年 | 37篇 |
2014年 | 33篇 |
2013年 | 106篇 |
2012年 | 64篇 |
2011年 | 83篇 |
2010年 | 73篇 |
2009年 | 68篇 |
2008年 | 95篇 |
2007年 | 97篇 |
2006年 | 70篇 |
2005年 | 70篇 |
2004年 | 67篇 |
2003年 | 61篇 |
2002年 | 60篇 |
2001年 | 58篇 |
2000年 | 62篇 |
1999年 | 92篇 |
1998年 | 459篇 |
1997年 | 302篇 |
1996年 | 208篇 |
1995年 | 114篇 |
1994年 | 95篇 |
1993年 | 106篇 |
1992年 | 47篇 |
1991年 | 49篇 |
1990年 | 35篇 |
1989年 | 50篇 |
1988年 | 51篇 |
1987年 | 33篇 |
1986年 | 40篇 |
1985年 | 55篇 |
1984年 | 42篇 |
1983年 | 35篇 |
1982年 | 41篇 |
1981年 | 57篇 |
1980年 | 31篇 |
1979年 | 30篇 |
1978年 | 31篇 |
1977年 | 78篇 |
1976年 | 152篇 |
1975年 | 30篇 |
1974年 | 18篇 |
1973年 | 31篇 |
1972年 | 20篇 |
排序方式: 共有3686条查询结果,搜索用时 127 毫秒
101.
This paper describes the mechanical properties of B2/L21 two-phase Fe30Ni20Mn20Al30 (at.%) in both the as-cast condition and after a 72 h anneal at 823 K. The temperature dependence of the compressive strength of Fe30Ni20Mn20Al30 showed three distinct regions: (1) brittle fracture at low temperature, (2) plastic flow with a rapid decline in yield strength from 1500 to 250 MPa from the brittle-to-ductile transition temperature (BDTT) to 873 K, and (3) a slight decrease in yield strength to ~150 MPa from 873 to 1073 K. Interestingly, the BDTT (573 K) exhibited by the coarser microstructure present in 72 h annealed material was lower than that of the as-cast alloy (623 K). Using both differential scanning calorimetry and in situ heating in a transmission electron microscope, an L21-to-B2 transition was found at 750 ± 25 K. A mixture of intergranular fracture and transgranular cleavage was observed after room temperature compression while only cleavage was found at 673 K. All the specimens deformed extensively without fracture when tested at temperatures higher than 673 K. The strain rate had little effect on the strength at 573 K and a moderate effect at 873 K with a strain-rate sensitivity exponent value of 0.1. 相似文献
102.
An overlapping composite track coating was produced on a steel surface by preplacing a 0·5 mm thick layer of TiC powder and then melting using a tungsten inert gas torch of constant energy input. The influence of the overlapping operation on preheating of the substrate, the dissolution of TiC particulates and the subsequent depth and hardness of the composite layer was analysed. The melt microstructure consisted of both undissolved and partially dissolved TiC particulates, together with a variety of morphologies and sizes of TiC particles precipitated during solidification. Preheating, resulting from the overlapping operation, occurred, producing additional melting of the TiC particulates and deeper melt depths but with a reduced volume fraction of TiC precipitates in the subsequent tracks. A maximum hardness of over 800 HV was developed in the composite layer. The high hardness was unevenly distributed in tracks melted at the initial and final stages, while it varied across the melt depths in other tracks. 相似文献
103.
The purpose of this study was to assess the microbial community in ground beef purchased from local stores in the north central area of South Korea. A total of 31 ground beef samples showed 3.77 ± 5.16 × 105 CFU/g on aerobic plate counts, and PCR-based denaturing gradient gel electrophoresis was performed to assess the microbial community, which revealed a diverse profile among the products. Four of these samples were selected for further analyses via 454 pyrosequencing. Pseudomonas was the most frequently identified genus in terms of relative abundance (4.54–73.8%), and other bacteria such as Lactobacillus, Acinetobacter, and Carnobacterium were also identified in the ground beef. The presence of bacteria can affect the spoilage of ground beef products depending on the microbial populations, and understanding these populations may help limit spoilage and/or foodborne illness. This study illustrates the microbial diversity occurring among Korean ground beef products, even within a local region, and provides a molecular approach for potential ground beef microbial quality assessment. 相似文献
104.
Rabya A. Lahmer David L. Jones Stacy Townsend Shenda Baker Arwel Prysor Williams 《International Journal of Food Science & Technology》2014,49(2):515-520
Here, we evaluated the impact of bacterial growth stage on the effect of chitosan‐arginine (Ch‐arg) on Escherichia coli O157:H7 cell numbers and metabolic activity within contaminated beef juice held at room temperature. Using a lux‐marked metabolic reporter strain of E. coli O157:H7, the results showed that Ch‐arg was most bioactive against cells in the lag phase and exponential phase. In comparison, there was a reduced, although still significant, inhibitory effect of Ch‐arg on the viability and metabolic activity of E. coli O157 held in stationary phase. Ch‐arg reduced, but did not eliminate E. coli O157 growth in the meat juice over 48 h. Based on the evidence presented here and elsewhere, we conclude that Ch‐arg can limit the growth and activity of food spoilage bacteria; however, it cannot completely eliminate bacterial contaminants originally present. Ch‐arg should therefore be viewed as a potentially protective measure rather than a biocidal agent that completely eliminates the risk of pathogen transfer in the food chain. 相似文献
105.
106.
Neill Taylor Dennis Baker Sergio Ciattaglia Pierre Cortes Joëlle Elbez-Uzan Markus Iseli Susana Reyes Lina Rodriguez-Rodrigo Sandrine Rosanvallon Leonid Topilski 《Fusion Engineering and Design》2011,86(6-8):619-622
An updated version of the ITER Preliminary Safety Report has been produced and submitted to the licensing authorities. It is revised and expanded in response to requests from the authorities after their review of an earlier version in 2008, to reflect enhancements in ITER safety provisions through design changes, to incorporate new and improved safety analyses and to take into account other ITER design evolution. The updated analyses show that changes to the Tokamak cooling water system design have enhanced confinement and reduced potential radiological releases as well as removing decay heat with very high reliability. New and updated accident scenario analyses, together with fire and explosion risk analyses, have shown that design provisions are sufficient to minimize the likelihood of accidents and reduce potential consequences to a very low level. Taken together, the improvements provided a stronger demonstration of the very good safety performance of the ITER design. 相似文献
107.
Airworthiness certification is required when bonded repairs are made to primary composite structure in situations where damage has reduced or has the potential to reduce residual strength to below the design ultimate strength. Generally, certification of bonded primary structure poses many difficulties. As most repairs are one-off events meeting these certification requirements is especially challenging since demonstration by testing will generally not be possible or cost-effective. This paper discusses options for addressing the two key issues relating to certification: (a) how to validate initial and enduring bond strength of adhesive bonds, mainly given the inability of conventional non-destructive inspection to provide this assurance and (b) how to develop acceptable generic design allowables for bonded repairs which represent actual failure modes – especially for cyclic loading, since validation by testing of simulated repairs will generally be infeasible. It is concluded that proof testing of bonded repair coupons is a promising approach for validating bond strength and fatigue testing of representative bond joint specimen can provide generic allowables for patch design. For hidden structure or very high value repairs structural health monitoring of repairs based on a strain-transfer approach offers considerable promise. 相似文献
108.
109.
A comparison has been made of the relationship between microstructure and microhardness developed by surface melting Nanosteel SHS 7170 Fe–Cr–B alloy powder onto a plain carbon steel surface. This powder was initially developed as a high velocity oxyfuel sprayed coating, giving a strength 10 times that of mild steel, and is particularly suitable for surface protection against wear and corrosion. In the present study, the alloy powder was injected into the laser melted surface, while a preplaced powder was melted using the gas tungsten arc welding (GTAW) technique. The laser track consisted of fine dendrites and needle-like microstructures, which produced a maximum hardness value of over 800 HV, while the GTAW track produced a mixture of equiaxed and columnar grain microstructures with a maximum hardness value of 670 HV. The lower hardness values are considered to be associated with dilution and grain size. 相似文献
110.
In Saccharomyces cerevisiae the GCR1 gene product is required for high-level expression of genes encoding glycolytic enzymes. In this communication, we extend our analysis of the DNA binding properties of Gcr1p. The DNA-binding domain of Gcr1p binds DNA with high affinity. The apparent dissociation constant of the Gcr1p DNA-binding domain for one of its specific binding sites (TTTCAGCTTCCTCTAT) is 2·9×10−10 M. However, competition experiments showed that Gcr1p binds this site in vitro with a low degree of specificity. We measured a 33-fold difference between the ability of specific competitor and DNA of random sequence to inhibit the formation of nucleoprotein complexes between Gcr1p and a radiolabeled DNA probe containing its binding site. DNA band-shift experiments, utilizing probes of constant length in which the positions of Gcr1p-binding sites are varied relative to the ends, indicated that Gcr1p–DNA nucleoprotein complexes contain bent DNA. The implications of these findings in terms of the combinatorial interactions that occur at the upstream activating sequence elements of genes encoding glycolytic enzymes are discussed. 相似文献