全文获取类型
收费全文 | 90562篇 |
免费 | 1318篇 |
国内免费 | 415篇 |
专业分类
电工技术 | 872篇 |
综合类 | 2319篇 |
化学工业 | 13381篇 |
金属工艺 | 5016篇 |
机械仪表 | 3187篇 |
建筑科学 | 2330篇 |
矿业工程 | 582篇 |
能源动力 | 1355篇 |
轻工业 | 4588篇 |
水利工程 | 1291篇 |
石油天然气 | 360篇 |
武器工业 | 1篇 |
无线电 | 9456篇 |
一般工业技术 | 17394篇 |
冶金工业 | 3762篇 |
原子能技术 | 284篇 |
自动化技术 | 26117篇 |
出版年
2023年 | 57篇 |
2022年 | 235篇 |
2021年 | 370篇 |
2020年 | 189篇 |
2019年 | 202篇 |
2018年 | 14668篇 |
2017年 | 13581篇 |
2016年 | 10241篇 |
2015年 | 860篇 |
2014年 | 568篇 |
2013年 | 759篇 |
2012年 | 3486篇 |
2011年 | 9744篇 |
2010年 | 8521篇 |
2009年 | 5814篇 |
2008年 | 6981篇 |
2007年 | 7966篇 |
2006年 | 267篇 |
2005年 | 1317篇 |
2004年 | 1239篇 |
2003年 | 1275篇 |
2002年 | 627篇 |
2001年 | 151篇 |
2000年 | 228篇 |
1999年 | 105篇 |
1998年 | 261篇 |
1997年 | 252篇 |
1996年 | 199篇 |
1995年 | 130篇 |
1994年 | 79篇 |
1993年 | 91篇 |
1992年 | 39篇 |
1991年 | 43篇 |
1987年 | 40篇 |
1985年 | 53篇 |
1984年 | 67篇 |
1983年 | 58篇 |
1982年 | 68篇 |
1981年 | 73篇 |
1980年 | 84篇 |
1979年 | 50篇 |
1978年 | 43篇 |
1977年 | 130篇 |
1976年 | 246篇 |
1968年 | 47篇 |
1967年 | 39篇 |
1966年 | 48篇 |
1965年 | 44篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
Mobile software applications have to cope with a particular environment that involves small size, limited resources, high autonomy requirements, competitive business models and many other challenges. To provide development guidelines that respond to these needs, several practices have been introduced; however, it is not clear how these guidelines may contribute to solve the issues present in the mobile domain. Furthermore, the rapid evolution of the mobile ecosystem challenges many of the premises upon which the proposed practices were designed. In this paper, we present a survey of the literature on software assurance practices for mobile applications, with the objective of describing them and assessing their contribution and success. We identified, organized and reviewed a body of research that spans in three levels: software development processes, software product assurance practices, and software implementation practices. By carrying out this literature survey, we reviewed the different approaches that researchers on Software Engineering have provided to address the needs that raise in the mobile software development arena. Moreover, we review the evolution of these practices, identifying how the constant changes and modernization of the mobile execution environment has impacted the methods proposed in the literature. Finally, we introduced discussion on the application of these practices in a real productive setting, opening an area for further research that may determine if practitioners have followed the proposed assurance paradigms. 相似文献
2.
3.
4.
Variations of Incomplete Open Cubes is the major project by the twentieth-century conceptual artist Sol LeWitt. In this paper we interpret the enumerative component of the project as embeddings of graphs. This formulation permits use of an algorithm to check the completeness of the list of the structures produced by the artist. Our conclusion is that the artist found the correct number of structures (that is, 122), but that his list contains a mistake in the presentation of a pair of incomplete cubes, a discovery that appears not to have been noted before. 相似文献
5.
Ferrite particles coated with biocompatible phases can be used for hyperthermia treatment of cancer. We have synthesized substituted
calcium hexaferrite, which is not stable on its own but is stabilized with small substitution of La. Hexaferrite of chemical
composition (CaO)0.75(La203)0.20(Fe2O3)6 was prepared using citrate gel method. Hydroxyapatite was prepared by precipitating it from aqueous solution of Ca(NO3)2 and (NH4)2HPO4 maintaining pH above 11. Four different methods were used for coating of hydroxyapatite on ferrite particles. SEM with EDX
and X-ray diffraction analysis shows clear evidence of coating of hydroxyapatite on ferrite particles. These coated ferrite
particles exhibited coercive field up to 2 kOe, which could be made useful for hysteresis heating in hyperthermia. Studies
by culturing BHK-21 cells and WBC over the samples show evidence of biocompatibility. SEM micrographs and cell counts give
clear indication of cell growth on the surface of the sample. Finally coated ferrite particle was implanted in Kasaulli mouse
to test its biocompatibility. The magnetic properties and biocompatibility studies show that these hydroxyapatite coated ferrites
could be useful for hyperthermia. 相似文献
6.
Chloride doped polyaniline conducting polymer films have been prepared in a protic acid medium (HCl) by potentiodynamic method
in an electrochemical cell and studied by cyclic voltammetry and FTIR techniques. The FTIR spectra confirmed Cl- ion doping in the polymers. The polymerization rate was found to increase with increasing concentration of aniline monomer.
But the films obtained at high monomer concentration were rough having a nonuniform flaky polyaniline distribution. Results
showed that the polymerization rate did not increase beyond a critical HCl concentration. Cyclic voltammetry suggested that,
the oxidation-reduction current increased with an increase in scan rate and that the undoped polyaniline films were not hygroscopic
whereas chloride doped polyaniline films were found to be highly hygroscopic. 相似文献
7.
Analysis of non-stationary electrochemical oscillations emerges utilization of the techniques allowing an observation of time evolution of their parameters such as instantaneous frequency, amplitude or frequency band. An algorithm of extraction of these parameters can be based on one of linear or quadratic time-frequency representations of oscillations record. In this paper, the authors present representatives of such transformations (Choi-Williams and Cone-shaped) originating from one of the most popular joint time-frequency energy distribution—Wigner-Ville distribution. Short introduction to the mathematics of the topic discussed is provided, then the authors present an example of application in the analysis of current oscillations of copper dissolution process. 相似文献
8.
9.
A Sawicka-Grzelak F Meisel-Miko?ajczyk SM S?otwińska M Wierzbicka 《Canadian Metallurgical Quarterly》1997,49(1-2):95-100
The aim of study was the evaluation of periodontal pockets microflora in patients with advanced periodontitis. From each subject 16-20 samples were taken using paper points. Pooled sample after 60 s. mixing was serially diluted in reduced BHI. For total cell counts and for the isolation of black pigmented anaerobes Brucella agar supplemented with 5% sheep blood, hemin, menadione, with and without Kanamycin-Vancomycin mixture and BM agar plates were used. For isolation of A. actinomycetemcomitans TSBV agar plates were used. Cultures were incubated in anaerobic chamber at 37 degrees C for 7 days and TSBV agar plates in an atmosphere of 95% air-5% CO2 at 37 degrees C for 5 days. Microorganisms were identified by Gram staining, colony morphology, fluorescence in UV-light, haemagglutination of 3% sheep erythrocytes, fermentation of sugars, production of indole, urease (API 20A), specific enzymes (Rapid ID 32A). Twenty seven subjects with clinically recognized periodontitis were examined. Microorganisms important in periodontitis were isolated from periodontal pockets of almost all examined subjects. The number of bacteria obtained from the sample of one patient ranged from 1 x 10(4) CFU/ml to 3,6 x 10(6) CFU/ml. Porphyromonas gingivalis was identified in the samples taken from 17 patients, Prevotella intermedia-19, Actinobacillus actinomycetemcomitans -11, Fusobacterium nucleatum-9, Peptostreptococcus spp.-22. 相似文献
10.
The purpose of this paper is to evaluate two methods of assessing the productivity and quality impact of Computer Aided Software Engineering (CASE) and Fourth Generation Language (4GL) technologies: (1) by the retrospective method; and (2) the cross-sectional method. Both methods involve the use of questionnaire surveys. Developers' perceptions depend on the context in which they are expressed and this includes expectations about the effectiveness of a given software product. Consequently, it is generally not reliable to base inferences about the relative merits of CASE and 4GLs on a cross-sectional comparison of two separate samples of users. The retrospective method that requires each respondent to directly compare different products is shown to be more reliable. However, there may be scope to employ cross-sectional comparisons of the findings from different samples where both sets of respondents use the same reference point for their judgements, and where numerical rather than verbal rating scales are used to measure perceptions. 相似文献