全文获取类型
收费全文 | 4176篇 |
免费 | 44篇 |
专业分类
电工技术 | 14篇 |
综合类 | 2篇 |
化学工业 | 278篇 |
金属工艺 | 31篇 |
机械仪表 | 34篇 |
建筑科学 | 61篇 |
矿业工程 | 1篇 |
能源动力 | 34篇 |
轻工业 | 137篇 |
水利工程 | 8篇 |
石油天然气 | 3篇 |
无线电 | 183篇 |
一般工业技术 | 315篇 |
冶金工业 | 2943篇 |
原子能技术 | 20篇 |
自动化技术 | 156篇 |
出版年
2022年 | 13篇 |
2021年 | 15篇 |
2019年 | 12篇 |
2018年 | 21篇 |
2017年 | 21篇 |
2016年 | 34篇 |
2015年 | 10篇 |
2014年 | 28篇 |
2013年 | 52篇 |
2012年 | 41篇 |
2011年 | 50篇 |
2010年 | 38篇 |
2009年 | 44篇 |
2008年 | 55篇 |
2007年 | 48篇 |
2006年 | 55篇 |
2005年 | 52篇 |
2004年 | 34篇 |
2003年 | 50篇 |
2002年 | 32篇 |
2001年 | 37篇 |
2000年 | 31篇 |
1999年 | 117篇 |
1998年 | 886篇 |
1997年 | 508篇 |
1996年 | 369篇 |
1995年 | 202篇 |
1994年 | 179篇 |
1993年 | 178篇 |
1992年 | 46篇 |
1991年 | 51篇 |
1990年 | 54篇 |
1989年 | 49篇 |
1988年 | 42篇 |
1987年 | 40篇 |
1986年 | 39篇 |
1985年 | 37篇 |
1984年 | 20篇 |
1983年 | 23篇 |
1982年 | 34篇 |
1981年 | 29篇 |
1980年 | 41篇 |
1979年 | 22篇 |
1978年 | 26篇 |
1977年 | 88篇 |
1976年 | 214篇 |
1975年 | 20篇 |
1974年 | 13篇 |
1973年 | 15篇 |
1965年 | 10篇 |
排序方式: 共有4220条查询结果,搜索用时 15 毫秒
91.
One of congress’ goals for 2007 was for 80% of all tax and informational returns to be filed electronically. However, to date
that lofty goal has fallen well short. This research proposes a model of e-filing adoption. To test the model a survey is
administered to 260 participants. The instrument assesses citizen perceptions of trust, risk and optimism bias. Structural
equation modeling is used to evaluate the relationships between these three concepts and intention to use. The results indicate
that trust of the Internet, trust of the e-filer, perceived risk and optimism bias all have an impact on intention to use
e-filing. Implications for practice and research are discussed. 相似文献
92.
Christian Collberg Andrew Huntwork Edward Carter Gregg Townsend Michael Stepp 《Information and Software Technology》2009,51(1):56-67
This paper presents an implementation of the watermarking method proposed by Venkatesan et al. in their paper [R. Venkatesan, V. Vazirani, S. Sinha, A graph theoretic approach to software watermarking, in: Fourth International Information Hiding Workshop, Pittsburgh, PA, 2001]. An executable program is marked by the addition of code for which the topology of the control-flow graph encodes a watermark. We discuss issues that were identified during construction of an actual implementation that operates on Java bytecode. We present two algorithms for splitting a watermark number into a redundant set of pieces and an algorithm for turning a watermark number into a control-flow graph. We measure the size and time overhead of watermarking, and evaluate the algorithm against a variety of attacks. 相似文献
93.
Gregory D. Larosiliere Lemuria D. Carter 《Journal of Computer Information Systems》2016,56(4):271-279
E-Government diffusion is of interest to both researchers and practitioners. The purpose of this study is to investigate electronic government maturity and its determinants. We use the Fit-Viability Framework to assess e-Government Maturity in diverse countries. We empirically test the impact of task-technology fit (TTF) and viability on e-government maturity at the country level. The results of partial least squares (PLS) analysis indicate that TTF characteristics along with a country’s viable resources and level of e-government development have a significant impact on e-government maturity. Implications for research and practice are discussed. 相似文献
94.
Jason M. Harley Cassia K. Carter Niki Papaionnou François Bouchet Ronald S. Landis Roger Azevedo Lana Karabachian 《User Modeling and User-Adapted Interaction》2016,26(2-3):177-219
The current study examined the relationships between learners’ (\(N = 123\)) personality traits, the emotions they typically experience while studying (trait studying emotions), and the emotions they reported experiencing as a result of interacting with four pedagogical agents (agent-directed emotions) in MetaTutor, an advanced multi-agent learning environment. Overall, significant relationships between a subset of trait emotions (trait anger, trait anxiety) and personality traits (agreeableness, conscientiousness, and neuroticism) were found for four agent-directed emotions (enjoyment, pride, boredom, and neutral) though the relationships differed between pedagogical agents. These results demonstrate that some trait emotions and personality traits can be used to predict learners’ emotions directed toward specific pedagogical agents (with different roles). Results provide suggestions for adapting pedagogical agents to support learners’ (with certain characteristics; e.g., high in neuroticism or agreeableness) experience of adaptive emotions (e.g., enjoyment) and minimize their experience on non-adaptive emotions (e.g., boredom). Such an approach presents a scalable and easily implementable method for creating emotionally-adaptive, agent-based learning environments, and improving learner-pedagogical agent interactions in order to support learning. 相似文献
95.
96.
IR Sanderson AJ Ouellette EA Carter PR Harmatz 《Canadian Metallurgical Quarterly》1993,105(4):974-980
BACKGROUND: Exogenous antigenic peptides are presented to T cells by class II major histocompatibility complex (Ia) molecules on the surface of antigen-presenting cells. Class II-associated invariant chain (Ii) is also required for effective antigen presentation. Because messenger RNAs (mRNAs) for Ii chain and for class II I-A beta chain appear in the mouse intestinal epithelium after weaning, experiments were conducted to test the effect of age of weaning and diet on the appearance of Ia and Ii mRNA. METHODS: Four litters were split at day 17; one half was weaned and the other remained with the mother until day 24. On day 23, 25, 27, and 29, enterocytes were isolated from full-length small intestine by vascular perfusion with 30 mmol/L ethylenediaminetetraacetic acid, and the RNA was extracted. RESULTS: Appearance of Ii and I-A beta was significantly delayed by late weaning, as judged by RNA hybridization blots (Ii chain) and complementary DNA amplification (I-A beta chain). In mice on elemental diets, the appearance of Ii and I-A beta chain was delayed compared with littermates reared on standard chow. Ii mRNA failed to appear in mice maintained on the elemental diet by day 40, despite normal growth. CONCLUSIONS: Appearance of mRNA for both Ia and Ii depends on the introduction of a complex diet and not the "stress" of weaning or elimination of breast milk. Introduction of foreign dietary antigens or development of an altered intestinal flora may contribute to this process. 相似文献
97.
Current methods used clinically to assess myocardial perfusion are invasive and expensive. As the technology of ultrasound imaging improves, CE may provide a relatively inexpensive, noninvasive means of quantitating myocardial perfusion. Issues regarding stability of microbubble contrast agents must be studied more closely under physiologic conditions. As such, encapsulated microbubbles may provide more stability under physiologic pressures than free gas microbubbles. Introducing high concentrations of contrast, either by hyperconcentrating the contrast agent or by increasing the injection rate, may provide greater stability under physiologic conditions. Further, before quantitative statement of tissue perfusion can be made, the relationship between tracer concentration and system response must be established. Further, a "linear" postprocessing ultrasound setting does not eliminate this requirement as data must still undergo nonlinear transformation during log compression and time-gain compensation. Additionally, issues regarding "electronic thresholding" must be explored more extensively in vivo. Commercial ultrasound scanners, in their present form, may not offer adequate sensitivity for absolute quantitative studies. Further development of modified ultrasound systems may provide sufficient sensitivity for quantitative perfusion imaging. CE offers a potentially powerful tool in the clinical management of patients with ischemic heart disease. Conventional coronary angiography provides information on the size of a lesion, but accompanying tissue perfusion distal to the lesion cannot be determined. Doppler ultrasonography determines velocity of blood flow in large vessels but does not offer the potential to quantitate tissue perfusion. Clearly, CE has a place in the future of diagnostic imaging. The recent work of Ito et al. demonstrated the qualitative potential of CE in the identification of "areas at risk" in patients who had undergone thrombolysis or percutaneous transluminal coronary angioplasty after an acute myocardial infarction. With further improvement in the ultrasound imaging techniques and microbubble stability, CE may offer an inexpensive, noninvasive means of assessing myocardial perfusion. 相似文献
98.
The beam radius along a focused, unapertured Gaussian beam was measured and used to calculate the dependence of the geometrical Fresnel number on the effective Fresnel number of the beam as it emerged from the focusing lens. The resulting data clearly demonstrate a focal shift away from the focal plane given by geometrical optics. The data agree very well with a theory due to Carter. This theory indicates that a significant focal shift occurs in these beams if the focusing lens is placed within the near field of the focal plane. 相似文献
99.
L.Carter Wellford S.M. Hamdan 《Computer Methods in Applied Mechanics and Engineering》1978,14(3):391-399
In part 11 a series of implicit finite element algorithms for the geometrically nonlinear structural problem were proposed. These algorithms were shown to be computationally efficient because they require the solution of a linear system rather than a nonlinear system at each time point. In addition, because of the conservative nature of the algorithms, their stability properties are easily assessed. One algorithm was shown to be unconditionally stable.In part 2 the convergence of the proposed algorithms is demonstrated. It is shown that for the consistent algorithm, additional conditions must be satisfied to insure convergence. In fact, one of the proposed algorithms is shown to be unconditionally stable but conditionally convergent. 相似文献
100.
We describe an assay system for measuring theophylline in 25 microliters of serum. The procedure involves extraction with a 95:5 mixture of chloroform:isopropanol containing beta-hydroxypropyltheophylline as internal standard, and reverse-phase chromatography on a 4 mm x 30 cm column containing "micron Bondapak C18." Theophylline and beta-hydroxypropyltheophylline are eluted with a 90:10 mixture of sodium acetate butter (20 mmoles/litre pH 4.0) and acetonitrile at a flow rate of 1.8 ml/min., are detected by their absorbance at 254 nm, and quantitated by measuring peak areas. Column temperature has not been found to be critical in this analysis. Each analysis requires 9 minutes of chromatography time with a total analysis time of 20 minutes. Analytical recoveries were found to be 71 to 75% for theophylline and 94% for beta-hydroxypropyltheophylline. This difference in recovery is corrected when determining the theophylline concentration in unknown samples. The method has good precision (coefficients of variation between 7.0% and 7.9% for therapeutic and toxic concentrations). The results obtained with this method compare favourably with results obtained by a published cation-exchange high-performance liquid chromatographic method. None of the metabolites of theophylline, common compounds related to theophylline in structure or drugs tested have been found to interfere with the analysis described. 相似文献