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   1811篇
  免费   13篇
电工技术   8篇
化学工业   55篇
金属工艺   8篇
机械仪表   6篇
建筑科学   30篇
能源动力   3篇
轻工业   83篇
石油天然气   1篇
无线电   80篇
一般工业技术   68篇
冶金工业   1434篇
原子能技术   8篇
自动化技术   40篇
  2022年   5篇
  2019年   7篇
  2018年   3篇
  2017年   9篇
  2016年   10篇
  2015年   9篇
  2014年   8篇
  2013年   13篇
  2012年   4篇
  2011年   16篇
  2010年   8篇
  2009年   16篇
  2008年   20篇
  2007年   18篇
  2006年   19篇
  2005年   18篇
  2004年   16篇
  2003年   20篇
  2002年   14篇
  2001年   13篇
  2000年   23篇
  1999年   48篇
  1998年   430篇
  1997年   260篇
  1996年   142篇
  1995年   101篇
  1994年   76篇
  1993年   103篇
  1992年   22篇
  1991年   33篇
  1990年   22篇
  1989年   22篇
  1988年   24篇
  1987年   30篇
  1986年   19篇
  1985年   20篇
  1984年   7篇
  1983年   10篇
  1982年   9篇
  1981年   12篇
  1980年   7篇
  1979年   6篇
  1978年   4篇
  1977年   35篇
  1976年   86篇
  1975年   4篇
  1973年   2篇
  1967年   2篇
  1966年   2篇
  1955年   3篇
排序方式: 共有1824条查询结果,搜索用时 31 毫秒
51.
Textile rental companies are poised on the edge of a boom in the healthcare marketplace. The detonator is outpatient care. Between 1984 and 1990, outpatient visits increased from almost 233 million to more than 310 million, or about 33 percent. The shift from traditional hospital care to outpatient or ambulatory service is creating a new market opportunity for textile rental services. Innovative medical services are cropping up in suburban malls and neighborhoods, and demand is high for high-quality linen supply or contract laundry services.  相似文献   
52.
Abstract

In an earlier study of preservice elementary and middle school teachers’ beliefs about repeating decimals, Weller, Arnon, and Dubinsky (2009) reported on a comparison of the mathematical performance of 77 preservice teachers who completed an APOS-based instructional unit with 127 preservice teachers who completed traditional instruction. The purpose of the current study, based on 47 interviews conducted 4 months after the instruction, during which time there was no further instruction on this topic, is to determine the strength and stability (over time) of the students’ beliefs, to uncover thinking that did not arise in the earlier study, and to see whether the interviews yield similar comparative results. The interviews did uncover changes in student thinking. The students who received the APOS-based instruction developed stronger and more stable (over time) beliefs that a repeating decimal is a number; a repeating decimal has a fraction or integer to which it corresponds; a repeating decimal in general equals its corresponding fraction or integer; and, in particular, In addition, a number of indices and categories were developed that may prove useful in other studies consisting of the comparison of interview and questionnaire data involving a large number of interview subjects.  相似文献   
53.
Herpes simplex virus type 1 (HSV-1) DNA replication intermediates exist in a complex nonlinear structure that does not migrate into a pulsed-field gel. Genetic evidence suggests that the product of the UL12 gene, termed alkaline nuclease, plays a role in processing replication intermediates (R. Martinez, R. T. Sarisky, P. C. Weber, and S. K. Weller, J. Virol. 70:2075-2085, 1996). In this study we have tested the hypothesis that alkaline nuclease acts as a structure-specific resolvase. Cruciform structures generated with oligonucleotides were treated with purified alkaline nuclease; however, instead of being resolved into linear duplexes as would be expected of a resolvase activity, the artificial cruciforms were degraded. DNA replication intermediates were isolated from the well of a pulsed-field gel ("well DNA") and treated with purified HSV-1 alkaline nuclease. Although alkaline nuclease can degrade virion DNA to completion, digestion of well DNA results in a smaller-than-unit-length product that migrates as a heterogeneous smear; this product is resistant to further digestion by alkaline nuclease. The smaller-than-unit-length products are representative of the entire HSV genome, indicating that alkaline nuclease is not inhibited at specific sequences. To further probe the structure of replicating DNA, well DNA was treated with various known nucleases; our results indicate that replicating DNA apparently contains no accessible double-stranded ends but does contain nicks and gaps. Our data suggest that UL12 functions at nicks and gaps in replicating DNA to correctly repair or process the replicating genome into a form suitable for encapsidation.  相似文献   
54.
Tobacco mosaic virus (TMV) derivatives that encode movement protein (MP) as a fusion to the green fluorescent protein (MP:GFP) were used in combination with antibody staining to identify host cell components to which MP and replicase accumulate in cells of infected Nicotiana benthamiana leaves and in infected BY-2 protoplasts. MP:GFP and replicase colocalized to the endoplasmic reticulum (ER; especially the cortical ER) and were present in large, irregularly shaped, ER-derived structures that may represent "viral factories." The ER-derived structures required an intact cytoskeleton, and microtubules appeared to redistribute MP:GFP from these sites during late stages of infection. In leaves, MP:GFP accumulated in plasmodesmata, whereas in protoplasts, the MP:GFP was targeted to distinct, punctate sites near the plasma membrane. Treating protoplasts with cytochalasin D and brefeldin A at the time of inoculation prevented the accumulation of MP:GFP at these sites. It is proposed that the punctate sites anchor the cortical ER to plasma membrane and are related to sites at which plasmodesmata form in walled cells. Hairlike structures containing MP:GFP appeared on the surface of some of the infected protoplasts and are reminiscent of similar structures induced by other plant viruses. We present a model that postulates the role of the ER and cytoskeleton in targeting the MP and viral ribonucleoprotein from sites of virus synthesis to the plasmodesmata through which infection is spread.  相似文献   
55.
The sensitivity and specificity of a urinary pregnanediol-3-glucuronide (PdG) ratio algorithm to identify anovulatory cycles was studied prospectively in two independent populations of women. Urinary hormone data from the first group was used to develop the algorithm, and data from the second group was used for its validation. PdG ratios were calculated by a cycles method in which daily PdG concentrations indexed by creatinine (CR) from cycle day 11 onward were divided by a baseline PdG (average PdG/Cr concentration for cycle days 6-10). In the interval method, daily PdG/CR concentrations from day 1 onward were divided by baseline PdG (lowest 5-day average of PdG/CR values throughout the collection period). Evaluation of the first study population (n = 6) resulted in cycles with PdG ratios > or = 3 for > or = 3 consecutive days being classified as ovulatory; otherwise they were anovulatory. The sensitivity and specificity of the PdG ratio algorithm to identify anovulatory cycles in the second population were 75% and 89.5%, respectively, for all cycles (n = 88); 50% and 88.3% for first cycles (n = 40) using the cycles method; 75% and 92.2%, respectively, for all cycles (n = 89); and 50% and 94.1% for first cycles (n = 40) using the interval method. The "gold standard" for anovulation was weekly serum samples < or = 2 ng/ml progesterone. The sensitivity values for all cycles and for the first cycle using both methods were underestimated because of apparent misclassification of cycles using serum progesterone due to infrequent blood collection. Blood collection more than once a week would have greatly improved the sensitivity and modestly improved the specificity of the algorithm. The PdG ratio algorithm provides an efficient approach for screening urine samples collected in epidemiologic studies of reproductive health in women.  相似文献   
56.
Summary We operationalize scientific output in a region by means of the number of articles (as in the SciSearch database) per year and technology output by means of the number of patent applications (as in the database of the European Patent Office) per priority year. All informetric analyses were done using the DIALOG online-system. The main research questions are the following: Which scientific and technological fields or topics are most influent within a region and which institutions or companies are mainly publishing articles or holding patents? Do the distributions of regional science and technology fields and of publishing institutions follow the well-known informetric function? Are there - as it is expected - only few fields and few institutions which dominate the region? Is there a connection between the economic power of a region and the regional publication and patent output? Examples studied in detail are seven German regions: Aachen, D&uuml;sseldorf, Hamburg, K&ouml;ln (Cologne), Leipzig - Halle - Dessau, M&uuml;nchen (Munich), and Stuttgart. Three different indicators were used, science and technology attraction of a region (number of scientific articles and patents), science and technology intensity (articles and patents per 1,000 inhabitants), and science and technology density (articles and patents per 1 billion EURO gross value added). Top region concerning both attraction and intensity is Munich, concerning density it is Aachen.  相似文献   
57.
This paper outlines the development of a new procedure for analysing continuum mechanics problems with a particular focus on fluid–structure interaction in flexible tubes. A review of current methods of fluid–structure coupling highlights common limitations of high computational cost and solution instability. It is proposed that these limitations can be overcome by an alternative approach in which both fluid and solid components are solved within a single discretized continuum domain. A single system of momentum and continuity equations is therefore derived that governs both fluids and solids and which are solved with a single mesh using finite volume discretization schemes. The method is validated first by simulating dynamic oscillation of a clamped elastic beam. It is then applied to study the case of interest—wave propagation in highly flexible tubes—in which a predicted wave speed of 8.58 m/s falls within 2% of an approximate analytical solution. The method shows further good agreement with analytical solutions for tubes of increasing rigidity, covering a range of wave speeds from those found in arteries to that in the undisturbed fluid. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   
58.
59.
60.
As part of a cooperation project, Daimler AG and the Institute of Machine Elements, Gears, and Transmissions (MEGT) at the Technische Universität Kaiserslautern are carrying out detailed investigations on a valve train with roller lever. The aim of the project is to develop a detailed simulation method that allows the secondary movement of the valve train to be modeled and thus represents an extension of the classic valve train dynamics simulation. This opens up the opportunity to evaluate the processes within the valve train in detail in the early stages of the product development process. This report introduces the validation of the methodology based on experimental results. First, the focus is placed on the cam-roller contact, which induces the secondary movement of the valve train. This movement takes place in the axial direction of the camshaft so that the validation takes place on the basis of the measured force in this direction. The force is dependent on the load, i.?e. the combination of speed and normal force, as well as the angular misalignments between cam and roller. Following the validation of the detailed modeling of the cam-roller contact, the gained knowledge is transferred into the extended valve train simulations. Finally the valve train simulation is validated.  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