首页 | 官方网站   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   1414篇
  免费   15篇
  国内免费   2篇
工业技术   1431篇
  2022年   9篇
  2021年   12篇
  2020年   9篇
  2019年   9篇
  2018年   21篇
  2017年   18篇
  2016年   12篇
  2015年   7篇
  2014年   18篇
  2013年   40篇
  2012年   38篇
  2011年   26篇
  2010年   24篇
  2009年   24篇
  2008年   28篇
  2007年   23篇
  2006年   16篇
  2005年   20篇
  2004年   21篇
  2003年   13篇
  2002年   17篇
  2001年   13篇
  2000年   9篇
  1999年   23篇
  1998年   261篇
  1997年   155篇
  1996年   116篇
  1995年   57篇
  1994年   49篇
  1993年   57篇
  1992年   26篇
  1991年   16篇
  1990年   21篇
  1989年   19篇
  1988年   11篇
  1987年   21篇
  1986年   9篇
  1985年   11篇
  1984年   8篇
  1983年   6篇
  1982年   13篇
  1981年   15篇
  1980年   13篇
  1979年   2篇
  1978年   8篇
  1977年   27篇
  1976年   43篇
  1975年   7篇
  1974年   4篇
  1970年   2篇
排序方式: 共有1431条查询结果,搜索用时 15 毫秒
961.
In this study, we reported the HLA-class II genotyping by PCR-SSP (PCR-Sequence Specific Primers). This method was used to subtype the HLA-DRB1*13 and 11 alleles in Dielmo population. The most frequent DRB1*11 subtypes were 1101, 1102 et 1104; the frequency of 1102 was the highest (18.5%). The DRB1*13 subtypes were more polymorphic in the population and DRB*1304 was the most common subtype (15%). There were significant differences between the ethnic groups for some subtypes of DRB1*11 and 13 alleles.  相似文献   
962.
The acquisition of echocardiographic images in harmonic mode (a frequency double than the transmitted, or fundamental) improves imaging quality. We assessed whether harmonic imaging improves the detection of endocardial borders, evaluation of ventricular function and diagnostic confidence in the clinical arena. We have studied in fundamental and harmonic imaging 45 patients (age 20-89 years, mean 53 years) using a multifrequency transthoracic probe transmitting at 1.75 MHz and receiving at 3.5 MHz (Acuson Sequoia). In 34 low echogenic patients we assessed left ventricular function. The remaining 11 patients represented selected cases (i.e. atrial septal aneurysm, aortic dissection, endocarditis and atrial septal defect). The echocardiographic images were recorded on a magneto-optical disk and analyzed by two blinded observers. Endocardial definition has been semiquantitatively evaluated assigning a 0-4 score for each of the 16 segments of the left ventricle. A score of 0 was allotted to the non-visualizable segments and a score of 4 to the best detectable segments. Ejection fraction was calculated in each patient from the apical 4-chamber view. We compared endocardial border definition and ejection fraction at rest, in fundamental and harmonic mode, and assessed the interobserver agreement in the calculation of ejection fraction. Harmonic images always showed a better definition and lower noise compared to fundamental. Endocardial border definition was significantly improved in all segments (from 1.3 +/- 1.1 fundamental to 2.9 +/- 1.0 harmonic). Forty-two segments were non detectable in fundamental (score 0) compared to 5 in harmonic. Of these 42 segments, 37 were detectable in harmonic, with a score of 2.0 +/- 1.0. Conversely, none of the 5 segments non detectable in harmonic could be visualized in fundamental. The interobserver agreement in calculating ejection fraction was improved by harmonic imaging compared to fundamental (r = 0.91 and r = 0.67, respectively). In the selected clinical cases the diagnosis was easier and faster by harmonic imaging. The harmonic mode drastically improves echocardiographic imaging, it may be used routinely and reduce the need for more invasive techniques such as transesophageal echocardiography.  相似文献   
963.
A genomic library of the extremely thermophilic eubacterial strain Rt8B.4 was constructed in lambda ZapII and screened for the expression of xylanase activity. One recombinant bacteriophage showed xylanase, xylosidase and arabinosidase activity. Sequence analysis and homology comparisons showed that this plasmid derivative, pNZ2011, was composed of 6.7 kb thermophilic DNA and contained what appeared to be an operon-like structure involving genes associated with xylose metabolism. The xylanase gene, xynA was shown to code for a multi-domain protein. Xylanase activity was shown to be associated with the carboxy-terminal domain (domain 2) by deletion analysis and also by selective polymerase chain reaction (PCR) amplification and expression of the individual domains. Denaturing polyacrylamide gel analysis of the protein encoded by the PCR product showed three main overexpressed proteins to be present in cell extracts, presumably caused by proteolytic degradation in the Escherichia coli host. The xylanase activity from domain 2 is associated with a 36-kDa protein, which is stable at 70 degrees C for at least 12 h at pH 7. The small size of this active enzymatic domain and its temperature stability suggest that it may be of value in the enzyme-enhanced bleaching of kraft pulp.  相似文献   
964.
BACKGROUND & AIMS: Detection of gastrointestinal damage can be accomplished by permeability testing. However, current methodology presents several problems, including lack of site specificity and an inability to easily measure colonic damage. We hypothesized that probes of similar size could be used to simultaneously assess permeability along the length of the gastrointestinal tract. Differentiation of damage sites could be made, provided that probes were selectively destroyed at different levels. Class 1 probes should be destroyed after leaving the stomach, class 2 after leaving the small intestine, and class 3 not at all. In this manner, class 1 probes would report damage to the proximal gut, class 2 to the small intestine, and class 3 to the gut as a whole. METHODS: We defined saccharide probes with these characteristics and observed their permeability patterns in defined models of localized gastrointestinal damage. RESULTS: With gastric damage, permeability of only class 1 probes increased, whereas with colonic damage only permeation rates of class 3 probes was increased. Furthermore, the permeation rates of both class 2 and 3 probes increased in the presence of small intestinal disease. CONCLUSIONS: These techniques allow a single screening test that is sensitive to damage at any level of the gastrointestinal tract and may be used in either animals or humans.  相似文献   
965.
We demonstrate that an aerosol can gel. This gelation is then used for a one-step method to produce an ultralow density porous carbon material. This material is named an aerosol gel because it is made via gelation of particles in the aerosol phase. The carbon aerosol gels have high specific surface area (200–350 m 2 /g), an extremely low density (2.5–5.0 mg/cc) and a high electrical conductivity, properties similar to conventional aerogels. The primary particles of the carbon aerosol gels are highly crystalline with a narrow (002) graphitic X-ray diffraction peak. Key aspects to form a gel from an aerosol are large volume fraction, ca. 10 ?4 or greater, and small primary particle size, 50 nm or smaller, so that the gel time is fast compared to other characteristic times.  相似文献   
966.
967.
We report Eu3+ doped transparent glass-ceramics (GCs) containing bismuth layer-structured ferroelectric (BLSF) CaBi2Ta2O9 (CBT) as the major crystal phase. The CBT crystal phase was generated in a silica rich glass matrix of SiO2-K2O-CaO-Bi2O3-Ta2O5 glass system synthesized by melt quenching technique followed by controlled crystallization through ceramming heat-treatment. Non-isothermal DSC study was conducted to analyze crystallization kinetics of the glass in order to understand the crystallization mechanism. The optimum heat-treatment protocol for ceramization of precursor glass that has been determined through crystallization kinetics analysis was employed to fabricate transparent GCs containing CBT nanocrystals, which was otherwise difficult. Structural analysis of the GCs was carried out using XRD, TEM, FESEM and Raman spectroscopy and results confirmed the existence of CBT nanocrystals. The transmittance and optical band gap energies of the GCs were found to be less when compared to the precursor glass. The refractive indices of the GCs were increased monotonically with increase in heat-treatment time, signaling densification of samples upon heat-treatment. The dielectric constants (εr) of the GCs were progressively increased with increase in heat-treatment duration indicating evolution of ferroelectric CBT crystals phase upon heat-treatment.  相似文献   
968.
969.
Neural Computing and Applications - This research article corroborates the working of a model reference adaptive model (MRAS) with fractional-order proportional-integral (FOPI $$^lambda $$ )-based...  相似文献   
970.
Metallurgical and Materials Transactions A - The present study demonstrates remarkable retardation of recrystallization and grain growth during sub-critical annealing of 60 pct cold-rolled ferritic...  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-23

京公网安备 11010802026262号