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   2596篇
  免费   95篇
  国内免费   1篇
电工技术   18篇
综合类   2篇
化学工业   462篇
金属工艺   33篇
机械仪表   28篇
建筑科学   142篇
矿业工程   6篇
能源动力   64篇
轻工业   215篇
水利工程   18篇
石油天然气   13篇
无线电   145篇
一般工业技术   475篇
冶金工业   734篇
原子能技术   20篇
自动化技术   317篇
  2022年   24篇
  2021年   27篇
  2020年   25篇
  2019年   20篇
  2018年   33篇
  2017年   46篇
  2016年   58篇
  2015年   45篇
  2014年   55篇
  2013年   122篇
  2012年   101篇
  2011年   126篇
  2010年   83篇
  2009年   93篇
  2008年   105篇
  2007年   79篇
  2006年   74篇
  2005年   77篇
  2004年   57篇
  2003年   63篇
  2002年   63篇
  2001年   41篇
  2000年   49篇
  1999年   42篇
  1998年   112篇
  1997年   76篇
  1996年   83篇
  1995年   62篇
  1994年   68篇
  1993年   62篇
  1992年   43篇
  1991年   28篇
  1990年   33篇
  1989年   42篇
  1988年   53篇
  1987年   43篇
  1986年   35篇
  1985年   51篇
  1984年   31篇
  1983年   21篇
  1982年   46篇
  1981年   40篇
  1980年   38篇
  1979年   31篇
  1978年   27篇
  1977年   35篇
  1976年   23篇
  1975年   20篇
  1974年   15篇
  1973年   17篇
排序方式: 共有2692条查询结果,搜索用时 15 毫秒
921.
This paper presents a review of the development of an expert system designed to assist in the teaching of statics. The system is intended to emulate manual solution methods for a range of problems typically used in introductory courses in structural mechanics. It provides the functionality of an expert tutor by offering advice as the student-users work their way through the problem as well as validating methods and results. FORCE-SOLVER has been developed for use on IBM-compatible micro-computers and demonstrates some of the necessary features required to make such an expert system easily used. The paper describes the various elements of the program, including the necessary problem-solving strategies.intended to capture the ‘expert’ knowledge necessary to determine efficient solution strategies.  相似文献   
922.
Wasabi (Wasabia japonica (Miq) Matsum) is a developing crop in New Zealand and is valued for its spicy taste and pungent smell. It is popular as a condiment for traditional and modern Japanese foods. However, the limited area suitable for wasabi production in Japan has resulted in inability to meet increasing market demand. Isothiocyanates (ITCs) are sulphur compounds responsible for the unique flavour of wasabi, with allyl isothiocyanate (AITC) being the main compound responsible for the pungency. In this study, AITC tissue concentration and yield were measured in three above‐ground tissues of the wasabi plant to investigate the effects of different fertiliser, manure and lime treatments. The highest tissue concentration of AITC was found in the rhizomes, ranging from 1564 to 3366 mg kg?1 (fresh weight basis), while the petioles and leaves contained 254–373 and 453–643 mg kg?1 respectively. Fertilisation with ammonium sulphate produced the highest‐quality rhizomes (72% increase in AITC yield) and petioles (64% increase), but the best response in the leaves (51%) resulted from the manure treatment. Nitrogen fertiliser alone reduced the AITC yield by up to 15%. These results should help in formulating guidelines for production of high‐quality wasabi tops containing high levels of AITC. © 2002 Society of Chemical Industry  相似文献   
923.
Chemical kinetics were determined for the reactions of ozone and hydroxyl radicals with the three cyanotoxins microcystin-LR (MC-LR), cylindrospermopsin (CYN) and anatoxin-a (ANTX). The second-order rate constants (k(O3)) at pH 8 were 4.1 +/- 0.1 x 10(5) M(-1) s(-1) for MC-LR, approximately 3.4 x 10(5) M(-1) s(-1) for CYN, and approximately 6.4 x 10(4) M(-1) s(-1) for ANTX. The reaction of ozone with MC-LR exhibits a k(O3) similar to that of the conjugated diene in sorbic acid (9.6 +/- 0.3 x 10(5) M(-1) s(-1)) at pH 8. The pH dependence and value of k(O3) for CYN at pH > 8 (approximately 2.5 +/- 0.1 x 10(6) M(-1) s(-1)) are similar to deprotonated amines of 6-methyluracil. The k(O3) of ANTX at pH > 9 (approximately 8.7 +/- 2.2 x 10(5) M(-1) s(-1)) agrees with that of neutral diethylamine, and the value at pH < 8 (2.8 +/- 0.2 x 10(4) M(-1) s(-1)) corresponds to an olefin. Second-order rate constants for reaction with OH radicals (*OH), k(OH) for cyanotoxins were measured at pH 7 to be 1.1 +/- 0.01 x 10(10) M(-1) s(-1) for MC-LR, 5.5 +/- 0.01 x 10(9) M(-1) s(-1) for CYN, and 3.0 +/- 0.02 x 10(9) M(-1) s(-1) for ANTX. Natural waters from Switzerland and Finland were examined for the influence of variations of dissolved organic matter, SUVA254, and alkalinity on cyanotoxin oxidation. For a Swiss water (1.6 mg/L DOC), 0.2, 0.4, and 0.8 mg/L ozone doses were required for 95% oxidation of MC-LR, CYN, and ANTX, respectively. For the Finnish water (13.1 mg/L DOC), >2 mg/L ozone dose was required for each toxin. The contribution of hydroxyl radicals to toxin oxidation during ozonation of natural water was greatest for ANTX > CYN > MC-LR. Overall, the order of reactivity of cyanotoxins during ozonation of natural waters corresponds to the relative magnitudes of the second-order rate constants for their reaction with ozone and *OH. Ozone primarily attacks the structural moieties responsible for the toxic effects of MC-LR, CYN, and ANTX, suggesting that ozone selectively detoxifies these cyanotoxins.  相似文献   
924.
Cold water extraction of gum ghatti gives a soluble gum and an insoluble gel. The two fractions were shown by potentiometric titration to have the same acid equivalent weight and to be largely in the salt form, with less than 10% in the free acid form. Quantitative analysis of the two fractions for metal ion content showed that the gel fraction was largely a calcium salt. The soluble fraction contains calcium, potassium, sodium, and magnesium. Precipitation of the calcium ions reduces the viscosity but the original viscosity is not restored by addition of calcium ions.  相似文献   
925.
The objectives of this work were to establish the effect of seven different forages with a varied condensed tannin (CT) content, plant maturity and nitrogen fertiliser application on the ruminal formation of skatole and indole using an in vitro method designed to mimic rumen fermentation conditions. After 10 h of incubation, the concentration of indole and skatole was highest when incubating white clover (P < 0.05). Polyethylene glycol addition, to inhibit CT, showed that CT significantly reduced the formation of indole and skatole when incubating Lotus corniculatus, sulla, Dorynium rectum and Lotus pedunculatus (P < 0.01). Mature forage growth resulted in a significantly lower concentration of indole and skatole being formed in vitro compared to the incubation of new spring growth (P < 0.001). A higher application of nitrogen fertiliser to perennial ryegrass‐based pasture resulted in a higher crude protein concentration in the plant and a significantly higher concentration of skatole formed in vitro (P < 0.001). Forages containing CT reduced the conversion of endogenous protein to indole and skatole and plants containing a higher CT concentration tended to be more effective, but compositional differences of CT between forages may also have had an influence. New forage growth or swards that had a high application of nitrogen fertiliser promoted the formation of indole and skatole. Copyright © 2007 Society of Chemical Industry  相似文献   
926.
A variety of natural vinegar products are found in civilizations around the world. A review of research on these fermented products indicates numerous reports of health benefits derived by consumption of vinegar components. Therapeutic effects of vinegar arising from consuming the inherent bioactive components including acetic acid, gallic acid, catechin, ephicatechin, chlorogenic acid, caffeic acid, p‐coumaric acid, and ferulic acid cause antioxidative, antidiabetic, antimicrobial, antitumor, antiobesity, antihypertensive, and cholesterol‐lowering responses. The aims of this article are to discuss vinegar history, production, varieties, acetic acid bacteria, and functional properties of vinegars.  相似文献   
927.
928.
We argue that mechanics and physics of solids rely on a fundamental exemplar: the apparent properties of a system depend on the building blocks that comprise it. Building blocks are referred to as archetypes and apparent system properties as the system genome. Three entities are of importance: the archetype properties, the conformation of archetypes, and the properties of interactions activated by that conformation. The combination of these entities into the system genome is called assembly. To show the utility of the archetype-genome exemplar, this work presents the mathematical ingredients and computational implementation of theories in solid mechanics that are (1) molecular and (2) continuum manifestations of the assembly process. Both coarse-grained molecular dynamics (CGMD) and the archetype-blending continuum (ABC) theories are formulated then applied to polymer nanocomposites (PNCs) to demonstrate the impact the components of the assembly triplet have on a material genome. CGMD simulations demonstrate the sensitivity of nanocomposite viscosities and diffusion coefficients to polymer chain types (archetype), polymer–nanoparticle interaction potentials (interaction), and the structural configuration (conformation) of dispersed nanoparticles. ABC simulations show the contributions of bulk polymer (archetype) properties, occluded region of bound rubber (interaction) properties, and microstructural binary images (conformation) to predictions of linear damping properties, the Payne effect, and localization/size effects in the same class of PNC material. The paper is light on mathematics. Instead, the focus is on the usefulness of the archetype-genome exemplar to predict system behavior inaccessible to classical theories by transitioning mechanics away from heuristic laws to mechanism-based ones. There are two core contributions of this research: (1) presentation of a fundamental axiom—the archetype-genome exemplar—to guide theory development in computational mechanics, and (2) demonstrations of its utility in modern theoretical realms: CGMD, and generalized continuum mechanics.  相似文献   
929.
1. We used intracellular current-clamp techniques to record from 33 ventral subicular neurons in slices or rat hippocampal formation. Presumed pyramidal neurons were characterized by their responses to depolarizing current pulses as either intrinsically burst firing (IB) or regular spiking (RS). Within the subiculum, IB cells were encountered most frequently in the deep cell layer, whereas RS cells were encountered most frequently in the superficial cell layer. IB cells had more depolarized resting potentials, lower input resistances, and more sag in their voltage responses to hyperpolarizing current pulses. 2. Somatostatin (5 microM) applied in the bathing medium caused a hyperpolarization and reduction in input resistance. These effects were of greater magnitude in IB cells. Somatostatin had no effect on sag in either cell type. These effects of somatostatin were unchanged in the presence of gamma-aminobutyric acid (GABA) receptor antagonists. 3. In a series of experiments conducted in RS cells only, somatostatin reduced the amplitude of the late but not the early component of evoked biphasic inhibitory postsynaptic potentials (IPSPs). 4. A second series of experiments was conducted in RS and IB cells. Somatostatin reduced the amplitude of pharmacologically isolated GABAA IPSPS in both cell types. In IB cells but not RS cells there was a correlation between this effect and the somatostatin-induced hyperpolarization. Somatostatin also reduced the amplitude of isolated GABAB IPSPS in both cell types, but more so in IB cells. 5. Somatostatin had no effect on the reversal potential of either IPSP in either cell type and no effect on the GABAA-mediated conductance in either cell type. In contrast, the GABAB-mediated conductance was reduced, especially in IB cells. 6. The effects of somatostatin on GABAA IPSPS are principally a result of membrane shunting and reductions in ionic driving force, but these mechanisms do not account for the reduction in GABAB IPSPS. 7. We suggest that the combined effects of somatostatin are likely to alter the balance between fast and slow inhibition and to do so more in IB cells than in RS cells.  相似文献   
930.
Two well-established phenomena in temporal performance—preference for simple ratios over complex ratios and the ability to proportionately rescale temporal patterns—were examined together. Unlike the case with simple ratios, participants (3 trained musicians) showed only a limited ability to learn complex ratios and no ability to proportionately rescale them. These differences suggest that different mechanisms are used to produce the 2 ratio types. Systematic biases observed in the performances of the simple ratios (6 trained musicians) were modeled by assuming that an additive timing component, which is unequal for short and long intervals and changes with base rate, overlays a system that preferentially produces simple ratios. The general pattern of results was consistent with the view that rhythmic learning builds on or modifies a system of innate preferences. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  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