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   760篇
  免费   14篇
  国内免费   2篇
电工技术   30篇
综合类   1篇
化学工业   222篇
金属工艺   18篇
机械仪表   5篇
建筑科学   4篇
能源动力   23篇
轻工业   58篇
无线电   51篇
一般工业技术   98篇
冶金工业   211篇
原子能技术   22篇
自动化技术   33篇
  2022年   11篇
  2021年   13篇
  2020年   8篇
  2019年   10篇
  2018年   13篇
  2017年   5篇
  2016年   12篇
  2015年   3篇
  2014年   24篇
  2013年   30篇
  2012年   31篇
  2011年   22篇
  2010年   25篇
  2009年   22篇
  2008年   24篇
  2007年   24篇
  2006年   35篇
  2005年   25篇
  2004年   26篇
  2003年   18篇
  2002年   21篇
  2001年   12篇
  2000年   18篇
  1999年   20篇
  1998年   74篇
  1997年   45篇
  1996年   36篇
  1995年   27篇
  1994年   19篇
  1993年   19篇
  1991年   9篇
  1990年   3篇
  1989年   7篇
  1988年   8篇
  1987年   5篇
  1986年   6篇
  1984年   7篇
  1983年   4篇
  1982年   5篇
  1981年   2篇
  1980年   3篇
  1979年   9篇
  1977年   4篇
  1976年   6篇
  1975年   4篇
  1974年   5篇
  1973年   3篇
  1972年   2篇
  1971年   2篇
  1970年   2篇
排序方式: 共有776条查询结果,搜索用时 15 毫秒
1.
Over 2000 electrocution deaths were identified among U.S. construction workers from 1980 to 1991, with the highest mean annual crude mortality rate (2.5 per 100,000 people), and second highest mean age-adjusted rate (2.7 per 100,000 people) of all industries. Although the crude fatality rates showed a downward trend, construction workers are still about four times more likely to be electrocuted at work than are workers in all industries combined. Nearly 40% of the 5083 fatal electrocutions in all industries combined occurred in construction, and 80% were associated with industrial wiring, appliances, and transmission lines. Electrocutions ranked as the second leading cause of death among construction workers, accounting for an average of 15% of traumatic deaths in the industry from 1980 to 1991. The study indicates that the workers most at risk of electrical injury are male, young, nonwhite, and electricians, structural metal workers, and laborers. The most likely time of injury is 11 a.m. to 3 p.m. from June to August. Focusing prevention on these populations and characteristics through better methods of worker and supervisor electrical safety training, use of adequate protective clothing, and compliance with established procedures could minimize the average annual loss of 168 U.S. construction workers.  相似文献   
2.
A physicochemical parameter, represented by the symbol Σs*, based on molar solubility in water and molar attraction constants of Small, has been developed to express quantitatively the relative hydrophobicity, or nonpolar character, of the hydrocarbon molecule. The value of Σs* can be calculated for a hydrocarbon from its chemical structure. The scale of Σs* is consistent within each group of aromatic, cyclic, and noncyclic hydrocarbons. Reverse osmosis data have been obtained at 250 psig for single-solute aqueous feed solution systems involving low concentrations of 39 different hydrocarbons (including 13 aromatics, 10 cyclic, and 16 noncyclic compounds) and several samples of cellulose acetate membranes of different surface porosities. The effect of operating pressure on membrane performance has also been studied for two aromatic hydrocarbon solutes. The values of Σs* for the solutes used were in the range of 425 to 924 for aromatic hydrocarbons, 521 to 931 for cyclic hydrocarbons, and 369 to 960 for noncyclic hydrocarbons. The reverse osmosis data have been correlated with Σs* for each group of hydrocarbons studied. In all cases, positive solute separations were obtained, and the ratio [PR]/[PWP] was less than 1. With respect to each film, solute separation increased with increase in Σs*, and decreased with increase in operating pressure. Also, solute separation decreased in the order aromatic hydrocarbon > cyclic hydrocarbon > noncyclic hydrocarbon at any given value of Σs*. At a given operating pressure, for low values of Σs* (~500 or less) solute separation increased with progressive decrease in average pore size on the membrane surface. For high values of Σs* (~800 or more), solute separation initially increased with decrease in average pore size, then passed through a maximum and minimum with further decrease in average pore size, and again increased with still further decrease in average pore size. The results are discussed on the basis of preferential sorption of solute at the membrane–solution interface under the experimental conditions studied.  相似文献   
3.
Reverse osmosis separations of eight polyethylene glycol (PEG) solutes in the average molecular weight range of 200 to 6750 in single-solute dilute aqueous solutions have been studied using porous cellulose acetate membranes at the operating pressures of 50, 75, and 100 psig. Diffusivity data for the above PEG solutes have also been obtained from experimental data on intrinsic viscosities. From an analysis of all experimental data, numerical values for the parameters representing the polar (?ΔΔG/RT), steric (δ*ΣEs), and nonpolar (ω*Σs*) forces governing reverse osmosis separations of PEG solutes have been generated. These numerical values are useful for precise characterization of cellulose acetate membranes for whose specifications sodium chloride is not the appropriate reference solute because of its low or practically negligible separation under reverse osmosis operating conditions. This work also illustrates that solute separation in reverse osmosis can predictably increase or decrease with increase in operating pressure depending on experimental conditions.  相似文献   
4.
Reverse osmosis separations of phenol (9.4 to 108 ppm), p-cresol (108 ppm), and p-chlorophenol (129 ppm) were studied using Loeb-Sourirajan-type porous cellulose acetate membranes, and single-solute aqueous feed solutions at 500 psig and the indicated solute concentrations. It was found that, by dissociating the solute by changing the pH of the feed solution, all the above phenols could be separated by reverse osmosis. Solute separation increased with increase in the degree of dissociation of the solute in the feed solution; and, by the appropriate choice of pore size on the membrane surface, separations of phenol approaching the degree of dissociation of phenol in the feed solution could be obtained under the operating conditions used. Similar experiments using aniline (93 ppm) as the solute showed that dissociation of solute molecules in the feed solution could be a technique generally applicable for the reverse osmosis separation of nonionic solutes in aqueous solution. The effects of operating pressure in the range 250 to 1500 psig and pore size on the membrane surface on the separation of un-ionized phenol and p-chlorophenol showed that, with respect to single-solute aqueous feed solutions of phenols, the component whose relative acidity was greater was preferentially sorbed at the cellulose acetate membrane—aqueous solution interface, and the solute concentration in the membrane-permeated product solution was a function of the extent and mobility of each of the sorbed species.  相似文献   
5.
An intensive chemotherapy combined with surgery, termed "intraoperative local infusion chemotherapy (ILIC)", was devised. The ILIC procedure is to infuse 50 mg of cisplatin through the feeding artery to the tumor, which has been isolated from blood flow by clamping the stomach and blood vessels, before radical surgery. An experiment in dogs showed considerable free Pt transfer into thoracic lymph after ILIC. Sum total Pt in the thoracic lymph after ILIC (294.5 micrograms/3 h) was 4.3 times that after IV administration (68.3 micrograms/3 h). In ILIC, AUC of free Pt in the thoracic lymph was about 5.8 times that in peripheral blood. ILIC was applied in 24 advanced gastric cancer patients, consisting of 6, 2, 4 and 12 in pTNM stage II, IIIA, IIIB and IV, respectively. Pt concentration of the tissues was high in the tumor (23.22 micrograms/g) and the regional lymph nodes (2.95 micrograms/g) compared to that in the serum (0.45 microgram/ml). The survival rate (Kaplan-Meier) in the ILIC patients was significantly higher than that of control patients (matched pair method) from among patients treated by surgery alone. No fatal complications were encountered in any patient treated by ILIC.  相似文献   
6.
Summary Detection and determination of traces of sulphites in foods was attempted by use of the modified Rankine apparatus and pararosaniline colorimetry. Replacement of alkaline titration reported previously by pararosaniline colorimetry lowered the absolute detection limit from 30 g (titration method) to 2 g. In view of clean analysis, in the color developing system, 0.1 N-sodium hydroxide was used in place of mercuric chloride solution commonly used as an absorbant of sulphites. In order to prevent oxidative decomposition of sulphites during operation, nitrogen gas was used as carrier instead of air. Dimedone and sodium azide were used for the elimination of aldehydes and nitrites, respecitvely, in the sample, which will disturb the color development of sulphites with pararosaniline-formaldehyde reagents. With this improved method, it was possible to determine the residual sulphites in frozen peeled shrimps, sugared beans and other foods with low sulphite contents accurately.
Colorimetrische Mikrobestimmung von Sulfiten in Lebensmitteln bei Anwendung der modifizierten IV. Rankine Apparatur
Zusammenfassung Geringe Sulfitmengen in Lebensmitteln (geschälte Garnelen, gezuckerte Bohnen) können colorimetrisch bestimmt werden. Die neuentwickelte Methode beruht auf einer Kombination von colorimetrischer Bestimmung mittels p-Rosanilin und der Bestimmungsmethode nach Rankine. Auf diese Weise lassen sich Gehalte von 2 g noch genau bestimmen. Bei der Farbentwicklung wurde das giftige Quecksilbertetrachlorid durch 0.1 n-NaOH ersetzt, anstelle von Luft Stickstoff als Trägergas verwendet und somit eine Oxydation des Sulfits während der Bestimmung vermieden. Da Nitrit und Aldehyde die Farbentwicklung stören, wurde ihr Einfluß durch Dimedon und Natriumazid ausgeschaltet.


Studies on the Analyses of Sulphites in Foods (IV)  相似文献   
7.
Although the melt structure of glass-forming ZnCl2 has so far been well studied, there exists quite little information on the structural change due to anion-substitution. In the present work, the short-range structure of ZnCl2–ZnBr2 mixture melts was analyzed systematically by time-of-flight pulsed neutron diffraction techniques, Raman spectroscopy, molecular orbital calculations, and molecular dynamics simulations. According to radial distribution analysis, it was found that there were tetrahedral structural units of ligand-substituted [ZnClnBr4−n]2− (n=0–4) in these melts, not implying the simple mixing of [ZnCl4]2− and [ZnBr4]2− units. Further detailed estimation indicated that the ligand-substituted complex anions were linked with each other by sharing a common anion.  相似文献   
8.
The performance of polymer electrolyte fuel cell (PEFC) is affected by an interfacial property between a proton exchange membrane (PEM) and electrodes. Thus, to develop a well-laminated membrane electrode assembly (MEA), a hybrid PEM (FN) was fabricated by mixing a radiation grafted membrane (sulfonated FEP) with ionomer (Nafion® dispersion) which is applied to coat the interface of the PEM and electrodes.The obtained FN, sulfonated FEP and Nafion®112 were characterized in terms of water uptake, ion exchange capacity (IEC), polarization performance and electrochemical impedance. FN showed high IEC and water uptake, which would induce the highest ionic conductivity (IC) among tested PEMs. In terms of FN, the interface between the PEM and electrodes should have been improved because FN showed the lowest charge transfer resistance than other tested PEMs. The high IC and improved interface between the PEM and electrodes resulted in the best cell performance of FN in tested PEMs.  相似文献   
9.
The role of the ligamentum flavum (LF) in the pathogenesis of adolescent idiopathic scoliosis (AIS) is not well understood. Using magnetic resonance imaging (MRI), we investigated the degrees of LF hypertrophy in 18 patients without scoliosis and on the convex and concave sides of the apex of the curvature in 22 patients with AIS. Next, gene expression was compared among neutral vertebral LF and LF on the convex and concave sides of the apex of the curvature in patients with AIS. Histological and microarray analyses of the LF were compared among neutral vertebrae (control) and the LF on the apex of the curvatures. The mean area of LF in the without scoliosis, apical concave, and convex with scoliosis groups was 10.5, 13.5, and 20.3 mm2, respectively. There were significant differences among the three groups (p < 0.05). Histological analysis showed that the ratio of fibers (Collagen/Elastic) was significantly increased on the convex side compared to the concave side (p < 0.05). Microarray analysis showed that ERC2 and MAFB showed significantly increased gene expression on the convex side compared with those of the concave side and the neutral vertebral LF cells. These genes were significantly associated with increased expression of collagen by LF cells (p < 0.05). LF hypertrophy was identified in scoliosis patients, and the convex side was significantly more hypertrophic than that of the concave side. ERC2 and MAFB genes were associated with LF hypertrophy in patients with AIS. These phenomena are likely to be associated with the progression of scoliosis.  相似文献   
10.
From the temperature dependence of the hole concentration in unirradiated lightly Al-doped 4H-SiC epilayers, an Al acceptor with EV + 0.2 eV, which is an Al atom (AlSi) at a Si sublattice site, and an unknown deep acceptor with EV + 0.35 eV are found, where EV is the top of the valence band. Both the densities are similar. With irradiation of 0.2 MeV electrons the Al acceptor density is reduced, while the unknown deep acceptor density is increased. Judging from the minimum electron energy required to displace a substitutional C atom (Cs) or the AlSi, the bond between the AlSi and its nearest neighbor Cs is broken due to the displacement of the Cs by this irradiation. Moreover, the displacement of the Cs results in the creation of a complex (AlSi-VC) of AlSi and a carbon vacancy (VC), indicating that the possible origin of the deep acceptor with EV + 0.35 eV is AlSi-VC.  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