全文获取类型
收费全文 | 15300篇 |
免费 | 237篇 |
国内免费 | 22篇 |
专业分类
电工技术 | 188篇 |
综合类 | 4篇 |
化学工业 | 1987篇 |
金属工艺 | 361篇 |
机械仪表 | 339篇 |
建筑科学 | 513篇 |
矿业工程 | 60篇 |
能源动力 | 308篇 |
轻工业 | 1214篇 |
水利工程 | 89篇 |
石油天然气 | 118篇 |
无线电 | 1509篇 |
一般工业技术 | 2333篇 |
冶金工业 | 4976篇 |
原子能技术 | 159篇 |
自动化技术 | 1401篇 |
出版年
2021年 | 98篇 |
2019年 | 102篇 |
2018年 | 135篇 |
2017年 | 101篇 |
2016年 | 94篇 |
2015年 | 109篇 |
2014年 | 225篇 |
2013年 | 569篇 |
2012年 | 304篇 |
2011年 | 408篇 |
2010年 | 332篇 |
2009年 | 336篇 |
2008年 | 450篇 |
2007年 | 439篇 |
2006年 | 434篇 |
2005年 | 389篇 |
2004年 | 323篇 |
2003年 | 360篇 |
2002年 | 309篇 |
2001年 | 322篇 |
2000年 | 314篇 |
1999年 | 388篇 |
1998年 | 1406篇 |
1997年 | 874篇 |
1996年 | 624篇 |
1995年 | 443篇 |
1994年 | 392篇 |
1993年 | 411篇 |
1992年 | 276篇 |
1991年 | 246篇 |
1990年 | 257篇 |
1989年 | 216篇 |
1988年 | 220篇 |
1987年 | 203篇 |
1986年 | 196篇 |
1985年 | 252篇 |
1984年 | 208篇 |
1983年 | 205篇 |
1982年 | 201篇 |
1981年 | 181篇 |
1980年 | 196篇 |
1979年 | 128篇 |
1978年 | 136篇 |
1977年 | 238篇 |
1976年 | 338篇 |
1975年 | 135篇 |
1974年 | 125篇 |
1973年 | 107篇 |
1972年 | 78篇 |
1970年 | 83篇 |
排序方式: 共有10000条查询结果,搜索用时 218 毫秒
101.
D.Z.-Y. Ting X. Cartoixà T.C. McGill D.L. Smith J.N. Schulman 《Journal of Computational Electronics》2002,1(1-2):147-151
The Rashba effect resonant tunneling diode is a candidate for achieving spin polarizing under zero magnetic field using only conventional non-magnetic III–V semiconductor heterostructures. We point out the challenges involved based on simple arguments, and offer strategies for overcoming these difficulties. We present modeling results that demonstrate the benefits of the InAs/GaSb/AlSb-based asymmetric resonant interband tunneling diode (a-RITD) for spin filtering applications. 相似文献
102.
This study examined the relationship of several maternal variables to the duration of exclusive breast-feeding and the total duration of breast-feeding, along with attitudes, perceptions, and beliefs about breast-feeding among women living in poor neighborhoods of Managua, Nicaragua. The field work was carried out in December 1992 and January 1993 using qualitative and quantitative methods. A structured questionnaire was administered in interviews with 556 mothers of children under 12 months of age, and meetings of four directed discussion groups were held, in which a total of 20 women participated. At one week of age, almost all the children of the mothers who were surveyed had been breast-fed, but only 45% had been exclusively breast-fed. At 12 weeks old, 30% were already completely weaned. The discussion groups revealed the coexistence of positive opinions about both breast-feeding and bottle-feeding. However, exclusive breast-feeding was considered harmful for the mother, and breast milk was not thought to be sufficient nourishment for the child. Previous experience was strongly related to the duration of exclusive breast-feeding and to total breast-feeding duration. Attitudes, social support, and work situation were important factors influencing the total length of time women breast-fed a child. In general, the results obtained through the interviews and in the discussion groups were in agreement and showed that the elements needed to promote exclusive breast-feeding were social support, a favorable community environment, and policies that dealt with problems faced by working mothers. The complementary research methods were useful for obtaining information about the relative importance of different factors that determine the duration of breast-feeding and for understanding that practice in greater depth from the mother's point of view. 相似文献
103.
104.
Mineralocorticoid action is facilitated by 11 beta-hydroxysteroid dehydrogenase type II (11 beta HSD2), which metabolizes glucocorticoids and allows aldosterone to bind to the nonselective mineralocorticoid receptor. We have recently demonstrated the presence of the 11 beta HSD2 protein in a wide range of human epithelia, suggesting that it is the sole isoform endowing specificity in man. In the present study we have used an immunopurified polyclonal antibody (RAH23) raised against a C-terminal peptide derived from the cloned rat 11 beta HSD2 protein to perform immunohistochemical and molecular analysis in rat tissues. In frozen sections of rat kidney, strong staining was seen with the RAH23 antibody in the distal tubule; weaker staining was observed in the thick ascending loop of Henle and the medullary and papillary collecting ducts. Punctate cortical staining was observed in the fetus at 20 days gestation and in 8-day-old rats, with a noticeable increase in the staining pattern at 16 days of age. The kidney did not attain the adult pattern of staining until 28 days of age. Epithelia of ileum and colon also stained with RAH23, as did excretory ducts of the submandibular gland. Intrahepatic and excretory bile ducts displayed strong immunoreactivity in the epithelial lining. Rat adrenal glands showed evidence of the 11 beta HSD2 antigen in the zona fasciculata and zona reticularis, but not in the zona glomerulosa or medulla. Western blot analysis with the RAH23 antibody revealed strong bands in the kidney, colon, adrenal gland, and submandibular gland at 40 kDa, colinear with the migration of the cloned 11 beta HSD2 enzyme. A band of medium intensity was also seen at this size in the pancreas, whereas a band of moderate intensity was seen in the bile duct, and weaker bands were noticed in the stomach, small intestine, and liver, with a diffuse band at 36-42 kDa in the prostate. Strong bands were seen in the pancreas and prostate at 78 kDa, with weaker signals in the colon, adrenal, stomach, and bile duct. A number of tissues also displayed multiple bands at about 30 kDa. Enzymatic assays on tissue homogenates showed extensive conversion of corticosterone to its 11-dehydro product in an NAD-dependent manner in the submandibular gland, adrenal gland, and kidney, but not in the pancreas or prostate. This study confirms the ubiquitous presence of 11 beta HSD2 in sodium-transporting epithelia, demonstrates the high level of 11 beta HSD2 protein and enzyme activity in the rat adrenal, and suggests a possible role for the enzyme in the biliary system. Further studies are required to determine the relevance of the various molecular species to the activity, latency, and processing of the enzyme. 相似文献
105.
Molybdenum black solar selective coatings have been produced on cobalt by immersion in a solution of ammonium paramolybdate
and nickel sulphate. The cobalt was electroplated on nickel-plated copper prior to immersion in the paramolybdate solution.
The maximum solar absorptance of the resulting molybdenum black coating was about 0.91. The minimum emittance was about 0.1
for coatings on cobalt deposited with an addition agent and about 0.23 for coatings on cobalt deposited without addition agent
in the 60 °C plating solution. These differences have been related to the coating morphology determined by scanning electron
microscopy. The emittance of coatings on cobalt deposited without addition agent decreases (or remains unchanged) during short-term
heat treatment while that of coatings deposited on cobalt plated with addition agent increases somewhat. Reduction of the
cobalt plating bath temperature to 45 °C can also lead to good initial coating properties but without the requirement for
an addition agent in the cobalt plating bath. XPS studies show that the oxidation state of molybdenum in the coatings is approximately
+ 5 corresponding to Mo4O11. This reduces to + 4 after argon ion bombardment. Some cobalt may be present in the coatings in the form of CoO. 相似文献
106.
MR Smith 《Canadian Metallurgical Quarterly》1997,176(6):1374-1375
Colposacropexy procedures restore anatomically correct apical vaginal support on the levator plate at the ischial spine level. Venous hemorrhage resulting from laceration of presacral veins during suture fixation is the major hazard of this procedure. Titanium orthopedic bone anchor fixation minimizes this risk through precision placement of the bone anchor-suture unit. 相似文献
107.
R. Smith 《Journal of Engineering Mathematics》1995,29(6):497-516
A two-dimensional horizontal dispersion equation is derived for the many ways that buoyancy modifies the spreading of a mono-disperse discharge of particles in a shallow-water flow. The particle-induced lateral density gradient gives rise to a secondary transverse flow which tends to carry particles outwards. Stratification with the consequent reduced mixing, changes the local vertical profile of the original flow and gives rise to a Burgers nonlinearity (slowing down of sinking particles or a speeding up of buoyant particles). Stratification also modifies the non-local horizontal distribution of the current, with an inflow towards particle-laden regions where the drag is reduced. A simple eddy-diffusivity turbulence model is used which permits explicit evaluation of most of the linear and nonlinear coefficients. Nonlinear effects are shown to be significant for particle volume concentrations of only 10 parts per million. 相似文献
108.
109.
S. H. Smith 《Journal of Engineering Mathematics》1995,29(2):195-202
The second order boundary layer effects are considered for the classical problem of the impulsive motion past a flat plate. Although Stewartson [1], [2] showed clearly how the effect of the leading edge requires a finite time to enter the boundary layer when only first order effects are included, we see here how this second order contribution is noticed immediately throughout the layer, and so becomes the dominant contribution after the Rayleigh behaviour during this time. 相似文献
110.
Totterdell Peter; Spelten Evelien; Smith Lawrence; Barton Jane; Folkard Simon 《Canadian Metallurgical Quarterly》1995,80(1):43
Although regulations on work hours usually include a minimum weekly rest period, there is little empirical evidence concerning recovery from work. Shift-working nurses (N?=?61) used a handheld computer for 28 days to complete self-ratings, cognitive-performance tasks, and a sleep diary. Many measures were worse on rest days that followed a night shift rather than a day shift and tended to be worse on 1st rest days compared with subsequent rest days. Alertness was lowest on the 1st rest day following a night shift. Social satisfaction was better on workdays that were preceded by 2 rather than 1 rest day. Reaction time (RT) decreased over consecutive night shifts and tended to increase over rest days following night shifts. The results are interpreted as being consistent with the combined adaptive costs of fatigue and adjustment to and from a nocturnal routine. The practical implications for scheduling rest days are considered. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献