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   1754篇
  免费   22篇
  国内免费   2篇
电工技术   7篇
化学工业   149篇
金属工艺   17篇
机械仪表   17篇
建筑科学   47篇
矿业工程   2篇
能源动力   17篇
轻工业   140篇
水利工程   3篇
无线电   82篇
一般工业技术   141篇
冶金工业   1053篇
原子能技术   9篇
自动化技术   94篇
  2022年   7篇
  2021年   11篇
  2020年   6篇
  2018年   6篇
  2017年   10篇
  2016年   9篇
  2015年   13篇
  2014年   19篇
  2013年   25篇
  2012年   25篇
  2011年   29篇
  2010年   34篇
  2009年   25篇
  2008年   26篇
  2007年   29篇
  2006年   30篇
  2005年   22篇
  2004年   23篇
  2003年   26篇
  2002年   19篇
  2001年   23篇
  2000年   20篇
  1999年   49篇
  1998年   321篇
  1997年   197篇
  1996年   123篇
  1995年   90篇
  1994年   60篇
  1993年   56篇
  1992年   15篇
  1991年   11篇
  1990年   13篇
  1989年   14篇
  1988年   16篇
  1987年   17篇
  1986年   22篇
  1985年   20篇
  1984年   8篇
  1983年   9篇
  1982年   12篇
  1981年   19篇
  1980年   17篇
  1979年   7篇
  1978年   10篇
  1977年   27篇
  1976年   75篇
  1974年   8篇
  1971年   9篇
  1970年   14篇
  1966年   10篇
排序方式: 共有1778条查询结果,搜索用时 0 毫秒
51.
Columnar liquid crystals (LCs) are found to spontaneously form homeotropically‐aligned films when deposited on surfaces fabricated via friction transfer of polytetrafluoroethylene. The inside cover schematically shows the film structure of a phthalocyanine derivative together with the corresponding X‐ray diffraction pattern (inset). The results, reported on p. 815 by Dmitri Ivanov and co‐workers, indicate that not only are the columns homeotropically oriented but also that they form a single monodomain of macroscopic size. These findings can have an important impact for fabrication of LC‐based organic solar cells.  相似文献   
52.
Etching of Al is studied in pure BCl3 as well as in mixtures with other gases in the reactive sputter etching mode in a cryopumped system. Etch rate, selectivity with respect to positive photoresist, SiO2 and Si and etch profiles are investigated as a function of gas composition, gas pressure, flow rate and plasma power. Plasma chemical processes are monitored by quadrupole mass spectroscopy as well as by optical emission spectroscopy. Perfectly square Al-profiles can be etched if etch rates are kept below 1000 A/min. Al-patterns running over steep steps can also be clearly defined if a certain amount of overetching can be tolerated. The experimental data indicate that the etch process is reactant supply limited. Anisotropic etching is achieved by either a ‘surface inhibitor mechanism’ or the formation of a sidewall protecting film.  相似文献   
53.
This exploratory study describes the nature and magnitude of the problem of health referrals, health-seeking behavior, perceptions, and knowledge at the district level in Zimbabwe. Data were obtained from focus groups with 159 persons in Tsholotsho and 132 persons in Murewa; from discussions with health personnel from the 6 health centers in Murewa and the 2 rural hospitals in Tsholotsho; and from records among a systematic sample of 400 new outpatients during October 1993 and March 1994 in Murewa district. Findings indicate that 71.8% in outpatient departments at Murewa Hospital had no access to a health center. 24.3% by-passed the health center for treatment at the hospital. 3.8% were referred by health centers. The absolute number of referrals did not change during 1991-93. However, the number directly accessing services from outside the district rose. Focus group participants reported their intention to use the nearest clinic for an illness. In Tsholotsho, people initially used the village community worker/headman. If illness was perceived as serious, patients would go to a hospital. For minor illness, people used traditional herbal remedies. If illness did not change after remedies, the clinic was consulted. Some illnesses were perceived as outside the realm of medicine. Most distinguished between a health center and a hospital, but were unaware of the important, superior functions of the health center. Most did not understand the logic behind the referral system, but appreciated referrals and not the cost of hospital treatment or transportation. The community was unaware of Ward Health Teams. Many did not understand the new fee policy introduced in 1994.  相似文献   
54.
A 2 x 3 factorial arrangement of treatments was used to elucidate the mechanism(s) by which prenatal androgenization improves postnatal rate and efficiency of growth and composition of gain in beef heifers. Fifteen control (C) and 15 prenatally androgenized (PA) Angus x Simmental heifers (prenatal treatment, Pretrt) received no (N), estrogen (E), or estrogen and testosterone (ET) implants postnatally (postnatal treatment, Posttrt) to evaluate whether the postpubertal growth response after prenatal androgenization could be induced in prepubertal heifers. Blood was collected from the heifers at 6 +/- 1, 9 +/- 1, and 12 +/- 1 mo of age and analyzed from serum concentrations of growth hormone (GH), IGF-I, IGF-II, insulin, thyroxine (T4), and triiodothyronine (T3). Season of the year had a greater effect on hormone concentrations than either Pretrt or Posttrt, and there were no Pretrt x Posttrt interactions. Prenatal treatment, PA, had no effect on GH; however, Posttrt E and ET increased (P < .001) GH concentrations. Prenatal treatment, PA, increased (P < .05) IGF-I concentrations, and there was a nonsignificant increase (P = .11) in IGF-I concentrations with Posttrt E and ET. Concentrations of IGF-II were unaffected by Pretrt PA; however, they were lower (P < .01) in the Posttrt E and ET groups. Insulin, T4, T3, BW, and ADG were not affected by Pretrt and Posttrt. Concentrations of GH and IGF-I were increased in heifers that received Pretrt PA and(or) Posttrt E and ET in a manner to support improved growth performance; however, BW and ADG were similar. In prepubertal beef heifers, factors in addition to increased GH and IGF-I seem to be necessary for improved growth performance.  相似文献   
55.
The site of S1-S2 root activation following percutaneous high-voltage electrical (ES) and magnetic stimulation were located by analyzing the variations of the time interval from M to H soleus responses elicited by moving the stimulus point from lumbar to low thoracic levels. ES was effective in activating S1-S2 roots at their origin. However supramaximal motor root stimulation required a dorsoventral montage, the anode being a large, circular surface electrode placed ventrally, midline between the apex of the xiphoid process and the umbilicus. Responses to magnetic stimuli always resulted from the activation of a fraction of the fiber pool, sometimes limited to the low-thresholds afferent component, near its exit from the intervertebral foramina, or even more distally. Normal values for conduction velocity in motor and 1a afferent fibers in the proximal nerve tract are provided.  相似文献   
56.
Fluorescent fatty acid labels have been incorporated into the palmitoylation sites of rhodopsin and used to probe the membrane accessibility and location of these sites. The fluorescence properties of anthroyloxy and pyrenyl fatty acids bound to rhodopsin were investigated in a reconstituted vesicle system. Collisional quenching of fluorescence by stearic acid (DSA) labeled with doxyls in the 16, 12, and 5 positions was used to determine the membrane accessibility and disposition of the modifying fatty acids. To properly determine the membrane concentration of these quenchers, the dependence of the Stern-Volmer parameters on both quencher and vesicle concentration was determined. An analysis of these dependences provided a correction for partitioning of the quencher between the aqueous phase and the membrane. After this correction, the relative effectiveness of doxyl quenchers was 16-DSA > 12-DSA > 5-DSA. Parallel studies on free anthroyloxy and pyrenyl fatty acids incorporated into the reconstituted system showed the same dependence on quencher position. These results indicate that the labels at the palmitoylation sites of rhodopsin are situated in the membrane much as a free fatty acid. This anchoring of the palmitates in the membrane results in the formation of a fourth cytoplasmic loop.  相似文献   
57.
The possibilities of using influenza A (Leningrad) 385/80 (H3N2) virus matrix protein-specific FITC-labeled D8 monoclonal antibodies in immunofluorescence assays were investigated. The virus antigen accumulation was detected in chorioallantoic cells of chick embryos. Exhibiting the type-specific properties, the fluorescent antibodies stain the perinuclear space, cytoplasmic membrane, and granular structures in the cytoplasm of infected cells. The haemagglutination test tires in the corresponding specimens were at least 1:16.  相似文献   
58.
This article describes the long-term consequences of successful cardiovascular disease (CVD) prevention and its influence on premature mortality in Finland, with special reference to North Karelia. Active community-based CVD prevention began in 1972 in the province of North Karelia (population, 180,000). Since 1977, active preventive work has been carried out nationwide, taking advantage of the experience from North Karelia, which continued as a demonstration area for integrated prevention of noncommunicable diseases. Comprehensive community-based interventions as part of WHO interhealth and CINDI programmes in North Karelia and nationwide aimed at changing the target risk factors and health behaviours (serum cholesterol, blood pressure, smoking, diet) at the population level. Age-adjusted mortality rates for CVD, coronary heart disease (CHD), cerebrovascular disease, all cancers, lung cancer, accidents and violence, and all causes in the population aged 35-64 years from the pre-programme period (1969-71) to 1995 were the main measures of the outcome. Among men there was a great reduction in deaths from CHD, CVD, cancer, and all causes in the whole country. From 1969-71 to 1995 the age-standardized CHD mortality (per 100,000) decreased in North Karelia by 73% (from 672 to 185) and nationwide by 65% (from 465 to 165). The reduction in CVD mortality was of the same magnitude. Among men, CHD mortality decreased in the 1970s, as did lung cancer mortality in the 1980s and 1990s, significantly more in North Karelia than in all of Finland. Among women there was a great reduction in CVD (including CHD and stroke) mortality and all-causes mortality, but only a small reduction in cancer mortality. These results show that a major reduction in CVD mortality among the working-age population can take place in association with active reduction of major risk factors, with a favourable impact on cancer and all-causes mortality.  相似文献   
59.
The survival capacity of Helicobacter pylori in artificially contaminated milk and tap water was investigated in the study. Helicobacter pylori could survive for up to 10 days in milk at 4 degrees C storage but only 4 days in tap water with a steady decrease of colony forming units. However, electron microscopy clearly showed that the non-culturable coccoid form was present in tap water which had been kept at 4 degrees C for 7 days. It is concluded that H. pylori may survive in tap water as well as in milk, with the implication that they may, thereby, act as a vehicle of transmission.  相似文献   
60.
Skeletal development of transgenic mice with a type II collagen mutation was analyzed and compared with wild-type littermates. The single base substitution in Col2a1 resulted in a glycine to serine mutation within the helical domain and corresponded to one previously identified in a patient with the lethal human chondrodysplasia, hypochondrogenesis (Horton et al. [1992] Proc. Natl. Acad. Sci. U.S.A. 89:4583-4587). Skeletal staining of embryos from 14.5 through 18.5 days of gestation demonstrated a dwarf phenotype in the transgenic embryos, most notably short limb bones and vertebral column that was first detected at 15.5 days post-coitus. In addition to the reduced length, the extent of ossification was less in the transgenic mice. The architecture of the long bone growth plate was abnormal in the transgenic tissue, in particular there was no discernible proliferative zone. There were few stacks of characteristically flattened cells and the overall length of the growth plate in the mutant embryos was reduced. At the ultrastructural level, there were fewer collagen fibrils present in the transgenic mouse cartilage compared to that of wild-type littermates. Ultrastructural localization of collagen types II, IX and XI revealed a similar pattern between the transgenic and wild-type pups, suggesting that the collagen fibrils present in the matrix of littermates with both phenotypes had a similar composition. Skeletal analysis and cartilage histochemistry indicated that effect of the type II collagen mutation was to reduce the density of the collagen fibrils within the cartilage matrix which was associated with delayed bone formation and resulted in a short-limbed phenotype.  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