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   2810篇
  免费   51篇
  国内免费   2篇
电工技术   23篇
综合类   2篇
化学工业   290篇
金属工艺   39篇
机械仪表   34篇
建筑科学   79篇
矿业工程   6篇
能源动力   61篇
轻工业   263篇
水利工程   20篇
石油天然气   2篇
无线电   233篇
一般工业技术   276篇
冶金工业   1310篇
原子能技术   23篇
自动化技术   202篇
  2023年   12篇
  2022年   16篇
  2021年   21篇
  2020年   10篇
  2019年   16篇
  2018年   25篇
  2017年   27篇
  2016年   37篇
  2015年   29篇
  2014年   23篇
  2013年   82篇
  2012年   51篇
  2011年   100篇
  2010年   63篇
  2009年   68篇
  2008年   90篇
  2007年   92篇
  2006年   97篇
  2005年   76篇
  2004年   55篇
  2003年   54篇
  2002年   68篇
  2001年   60篇
  2000年   35篇
  1999年   74篇
  1998年   365篇
  1997年   224篇
  1996年   158篇
  1995年   92篇
  1994年   79篇
  1993年   81篇
  1992年   38篇
  1991年   34篇
  1990年   33篇
  1989年   42篇
  1988年   28篇
  1987年   33篇
  1986年   26篇
  1985年   25篇
  1984年   26篇
  1983年   21篇
  1982年   26篇
  1981年   12篇
  1980年   24篇
  1979年   12篇
  1978年   13篇
  1977年   36篇
  1976年   78篇
  1975年   12篇
  1973年   10篇
排序方式: 共有2863条查询结果,搜索用时 15 毫秒
41.
A series of ergot alkaloids, together with the DA agonists apomorphine and piribedil, were tested for protective effects against audiogenic seizures in an inbred strain of mice (DBA/2) and for induction of circling behaviour in mice with unilateral destruction of one nigrostriatal DA pathway. The order of potency against audiogenic seizures was apomorphine greater than ergocornine greater than bromocryptine greater than ergometrine greater than LSD greater than methysergide greater than piribedil while that observed in the rotating mouse model was apomorphine greater than ergometrine greater than ergocornine greater than bromocryptine greater than piribedil. LSD caused only weak circling behaviour even when administered in high doses (greater than 1 mg/kg). Methysergide was ineffective. Prior administration of the neuroleptic agent haloperidol blocked the effect of DA agonists and of ergot alkaloids in both animal models. The possible action of ergot alkaloids as DA agonists is discussed.  相似文献   
42.
Experimental studies on the aetiology of acute scorpion pancreatitis   总被引:1,自引:0,他引:1  
This study examined the action of the venom of the scorpion Tityus trintatis on the pancreas and gastrointestinal tract in anaesthetized dogs, on the isolated extracorporeal haemoperfused canine pancreas and on the isolated canine sphincter of Oddi. The venom induces exocrine secretion in both the isolated and intact pancreas and causes contraction of the isolated sphincter of Oddi. These results are discussed in relation to the pathogenesis of acute scorpion pancreatitis and possibly of some other forms of acute pancreatitis.  相似文献   
43.
Cytogenetic studies were done on 18 patients with myelofibrosis or the closely related syndrome, undifferentiated myeloproliferative disorder (MPD). Clones of cells with chromosome abnormalities were demonstrated in the blood of eight individuals, including two with a history of radiation therapy and two with "acute myelofibrosis". Trisomy 8 was present in the latter two patients, but otherwise, there was no consistent cytogenetic pattern or correlation with specific hematologic findings. Sixteen of these patients have been followed for more than 1 year or until death; none has progressed to leukemia. The results indicate that chromosome abnormalities are relatively common in this disorder, but as with polycythemia vera, and unlike some other "preleukemic" states, the aberrant clones in myelofibrosis do not appear to indicate that clinical leukemia is imminent.  相似文献   
44.
During blue-green algal blooms, other algae can be completely suppressed. This ability of blue-green algae to suppress other algae may be determined by the abailability of iron. Iron deprivation induces the production of hydroxamate chelators, which appear to be the agent suppressing other algae.  相似文献   
45.
Individually interviewed 50 male undergraduates for 20 min about procedures used in confronting decision-making situations. The interviewer disclosed experiences, attitudes, and feelings similar to those revealed by the students 0, 2, 4, 8, or 12 times during the interviews. An intermediate number of interviewer self-disclosures resulted in significantly more students returning for a 2nd interview than did few or many self-disclosures. Results suggest that the use of a moderate number of interviewer self-disclosures may be an effective means of increasing counselor attractiveness and client approach responses to the counselor. (16 ref.) (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
46.
The objective of this study was to determine the effect of temperature on whole milk density measured at four different temperatures: 5, 10, 15, and 20 °C. A total of ninety-three individual milk samples were collected from morning milking of thirty-two Holstein Friesian dairy cows, of national average genetic merit, once every two weeks over a period of 4 weeks and were assessed by Fourier transform infrared spectroscopy for milk composition analysis. Density of the milk was evaluated using two different analytical methods: a portable density meter DMA35 and a standard desktop model DMA4500M (Anton Paar GmbH, UK). Milk density was analysed with a linear mixed model with the fixed effects of sampling period, temperature and analysis method; triple interaction of sampling period x analysis method x temperature; and the random effect of cow to account for repeated measures. The effect of temperature on milk density (ρ) was also evaluated including temperature (t) as covariate with linear and quadratic effects within each analytic method. The regression equation describing the curvature and density–temperature relationship for the DMA35 instrument was ρ = 1.0338−0.00017T−0.0000122T2 (R2 = 0.64), while it was ρ = 1.0334 + 0.000057T−0.00001T2 (R2 = 0.61) for DMA4500 instrument. The mean density determined with DMA4500 at 5 °C was 1.0334 g cm−3, with corresponding figures of 1.0330, 1.0320 and 1.0305 g cm−3 at 10, 15 and 20 °C, respectively. The milk density values obtained in this study at specific temperatures will help to address any bias in weight–volume calculations and thus may also improve the financial and operational control for the dairy processors in Ireland and internationally.  相似文献   
47.
Identifying the sources and impacts of organic and inorganic contaminants at the watershed scale is a complex challenge because of the multitude of processes occurring in time and space. Investigation of geochemical transformations requires a systematic evaluation of hydrologic, landscape, and anthropogenic factors. The 1160 km2 Boulder Creek Watershed in the Colorado Front Range encompasses a gradient of geology, ecotypes, climate, and urbanization. Streamflow originates primarily as snowmelt and shows substantial annual variation. Water samples were collected along a 70-km transect during spring-runoff and base-flow conditions, and analyzed for major elements, trace elements, bulk organics, organic wastewater contaminants (OWCs), and pesticides. Major-element and trace-element concentrations were low in the headwaters, increased through the urban corridor, and had a step increase downstream from the first major wastewater treatment plant (WWTP). Boron, gadolinium, and lithium were useful inorganic tracers of anthropogenic inputs. Effluent from the WWTP accounted for as much as 75% of the flow in Boulder Creek and was the largest chemical input. Under both hydrological conditions, OWCs and pesticides were detected in Boulder Creek downstream from the WWTP outfall as well as in the headwater region, and loads of anthropogenic-derived contaminants increased as basin population density increased. This report documents a suite of potential endocrine-disrupting chemicals in a reach of stream with native fish populations showing indication of endocrine disruption.  相似文献   
48.
ABSTRACT Polyphenol oxidase (PPO) was isolated from Bramley's Seedling apples with 75.7‐fold purification and 26.5% recovery by ammonium sulfate precipitation, phenyl sepharose chromatography, ion exchange chromatography, and hydroxyapatite chromatography. Molecular weight was estimated to be about 45 kDa by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis (SDS PAGE). Optimum PPO activity was at pH 6.5 and greater than 50% activity was retained during storage for 72 h at pH 5.5 to 6.5. Optimum temperature for activity was 30 °C and the enzyme had residual activity of greater than 50% during storage for 72 h at 20 °C to 30 °C and for 24 h at 40 °C to 50 °C. Of the substrates tested, activity was greatest with 4‐methylcatechol followed by catechol, pyrogallol, and (?)epicatechin. The most effective inhibitors tested were sodium metabisulfite and ascorbic acid.  相似文献   
49.
Enrichment of milk fat with n-3 fatty acids, in particular eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), may be advantageous because of their beneficial effects on human health. In addition, these fatty acids play an important role in reproductive processes in dairy cows. Our objective was to evaluate the protection of EPA and DHA against rumen biohydrogenation provided by Ca salts of fish oil. Four Holstein cows were assigned in a Latin square design to the following treatments: 1) ruminal infusion of Ca salts of fish oil and palm fatty acid distillate low dose (CaFO-1), 2) ruminal infusion of Ca salts of fish oil and palm fatty acid distillate high dose (CaFO-2), 3) ruminal infusion of fish oil high dose (RFO), and 4) abomasal infusion of fish oil high dose (AFO). The high dose of fish oil provided ∼16 and ∼21 g/d of EPA and DHA, respectively, whereas the low dose (CaFO-1) provided 50% of these amounts. A 10-d pretreatment period was used as a baseline, followed by 9-d treatment periods with interceding intervals of 10 d. Supplements were infused every 6 h, milk samples were taken the last 3 d, and plasma samples were collected the last day of baseline and treatment periods. Milk fat content of EPA and DHA were 5 to 6 times greater with AFO, but did not differ among other treatments. Milk and milk protein yield were unaffected by treatment, but milk fat yield and DM intake were reduced by 20 and 15%, respectively, by RFO. Overall, results indicate rumen biohydrogenation of long chain n-3 fatty acids was extensive, averaging >85% for EPA and >75% for DHA for the Ca salts and unprotected fish oil supplements. Thus, Ca salts of fish oil offered no protection against the biohydrogenation of EPA and DHA beyond that observed with unprotected fish oil; however, the Ca salts did provide rumen inertness by preventing the negative effects on DM intake and milk fat yield observed with unprotected fish oil.  相似文献   
50.
Sixteen multiparous cows, including eight rumen fistulated cows, were used in a 4x4 Latin square experiment designed to study dietary effects on rumen and blood parameters and milk production in cows differing in genetic capacity for milk fat content. Diets contained forage to concentrate ratios of 50:50 or 30:70 with either grass hay or silage as the forage. Ruminal fermentation was characterized by a high molar percentage of butyrate, 14 to 17%. Forage to concentrate ratio affected most rumen parameters, with the exception of the molar percentage of propionate (18 to 19%). The silage had a higher fiber degradation rate compared with hay. Compared to hay diets, silage diets had higher ruminal outflow rates, lower acetate:propionate ratios, and greater milk production with no differences in milk composition. Cows selected for low milk fat had higher molar percentages of propionate in the rumen. The low milk fat cows had higher milk production than cows selected for high milk fat but did not differ in milk fat yield. Cows fed the 30:70 diets had higher plasma insulin concentrations in response to a glucose challenge. The low milk fat cows had lower basal concentrations of insulin and lower insulin responses to a glucose challenge. Small changes in nutrient metabolism and supply were sufficient to influence milk production.  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