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   987篇
  免费   56篇
电工技术   2篇
化学工业   247篇
金属工艺   4篇
机械仪表   17篇
建筑科学   67篇
矿业工程   3篇
能源动力   22篇
轻工业   157篇
水利工程   7篇
石油天然气   3篇
无线电   37篇
一般工业技术   129篇
冶金工业   232篇
原子能技术   4篇
自动化技术   112篇
  2023年   14篇
  2022年   39篇
  2021年   75篇
  2020年   26篇
  2019年   28篇
  2018年   30篇
  2017年   37篇
  2016年   35篇
  2015年   18篇
  2014年   30篇
  2013年   58篇
  2012年   55篇
  2011年   54篇
  2010年   53篇
  2009年   50篇
  2008年   60篇
  2007年   35篇
  2006年   23篇
  2005年   23篇
  2004年   22篇
  2003年   24篇
  2002年   26篇
  2001年   12篇
  2000年   8篇
  1999年   11篇
  1998年   12篇
  1997年   8篇
  1996年   16篇
  1995年   13篇
  1994年   12篇
  1993年   8篇
  1992年   13篇
  1991年   5篇
  1990年   10篇
  1989年   8篇
  1988年   7篇
  1987年   9篇
  1986年   7篇
  1985年   8篇
  1984年   8篇
  1983年   7篇
  1982年   4篇
  1981年   5篇
  1979年   3篇
  1978年   6篇
  1977年   3篇
  1976年   3篇
  1974年   3篇
  1970年   3篇
  1964年   2篇
排序方式: 共有1043条查询结果,搜索用时 78 毫秒
51.
52.
The isolation and purification of polyphenol oxidase from purple eggplant (Solanum melongena L.) is described. A 15-fold purified preparation has been obtained with 15% yield by a procedure involving (NH4)2SO4 precipitation, DEAE cellulose chromatography and Sephadex G 100 gel filtration.The enzyme has an optimum pH of 6·4 and a molecular weight of 79 000 ± 5000.Inhibition studies with sodium diethyl dithiocarbamate and potassium cyanide and dialysis against the latter show that the enzyme requires copper.Eggplant polyphenol oxidase exhibits only a catecholase activity and presents a great affinity for catechol and caffeic acid. This latter compound is no doubt its natural substrate.These results show that conversely to tomato, where a polyphenol oxidase activity was tightly bound to the peroxidase, in the case of eggplant there is a true polyphenol oxidase enzyme.  相似文献   
53.
Pharmaceuticals are designed and used because of their specific biological effects. Over the past decade, compounds from various classes of pharmaceuticals have been detected in the environment. Concern has grown about the adverse effects pharmaceuticals in the environment might potentially have on human and ecological health. A sound risk assessment is therefore urgently needed for pharmaceuticals. Standardized tests for assessing the effects of chemicals on environmental organisms are widely used for this purpose. However it is questionable whether classical standardized tests give reliable data needed for environmental risk assessment. In this study we investigated the suitability of the respiration inhibition test OECD 209 for the assessment of the effects of antibiotics, disinfectants and cytotoxics on sewage sludge bacteria. We found that inhibition concentrations can strongly depend on the test period and the type of compound. We conclude that tests to assess the effects of pharmaceuticals on environmental organisms such as bacteria have to be evaluated before their results can be used in environmental risk assessment.  相似文献   
54.
Capillary electrophoresis (CE) was coupled to ICPMS in order to combine the good performance of this separation technique with the high sensitivity of the ICPMS for the analysis of plutonium and neptunium oxidation states. The combination of a fused-silica capillary with a MicroMist AR 30-I-FM02 nebulizer and a Cinnabar small-volume cyclonic spray chamber yielded the best separation results. With this setup, it was possible to separate a model element mixture containing neptunium (NpO2(+)), uranium (UO2(2+)), lanthanum (La3+), and thorium (Th4+) in 1 M acetic acid. The same conditions were also suitable for the separation of various oxidation states of plutonium and neptunium in different aqueous samples. All separations were obtained within less than 15 min. A detection limit of 50 ppb identical with 2 x 10(-7) M (3-fold standard deviation of a blank) was achieved. To prove the negligible disturbance of the plutonium and neptunium redox equilibria during the CE separations, plutonium and neptunium speciation by CE-ICPMS in acidic solutions was compared with the results of UV/visible absorption spectroscopy and was found to be in good agreement. The CE-ICPMS system was also applied to study the reduction of Pu(VI) in a humic acid-containing groundwater at different pH values.  相似文献   
55.
This study compared patterns of self-disclosure in psychotherapy and marriage. Participants (48 married, current psychotherapy patients, mean age = 42) completed a 101-item Disclosure to Therapist Inventory and a companion measure, a 101-item Disclosure to Spouse Inventory. Results indicated a pattern of greater disclosure to one's therapist in regard to issues involving despair (e.g., feelings of depression), and to one's spouse in regard to procreation and body concerns (e.g., birth control) and values (e.g., feelings about religion, race, or politics). Issues involving sex were infrequently discussed in either context. Discrepancy scores (differences between extent of disclosure and perceived importance) were greater in the spouse condition. Outcome predictors varied by situation, with overall disclosure predicting therapeutic outcome and both overall disclosure and discrepancy scores predicting marital satisfaction. Findings suggest that although there is substantial overlap in issues discussed in these two contexts, certain intimate disclosures are perceived as situation-specific. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
56.
Less is known about depression in children than in adults. This study integrates fields by combining cognitive and interpersonal research investigating childhood depression symptoms through the use of a genetic framework. Three research questions are addressed. First, what are the associations among interpersonal cognitions, anxiety, and depression? Second, what are the relative magnitudes of genetic and environmental influences on interpersonal cognitions? Third, to what extent do genetic and environmental influences explain associations between interpersonal cognitions and depression? Three hundred pairs of 8-year-old twins reported on symptoms of depression and anxiety by completing the Children's Depression Inventory and the Screen for Childhood Anxiety-Related Emotional Disorders. The authors examined interpersonal cognitions with the Children's Expectation of Social Behaviors and the Perceptions of Peers and Self Questionnaires. Interpersonal cognitions were more strongly correlated with depression (mean r = .35) than with anxiety (mean r = .13). Genetic influence on interpersonal cognitions was small (M = 3%), and associations between interpersonal cognitions and depression were mainly explained by environmental influences. These latter findings may result from interpersonal cognitions in young children, reflecting life experiences as opposed to trait-like cognitive biases. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
57.
Childhood depressive conditions have been explored from multiple theoretical approaches but with few empirical attempts to address the interrelationships among these different domains and their combined effects. In the present study, the authors examined different pathways through which social, cognitive, and genetic risk factors may be expressed to influence depressive symptoms in 300 pairs of child twins from a longitudinal study. Path analysis supported several indirect routes. First, risks associated with living in a step- or single-parent family and punitive parenting did not directly influence depressive outcome but were instead mediated through maternal depressive symptoms and child negative attributional style. Second, the effects of negative attributional style on depressive outcome were greatly exacerbated in the presence of precipitating negative life events. Third, independent of these social and cognitive risk mechanisms, modest genetic effects were also implicated in symptoms, with some indication that these risks are expressed through exposure to negative stressors. Together, these routes accounted for approximately 13% of total phenotypic variance in depressive symptoms. Theoretical and analytical implications of these results are discussed in the context of several design-related caveats. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
58.
What unique assessment issues do evaluators face when conducting mental retardation evaluations in capital cases? This article describes 4 assessment issues and reports how 20 evaluators in 1 state have approached them: (a) What methods are appropriate for evaluating adaptive functioning in this context? (b) Should information about criminal behavior be used as evidence of adaptive functioning? (c) Should correctional officers serve as informants regarding adaptive behavior? and (d) How should the Flynn effect influence the interpretation of intelligence test scores? Implications for practice are that practitioners should expect to be asked about the relation between criminal behavior and adaptive functioning, must think carefully about the validity of tests of adaptive functioning for inmates, and must be knowledgeable about the current state of Flynn effect research. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
59.
Water-leaving reflectance (ρw) data from the European Space Agency ocean colour sensor Medium Resolution Imaging Spectrometer (MERIS) was validated with in situ ρw between October 2008 and November 2011, off Sagres on the southwest coast of the Iberian Peninsula. The study area is exceptional, since Stations A, B, and C at 2, 10, and 18 km offshore are in optically deep waters at approximately 40, 100, and 160 m, respectively. These stations showed consistently similar bio-optical properties, characteristic of Case 1 waters, enabling the evaluation of adjacency effects independent of the usual co-varying inputs of coastal waters. Using the third reprocessing of MERIS with the standard MEGS 8.1 processor, four different combinations of procedures were tested to improve the calibration between MERIS products and in situ data. These combinations included no vicarious adjustment (NoVIC), vicarious adjustment (VIC), and, for mitigating the effects of land adjacency on MERIS ρw, the improved contrast between ocean and land (ICOL) processor (version 2.7.4) and VIC + ICOL. Out of approximately 130 potential matchups for each station, 38–77%, 74–86%, and 88–90% were achieved at Stations A, B, and C, respectively, depending on which of the four combinations were used. Analyses of ρw comparing these various procedures, including statistics, scatter plots, histograms, and MERIS full-resolution images, showed that the VIC procedure compared with NoVIC produced minimal changes to the calibration. For example, at the oceanic Station C, the regression slope was closer to unity at all wavelengths with NoVIC compared to VIC, whereas, with the exception of wavelengths 412 and 443 nm, the intercept, mean ratio (MR), absolute percentage difference (APD), and relative percentage difference (RPD) were better with NoVIC. The differences for MR and APD indicate that there was marginal improvement for these two bands with VIC, and an over-adjustment with RPD. ICOL also showed inconsistent results for improving the retrieval of the near-shore conditions, but under some conditions, such as ρw at wavelength 560 nm, the improvement was striking. VIC + ICOL showed results intermediate between those of VIC and ICOL implemented separately. In relation to other validation sites, the offshore Station C at Sagres had much in common with the Mediterranean deep water, BOUSSOLE buoy, although the matchup statistics between MERIS ρw and in situ ρw were much better for Sagres than for BOUSSOLE. Strikingly, the matchup statistics for ρw at Sagres were very similar to those for the Acqua Alta Oceanographic Tower (AAOT), where the AAOT showed more scatter at 412 nm, probably because of the atmospheric correction where the aerosol optical thickness is higher at the AAOT. Conversely, Sagres showed much greater scatter at 665 nm in the red as the values were generally close to the limits of detection owing to the clearer waters at Sagres compared to the more turbid waters at the AAOT.  相似文献   
60.
The aim of this study was to determine if changes in omega‐3 polyunsaturated fatty acid status following tuna oil supplementation correlated with changes in scores of depression. A total of 95 volunteers receiving treatment for major depression were randomised to consume 8 × 1 g capsules per day of HiDHA (2 g DHA, 0.6 g EPA and 10 mg Vitamin E) or olive oil (placebo) for 16 weeks, whilst undergoing weekly counseling sessions by trained clinical psychologists using a standard empirically validated psychotherapy. Depression status was assessed using the 17 item Hamilton rating scale for depression and the Beck Depression Inventory by a psychodiagnostician who was blind to the treatment. Blood was taken at baseline and 16 weeks (n = 48) for measurement of erythrocyte fatty acids. With HiDHA supplementation, erythrocyte DHA content rose from 4.1 ± 0.2 to 7.9 ± 0.4 % (mean ± SEM, p < 0.001) of total fatty acids but did not change (4.0 ± 0.2 to 4.1 ± 0.2 %) in the olive oil group. The mean changes in scores of depression did not differ significantly between the two groups (?12.2 ± 2.1 for tuna oil and ?14.4 ± 2.3 for olive oil). However, analysis of covariance showed that in the fish oil group there was a significant correlation (r = ?0.51) between the change in erythrocyte DHA and the change in scores of depression (p < 0.05). Further study of the relationship between DHA and depression is warranted.  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