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   4173篇
  免费   126篇
  国内免费   88篇
电工技术   41篇
综合类   123篇
化学工业   191篇
金属工艺   120篇
机械仪表   80篇
建筑科学   256篇
矿业工程   31篇
能源动力   29篇
轻工业   131篇
水利工程   38篇
石油天然气   44篇
武器工业   24篇
无线电   174篇
一般工业技术   342篇
冶金工业   2523篇
原子能技术   37篇
自动化技术   203篇
  2023年   24篇
  2022年   49篇
  2021年   56篇
  2020年   40篇
  2019年   49篇
  2018年   56篇
  2017年   44篇
  2016年   28篇
  2015年   61篇
  2014年   57篇
  2013年   110篇
  2012年   95篇
  2011年   97篇
  2010年   97篇
  2009年   101篇
  2008年   87篇
  2007年   111篇
  2006年   118篇
  2005年   83篇
  2004年   68篇
  2003年   67篇
  2002年   60篇
  2001年   40篇
  2000年   38篇
  1999年   108篇
  1998年   721篇
  1997年   445篇
  1996年   293篇
  1995年   176篇
  1994年   156篇
  1993年   194篇
  1992年   36篇
  1991年   34篇
  1990年   48篇
  1989年   48篇
  1988年   32篇
  1987年   32篇
  1986年   28篇
  1985年   29篇
  1984年   9篇
  1983年   10篇
  1982年   21篇
  1981年   14篇
  1980年   22篇
  1978年   15篇
  1977年   69篇
  1976年   145篇
  1975年   9篇
  1974年   6篇
  1966年   6篇
排序方式: 共有4387条查询结果,搜索用时 640 毫秒
991.
992.
Utilization of psychiatric in-patient care among 537 new patients was studied in the Department of Psychiatry in Oulu, Finland, during a 3-year follow-up period. Hospitalization during the second and third years of the follow-up was predicted by hospitalization and number of emergency out-patient contacts during the first year of the study, diagnosis of functional psychosis or personality disorder, and previous in-patient care. In total, 5% of the cohort fulfilled our criteria for 'revolving-door' patients. The 'revolving-door' phenomenon was associated with in-patient care at the first contact with the psychiatric services and diagnosis of psychosis or personality disorder. In total, 2% of the cohort became long-stay hospital patients, and this was predicted by psychosis diagnosis. The clinical implications of these findings are that increased attention should be paid to the first assessment of new patients and to the interaction between psychiatric services and patients during the first year of care.  相似文献   
993.
Using functional analysis results to prescribe treatments is the preferred method for developing behavioral interventions. Little is known, however, about the reliability and validity of visual inspection for the interpretation of functional analysis data. The purpose of this investigation was to develop a set of structured criteria for visual inspection of multielement functional analyses that, when applied correctly, would increase interrater agreement and agreement with interpretations reached by expert consensus. In Study 1, 3 predoctoral interns interpreted functional analysis graphs, and interrater agreement was low (M = .46). In Study 2, 64 functional analysis graphs were interpreted by a panel of experts, and then a set of structured criteria were developed that yielded interpretive results similar to those of the panel (exact agreement = .94). In Study 3, the 3 predoctoral interns from Study 1 were trained to use the structured criteria, and the mean interrater agreement coefficient increased to .81. The results suggest that (a) the interpretation of functional analysis data may be less reliable than is generally assumed, (b) decision-making rules used by experts in the interpretation of functional analysis data can be operationalized, and (c) individuals can be trained to apply these rules accurately to increase interrater agreement. Potential uses of the criteria are discussed.  相似文献   
994.
OBJECTIVE: To identify the independent and differential diagnostic and symptom correlates of suicidal ideation and suicide attempts and determine whether there are gender- and age-specific diagnostic profiles. METHOD: The relationships between suicidal ideation, suicide attempts, and psychiatric disorders were examined among 1,285 randomly selected children and adolescents, aged 9 to 17 years, of whom 42 had attempted suicide and 67 had expressed suicidal ideation only. Youths and their parents were interviewed as part of the Methods for the Epidemiology of Child and Adolescent Mental Disorders (MECA) Study, using the Diagnostic Interview Schedule for Children Version 2.3 (DISC-2.3). RESULTS: Logistic regression analyses indicated that mood, anxiety, and substance abuse/dependence disorders independently increased the risk of suicide attempts, after controlling for sociodemographic characteristics. There was no significant independent contribution of disruptive disorders to suicide attempts, although its association with suicidal ideation was significant. Substance abuse/dependence independently differentiated suicide attempters from ideators. Noncriterion symptoms that remained significant predictors of suicide risk, after adjusting for psychiatric disorder, included panic attacks and aggressiveness. Perfectionism did not significantly increase suicide risk after adjusting for psychiatric disorder. The association of specific disorders and noncriterion symptoms with suicidality varied as a function of gender and age. CONCLUSION: A monolithic diagnostic risk profile for suicidality, ignoring gender- and age-specific risks, is inadequate. The contribution of substance abuse/dependence in the escalation from suicidal thoughts to suicide attempts is underscored.  相似文献   
995.
Familial aggregation of speech and language disorders was examined as a basis of subgrouping children with phonologic disorders. Fifty-nine children with phonologic disorders were subgrouped according to whether or not other nuclear family members reported a history of speech/language disorders. Thirty-four subjects (58%) reported at least one other nuclear family member affected and 25 subjects (42%) reported no other nuclear family members affected. Groups were compared on measures of articulation, phonology, language, and oral motor skills to determine if the familial phonologic subgroup presented a unique profile of speech and language deficits. Significant group differences were not observed. However, children with positive nuclear family histories tended to perform more poorly than children without histories on all tasks, although not reaching significance. Although all parents were considered to have achieved normal adult articulation, parents of children with positive family histories also tended to perform more poorly than parents of children with negative histories. Results suggested that poorer oral motor coordination and productive phonology may distinguish individuals with familial phonologic disorders from individuals with phonologic disorders of unknown origin.  相似文献   
996.
997.
To investigate the role of various lipoproteins in plasma to promote cholesterol efflux from cell membranes, potencies of lipoproteins in normolipidemic fasting and postprandial (PP) plasmas to accept additional cholesterol molecules from cell membranes were determined. We used red blood cells (RBCs) and lipoproteins in fresh blood as donors and acceptors of cell membrane cholesterol, respectively. When fresh fasting plasma (n=24) containing active lecithin:cholesterol acyltransferase (LCAT) and cholesteryl ester transfer proteins (CETP) was incubated with a 3-fold excess of autologous RBCs at 37 degrees C for 18 hours, plasma cholesterol levels increased by 19.6% (38.5+/-14.2 mg/dL) owing to an exclusive increase in the CE level. Very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) fractions retained 48.1%, 26.3%, and 25.6% of the net cholesterol mass increase in fasting plasma, resulting in 91%, 8%, and 21% increases in their cholesterol contents, respectively. The PP plasma was 1.3-fold more potent than fasting plasma in promoting cholesterol efflux from RBCs by associating excess cholesterol with chylomicrons, resulting in a 356% increase in the cholesterol content of chylomicrons. These increases in lipoprotein cholesterol content indicate that chylomicrons were about 3.9x, 44x, and 17x more potent than fasting VLDL, LDL, and HDL, respectively, in accepting additional cholesterol molecules released from RBCs. The capacity of PP plasma to promote cholesterol efflux from RBCs was significantly correlated with plasma cholesterol levels (r=0.60, P<0.005), triglycerides (r=0.68, P<0.001), chylomicrons (r=0.90, P<0.001), VLDL (r=0.65, P<0.001), and LDL (r=0.47, P<0.025) but not with the levels of HDL (r= -0.34, P<0.20). In fasting plasma containing a low level of VLDL and HDL, isolated chylomicrons supplemented to the plasma were approximately 9x more potent than HDL in boosting the capacity of plasma to promote cholesterol efflux from RBCs. This study indicates that chylomicrons in PP plasma are the most potent ultimate acceptors of cholesterol released from cell membranes and that a low HDL level is not a factor that limits the ability of PP plasma to promote cholesterol efflux from cell membranes. Our data obtained from an in-vitro system suggest that PP chylomicrons may play a major role in promoting reverse cholesterol transport in vivo, since the transfer of cholesterol from cell membranes to chylomicrons will lead to the rapid removal of this cholesterol by the liver. HDL in vivo may promote reverse cholesterol transport by enhancing the rapid removal of chylomicrons from the circulation, since the rate of clearance of chylomicrons is positively correlated with the HDL level in plasma.  相似文献   
998.
999.
To further our knowledge of the structure-function relationship and mechanism of action of the Clostridium perfringens enterotoxin (CPE), a series of recombinant CPE (rCPE) species containing N- and C-terminal CPE deletion fragments was constructed by recombinant DNA approaches. Each rCPE species was characterized for its ability to complete the first four early steps in the action of CPE, putatively ordered as specific binding, a postbinding physical change to bound CPE, large-complex formation, and induction of alterations in small-molecule membrane permeability. These studies demonstrated that (i) at least 44 amino acids can be removed from the N terminus of CPE without loss of cytotoxicity, (ii) removal of the first 53 amino acids from the N terminus of CPE produces a fragment that appears to be noncytotoxic because it cannot undergo the post-binding physical change step in CPE action, (iii) removal of as few as five amino acids from the C terminus of CPE produces a noncytotoxic fragment lacking receptor binding activity, and (iv) a fragment lacking the first 44 N-terminal amino acids of native CPE formed twice as much large complex and was twice as cytotoxic as native CPE. From these structure-function results, it appears that the minimum-size cytotoxic CPE fragment comprises approximately residues 45 to 319 of native CPE. Results from these deletion fragment studies have also contributed to our understanding of CPE action by (i) independently supporting previous suggestions that binding, the postbinding physical change step, and large-complex formation represent important steps in CPE cytotoxicity and (ii) providing independent evidence confirming the putative sequential order of these early events in CPE action.  相似文献   
1000.
OBJECTIVE: To assess the educational value of a practice-based ambulatory program used at a school of veterinary medicine. DESIGN: Retrospective cohort study. SAMPLE POPULATION: Graduates of US veterinary medical schools between 1987 and 1994. PROCEDURE: Phase I involved use of interviews and focus groups to assist in development of the questionnaire used in phase II, a retrospective cohort study. The pretested questionnaire was sent to a study population consisting of all graduates of the University of Wisconsin-Madison, School of Veterinary Medicine, between 1987 and 1994 as well as a control group who were randomly selected from the 1994 AVMA list of veterinarians. Control-group veterinarians were matched on the basis of professional activity, region, and year of graduation. RESULTS: 728 of 1,067 veterinarians completed the questionnaire in phase II of the study (response rate, 68%). The practice-based ambulatory program at the University of Wisconsin-Madison compared favorably with university-based ambulatory programs in volume of experiences and perceived educational quality. Regardless of rotation type, female students were significantly less likely to observe or perform 12 specific clinical procedures and were significantly less likely to rate instructional quality as excellent or very good, compared with male students. CLINICAL IMPLICATIONS: Practice-based ambulatory rotations can be a good alternative to existing university-based ambulatory rotations. Implementation of these programs should emphasize performance of procedures while striving to ensure participation of female students.  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