全文获取类型
收费全文 | 658380篇 |
免费 | 8524篇 |
国内免费 | 1948篇 |
专业分类
电工技术 | 12226篇 |
综合类 | 615篇 |
化学工业 | 97370篇 |
金属工艺 | 25019篇 |
机械仪表 | 18874篇 |
建筑科学 | 15591篇 |
矿业工程 | 2836篇 |
能源动力 | 17791篇 |
轻工业 | 55846篇 |
水利工程 | 6359篇 |
石油天然气 | 11160篇 |
武器工业 | 43篇 |
无线电 | 77779篇 |
一般工业技术 | 126716篇 |
冶金工业 | 133518篇 |
原子能技术 | 12817篇 |
自动化技术 | 54292篇 |
出版年
2021年 | 5713篇 |
2020年 | 4142篇 |
2019年 | 5417篇 |
2018年 | 9144篇 |
2017年 | 8917篇 |
2016年 | 9312篇 |
2015年 | 6520篇 |
2014年 | 10930篇 |
2013年 | 29726篇 |
2012年 | 17404篇 |
2011年 | 24055篇 |
2010年 | 19075篇 |
2009年 | 21459篇 |
2008年 | 22054篇 |
2007年 | 21763篇 |
2006年 | 19139篇 |
2005年 | 17662篇 |
2004年 | 17067篇 |
2003年 | 16725篇 |
2002年 | 15912篇 |
2001年 | 16153篇 |
2000年 | 15099篇 |
1999年 | 16186篇 |
1998年 | 42398篇 |
1997年 | 29416篇 |
1996年 | 22640篇 |
1995年 | 16911篇 |
1994年 | 14679篇 |
1993年 | 14403篇 |
1992年 | 10230篇 |
1991年 | 9716篇 |
1990年 | 9498篇 |
1989年 | 9159篇 |
1988年 | 8472篇 |
1987年 | 7375篇 |
1986年 | 7280篇 |
1985年 | 8217篇 |
1984年 | 7579篇 |
1983年 | 6706篇 |
1982年 | 6285篇 |
1981年 | 6480篇 |
1980年 | 6052篇 |
1979年 | 5857篇 |
1978年 | 5632篇 |
1977年 | 6842篇 |
1976年 | 9022篇 |
1975年 | 4887篇 |
1974年 | 4605篇 |
1973年 | 4658篇 |
1972年 | 3844篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
OBJECTIVE: To describe an outbreak of hepatitis A virus (HAV) infection that occurred in Melbourne in 1991. DESIGN: (i) We reviewed Health Department Victoria surveillance data from 1989 to 1991 (primary surveillance) and identified all notified cases of HAV infection with an onset of illness between 1 January and 31 December 1991 (secondary surveillance data). (ii) We studied HAV seroprevalence in Melbourne among homosexual men attending gay venues. RESULTS: (i) In the two years 1989-1990 there were only 55 notifications of HAV infection to Health Department Victoria, with a male:female ratio of 1.2:1. Of the 495 notifications for 1991, 407 (82%) were male, of whom 210 (52%) were known to be homosexual. Many reported sexual and social contact at Melbourne's gay venues and with the Sydney gay community. (ii) Of men attending gay venues, 27% had serological evidence of past infection with HAV, compared with 45.8% in a 1981 Melbourne study. CONCLUSION: The outbreak of HAV infection in Victoria largely affected homosexual males in their sexually active years. Contact with the Sydney gay community was commonly reported. 相似文献
922.
A remarkable nonlinearity has been observed for electric transport in K-TCNQ (potassium tetracyanoquinodimethane) quasi-one-dimensional
conductor. The negative differential resistance region appears afterV
m (turn over voltage), beyond which voltage goes down with increase of current. A possible mechanism is discussed in terms
of dynamics of charged solitons and domain walls in one-dimensional molecular stacks of these types of crystals. 相似文献
923.
I. T. Denisyuk 《Materials Science》1993,28(6):487-494
Summary A planar problem is considered for an isotropic body containing elastic angular inclusions, whose contours have angular points, which is reduced to a system of singular integral equations allowing numerical integration. The complex potentials are represented as sums of principal parts having power-law singularities at the angular points and regular parts that are derived from the integral equation system. Characteristic equations are given that define the order of the stress singularity.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 6, pp. 20–29, November–December, 1992. 相似文献
924.
Hong Lin H. Nayeb-Hashemi R. M. Pelloux 《Fatigue & Fracture of Engineering Materials & Structures》1993,16(7):723-742
A new multiaxial fatigue damage model for orthotropic materials is proposed based on the strain vector. Six material constants are included in the model. These material constants represent the dependence of fatigue resistance on material orientation, and they can be obtained by conducting strain-controlled uniaxial fatigue tests along the three principal orthotropic directions of an orthotropic material. The model can also be transformed in new coordinate systems to predict the fatigue lives of new material orientations. Biaxial low-cycle fatigue tests are conducted to verify the model. The prediction of the model agrees with the experimental results reasonably well. 相似文献
925.
The authors examine effects on the torque that simple variations of the standard rotor pole face profile could have, basically from an elementary viewpoint of how the modified profiles affect the distribution and magnitude of the radial and tangential components of the flux density. It is observed that the average torque is mostly affected by changes that alter the dominant radial component, such as changes in the effective air gap length. The other observation that an increase in the average torque can also be obtained by a favorable shift in the torque versus angle characteristics by relatively simple changes of the rotor pole profile is not so well known. Such a shift that reduces the slope of the torque angle characteristic, skewing the curve towards the unaligned position, has two advantages: one is that the phase inductance is at its maximum positive slop and, hence, the maximum torque, when the phase is energized; second, the flatter inductance profile near the aligned position when the phase current is to be commutated would allow a faster drop off of the commutated current and, thus, a smaller negative torque 相似文献
926.
BACKGROUND: The effects of stroke on classically nondominant hemisphere functions have received less attention than those on the dominant hemisphere, but visuospatial neglect and denial of illness both produce significant morbidity. SUMMARY OF COMMENT: The early literature on denial of illness is discussed and the etiological theories are examined. These are explanations based on deficits of higher mental function, impaired sensory input (especially proprioceptive), an abnormal representation of body image, psychodynamic defense mechanisms, and/or premorbid personality factors. CONCLUSIONS: Denial of illness is an important consequence of stroke. No explanation thus far proposed is entirely satisfactory. The consequences on rehabilitation and strategies for therapy have not been adequately investigated. 相似文献
927.
CP Kubicek R Messner F Gruber M Mandels EM Kubicek-Pranz 《Canadian Metallurgical Quarterly》1993,268(26):19364-19368
We prepared [U-14C]cellobiose by cultivating Acetobacter pasteurianus in the presence of [U-14C]glucose and hydrolyzing the [U-14C]cellulose formed with beta-glucosidase-free cellulase from Trichoderma reesei. This 14C-labeled cellobiose was used to investigate the presence of an uptake system for cellobiose in T. reesei. Evidence was obtained for the presence of a high affinity (Km for cellobiose 0.3 microM) but low activity (2.5 milliunits/mg fungal dry weight) cellobiose permease. The permease is formed constitutively, but higher levels are formed after addition of sophorose (glucosyl-beta-1,2-diglucoside), a reputed cellulase inducer. The permease appears to be specific for beta-diglucosides, as the uptake of [U-14C]cellobiose is inhibited by sophorose, gentiobiose (glucosyl-beta-1,3-glucoside), and cellobiose. Under these conditions, cellooligodextrines (n, 4-7; final concentration, 1 mM) are not inhibitors. Glucose, but no other monosaccharides, inhibits the permease. The hypersecretory mutant T. reesei RUT C-30 exhibits elevated permease activities, whereas in T. reesei QM 9979, a mutant strain defective in the induction of cellulases by cellulose or sophorose, strongly reduced permease activities were demonstrated. The results stress a hitherto not recognized point of control in the induction of cellulases by T. reesei at the level of uptake of cellulose oligosaccharides. 相似文献
928.
GM Gazzaniga G Angelini G Pastorino E Santoro M Lucchini ML Dal Prà 《Canadian Metallurgical Quarterly》1993,78(3):271-275
Four hundred forty-six patients with Tourette Syndrome (TS) and/or their parents completed a 52-item self-report survey about vocal and motor tics, and the frequency of associated co-morbid conditions of aggression, obsessions and compulsions, attentional problems, sleep disturbance, mood disturbance, anxiety, and self-mutilative behaviours which have been frequently reported in the literature on TS. Respondents also responded to an open-ended question regarding the most disabling aspects of TS. Results were analyzed within two age groups; under 18 years of age (N = 245) and 18 years of age or older (N = 177). Tics and associated conditions were the most frequently reported disabling aspect by both age groups. Subjects under age 18, however, reported significantly more frequent problems with hyperactivity, temper control, aggressive behaviours and sleepwalking than adults with TS. 相似文献
929.
930.
The morphologic changes and signal intensity of the spinal cord on preoperative magnetic resonance images were correlated with postoperative outcomes in 74 patients undergoing decompressive cervical surgery for compressive myelopathy. The transverse area of the spinal cord on T1-weighted images at the level of maximum compression was closely correlated with the severity of myelopathy, duration of disease, and recovery rate as determined by the Japanese Orthopaedic Association score. In patients with ossification of the posterior longitudinal ligament or cervical spondylotic myelopathy, the increased intramedullary T2-weighted magnetic resonance imaging signal at the site of maximal cord compression and duration of disease significantly influenced the rate of recovery. A multiple regression equation was then developed with these three variables to predict surgical outcomes. 相似文献