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   5687篇
  免费   70篇
  国内免费   2篇
电工技术   38篇
化学工业   318篇
金属工艺   37篇
机械仪表   20篇
建筑科学   40篇
矿业工程   1篇
能源动力   50篇
轻工业   219篇
水利工程   8篇
石油天然气   3篇
无线电   179篇
一般工业技术   224篇
冶金工业   4355篇
原子能技术   6篇
自动化技术   261篇
  2023年   10篇
  2022年   16篇
  2021年   22篇
  2020年   23篇
  2019年   35篇
  2018年   43篇
  2017年   39篇
  2016年   51篇
  2015年   26篇
  2014年   53篇
  2013年   84篇
  2012年   72篇
  2011年   88篇
  2010年   71篇
  2009年   54篇
  2008年   76篇
  2007年   70篇
  2006年   61篇
  2005年   55篇
  2004年   54篇
  2003年   47篇
  2002年   39篇
  2001年   32篇
  2000年   33篇
  1999年   166篇
  1998年   1485篇
  1997年   835篇
  1996年   506篇
  1995年   298篇
  1994年   254篇
  1993年   266篇
  1992年   43篇
  1991年   70篇
  1990年   52篇
  1989年   50篇
  1988年   59篇
  1987年   52篇
  1986年   44篇
  1985年   48篇
  1984年   11篇
  1983年   19篇
  1982年   18篇
  1981年   25篇
  1980年   35篇
  1979年   4篇
  1978年   12篇
  1977年   81篇
  1976年   146篇
  1975年   7篇
  1973年   6篇
排序方式: 共有5759条查询结果,搜索用时 15 毫秒
991.
Visually evoked potentials (VEPs) measured under conditions of retinal image stabilization that minimized the influences of visual masking and smearing were averaged from electroencephalographic records measured from striate cortex of three cats. The amplitudes of the VEPs increased around saccade initiation. The grating-evoked potentials obtained at different times relative to the saccade exhibited changes in waveform shape that could be attributed to a saccade-evoked potential. The changes in the shape of the waveform were reasonably accounted for by the summation of the grating-evoked potential (produced when the cat did not make a saccade) and an appropriately timed saccade-evoked potential. The fundamental amplitudes of the residual potentials were computed and found to vary across the time course of the saccade. These observations suggest that there are other influences besides visual masking that are exerted early in the visual pathway to modulate visual processing during saccadic eye movements. A corollary discharge process is the most likely candidate to exert these influences.  相似文献   
992.
Nurses with critical care knowledge and experience increasingly provide technologically advanced care to patients at home who often present comorbid psychiatric symptoms. Failure to accurately recognize and provide appropriate intervention can be devastating for patients because these symptoms can exacerbate the medical condition and may lead to mortality. The Biopsychosocial Model and the Transactional Model of Stress were presented as guides for nursing practice. Practice is directed at 1) understanding the patient's perception of events, 2) involving the caregiver(s) in the patient's recovery, and 3) enhancing the individual's adaptive coping efforts and resources. The article attempts to guide nurses in making decisions on when to consult and collaborate with an APPN to achieve desired patient outcomes.  相似文献   
993.
The neuronal-specific toxin alpha-conotoxin ImI (CTx ImI) has the sequence Gly-Cys-Cys-Ser-Asp-Pro-Arg-Cys-Ala-Trp-Arg-Cys-NH2, in which each cysteine forms a disulfide bridge to produce a constrained two-loop structure. To investigate the structural basis for bioactivity we mutated individual residues in CTx ImI and determined bioactivity. Bioactivity of the toxins was determined by their competition against 125I-labeled alpha-bungarotoxin binding to homomeric receptors containing alpha7 sequence in the major extracellular domain and 5HT-3 sequence elsewhere. The results reveal two regions in CTx ImI essential for binding to the alpha7/5HT-3 receptor. The first is the triad Asp-Pro-Arg in the first loop, where conservative mutations of each residue diminish affinity by 2-3 orders of magnitude. The second region is the lone Trp in the second loop, where an aromatic side chain is required. The overall results suggest that within the triad of the first loop, Pro positions the flanking Asp and Arg for optimal interaction with one portion of the binding site, while within the second loop, Trp stabilizes the complex through its aromatic ring.  相似文献   
994.
OBJECTIVE: The audiologic presentation of vestibular schwannoma (VS) associated with neurofibromatosis type 2 (NF2) has not been well characterized. The goal of this study was to investigate the audiologic features of NF2-associated VS and to determine their relationship to the size of the tumor. STUDY DESIGN: A retrospective case review. SETTING: Quaternary governmental medical research institute evaluating patients fitting specific criteria for ongoing clinical studies. PATIENTS: Audiologic and magnetic resonance imaging data were available for 40 patients (25 males, 15 females), with an average age of 32 years, who had been recruited for ongoing clinical and genetic studies of NF2. MAIN OUTCOME MEASURES: The audiologic profile and magnetic resonance imaging characteristic of tumor were retrospectively reviewed. RESULTS: The average size of the tumor at presentation was 7.26 +/- 16.58 cm3 and measured 1.2, 1.6, and 1.1 cm in the anterior/posterior, lateral/medial, and superior/inferior dimensions, respectively. An increase in lateral/medial size of the tumor most significantly correlated with deterioration in mid- (1,000-2,000 Hz) and high- (4,000-8,000 Hz) frequency hearing levels, elevated speech reception threshold, and prolonged auditory brain stem response waves III and V latency. CONCLUSIONS: Patients with NF2 demonstrate a more predictable audiologic profile for a given size tumor than has been previously described with spontaneous or sporadic VS.  相似文献   
995.
Patients with craniocervical mandibular (TMD) disorders can present with tinnitus as a primary or secondary complaint. The embryology and functional anatomy of the middle ear, temporomandibular joint, muscles of mastication and associated tendons, ligaments, blood vessels, nerves and lymphatics was found to be helpful in establishing etiologic concepts which relate tinnitus to these temporomandibular disorders. In addition to etiologic concepts, treatment modalities are described. The authors relate their experiences as well as those of others with different patient populations.  相似文献   
996.
997.
In children, recurrent cough is a common presenting symptom that may represent asthma. We tested the hypotheses that children with recurrent cough have increased cough-receptor sensitivity (CRS) or airway hyperresponsiveness (AHR). Skin prick testing, the capsaicin CRS test, and hypertonic saline (HS) challenge were performed in 44 children (median age: 8.9 yr) with recurrent dry cough (> or = 2 episodes of cough, each lasting > or = 2 wk, within a period of 12 mo) and 44 controls. Measures of CRS were the concentration of capsaicin required to stimulate > or = 2 coughs (Cth) and > or = 5 coughs (C5). During the coughing period, Cth (mean log: 0.62 [95% CI: 0.43 to 0.81]) and C5 (mean log: 1.15 [95% CI: 0.86 to 1.44]) of the subjects without AHR were significantly lower (p = 0.0026, 0.027, respectively) than Cth (mean log: 1.27 [95% CI: 0.88 to 1.66]) and C5 (mean log: 1.79 [95% CI: 1.21 to 2.37]) of the subjects with AHR and those of the controls (p = 0.0002 and 0.0001). During the cough-free period, there was no difference in CRS among the groups. In subjects who demonstrated AHR, the provocation dose causing a > or = 15% fall in FEV1 (PD15) during the cough period was significantly lower (p = 0.005) than that during the cough-free period. We conclude that AHR or increased CRS is present during the coughing phase in children with recurrent cough.  相似文献   
998.
PURPOSE: To evaluate the effectiveness of two-incision radial keratotomy (RK) in correcting low-magnitude refractive myopic astigmatism. SETTING: Two clinical study sites, one in St. Louis, Missouri, USA, the other in Caracas, Venezuela. METHODS: Fifty-seven eyes of 43 patients with low-magnitude myopic astigmatism had two-incision RK at one of two clinical study sites. In the initial phase of this series, 10 eyes with amblyopia at the 20/30 level had surgery at one center. Refractive keratotomy was performed with the radial incision placed in the plus cylinder axis of refraction. This axis was verified as the meridian of greatest corneal curvature by standard keratometry and computer-assisted corneal topographic analysis. Two eyes received a second operation (enhancement). RESULTS: Mean follow-up was 11.1 months (range 6 to 12 months). Mean preoperative and postoperative myopic spherical equivalent measured -1.42 diopters (D) +/- 0.51 (SD) and -0.14 +/- 0.39 D, respectively; the mean reduction was 1.28 +/- 0.59 D (P = .0001). Mean preoperative and postoperative refractive astigmatism was 1.41 +/- 0.45 D and 0.48 +/- 0.33 D, respectively (P = .0001). Mean preoperative and postoperative keratometric astigmatism was 1.26 +/- 0.54 D and 0.31 +/- 0.35 D, respectively, a mean reduction of 0.95 D (P = .0001). The surgical meridian was flattened by an average of 2.06 D by keratometry and the orthogonal meridian, by an average of 1.10 D. Preoperative uncorrected visual acuity (UCVA) was 20/40 or better in five (9%) eyes (range counting fingers to 20/40). Postoperative UCVA acuity was 20/40 or better in all eyes (mean acuity 20/25). In the nonamblyopic subgroup mean postoperative UCVA was 20/24. CONCLUSIONS: A limited number of radial incisions placed in the topographically confirmed axis of greatest curvature are effective in the treatment of low-magnitude myopic astigmatism.  相似文献   
999.
Pulmonary alveolar microlithiasis was found in three siblings. Only the youngest of them, a former smoker, developed endstage lung disease. The other two are asymptomatic with normal lung function despite impressive changes on all chest radiographs. The role of smoking in perpetuating microlithiasis and furthering the progression of this disease is discussed.  相似文献   
1000.
The glnA gene from the human pathogen Streptococcus agalactiae was cloned from a genomic library prepared with the lambda phage vector lambdaDASHII. A 4.6-kb DNA fragment of one of the recombinant phages was subcloned in pUC18. This Escherichia coli clone expressed a 52-kDa protein encoded by a 1,341-bp open reading frame. The nucleotide sequence of the open reading frame and the deduced amino acid sequence shared a significant degree of homology with the sequences of other glutamine synthetases (GS). The highest homology was between our deduced protein and GS of gram-positive bacteria such as Bacillus subtilis, Bacillus cereus, and Staphylococcus aureus. Plasmids with the cloned streptococcal glnA were able to complement E. coli glnA mutants grown on minimal media. Rabbit antisera to streptococcal GS recombinant protein recognized not only the recombinant protein but also a similar-sized band in mutanolysin extracts of all group B streptococcal strains tested, regardless of polysaccharide type or surface protein profile. The amino acid sequence of the deduced protein had similarities to other streptococcal cell-surface-bound proteins. The possible functional role of the immunological features of streptococcal GS is discussed.  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