全文获取类型
收费全文 | 1041篇 |
免费 | 23篇 |
专业分类
电工技术 | 3篇 |
化学工业 | 64篇 |
金属工艺 | 7篇 |
机械仪表 | 6篇 |
建筑科学 | 9篇 |
能源动力 | 10篇 |
轻工业 | 67篇 |
无线电 | 25篇 |
一般工业技术 | 36篇 |
冶金工业 | 812篇 |
原子能技术 | 5篇 |
自动化技术 | 20篇 |
出版年
2022年 | 9篇 |
2021年 | 11篇 |
2020年 | 7篇 |
2019年 | 4篇 |
2018年 | 5篇 |
2017年 | 10篇 |
2016年 | 8篇 |
2015年 | 8篇 |
2014年 | 12篇 |
2013年 | 12篇 |
2012年 | 20篇 |
2011年 | 20篇 |
2010年 | 10篇 |
2009年 | 12篇 |
2008年 | 14篇 |
2007年 | 11篇 |
2006年 | 13篇 |
2005年 | 2篇 |
2004年 | 3篇 |
2003年 | 5篇 |
2002年 | 5篇 |
2001年 | 4篇 |
2000年 | 1篇 |
1999年 | 32篇 |
1998年 | 278篇 |
1997年 | 143篇 |
1996年 | 122篇 |
1995年 | 47篇 |
1994年 | 42篇 |
1993年 | 49篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 7篇 |
1979年 | 2篇 |
1978年 | 3篇 |
1977年 | 21篇 |
1976年 | 44篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有1064条查询结果,搜索用时 15 毫秒
71.
72.
73.
The bag cell neurons of Aplysia are a cluster of cells that control egg laying behavior. After brief synaptic stimulation, they depolarize and fire spontaneously for up to 30 min. During the first few seconds of this afterdischarge, the action potentials of the bag cell neurons undergo pronounced broadening. Single bag cell neurons in culture also show spike broadening in response to repeated depolarizations. This broadening is frequency-dependent and associated with the induction of a depolarizing afterpotential lasting minutes. In some neurons the depolarizing afterpotential is sufficient to trigger spontaneous firing. To test the possibility that spike broadening during stimulation is required to trigger the depolarizing afterpotential, we eliminated frequency-dependent broadening by heterologous expression of the Kv3.1 potassium channel. This channel has rapid activation and deactivation kinetics and no use-dependent inactivation. Expression of Kv3.1 prevented spike broadening and also eliminated the depolarizing afterpotential. Measurements of the integral of calcium current during voltage commands, which simulated the action potentials of the control neurons and those expressing Kv3.1, indicate that spike broadening produces up to a fivefold increase in calcium entry. Manipulations that limit calcium entry during action potentials or chelation of intracellular calcium using BAPTA AM prevented the induction of the depolarizing afterpotential. We conclude that spike broadening is essential for the induction of the depolarizing afterpotential probably by regulating calcium influx and suggest that one of the physiological roles of spike broadening may be to regulate long-term changes in neuronal excitability. 相似文献
74.
F Parazzini L Chatenoud C La Vecchia E Negri S Franceschi G Bolis 《Canadian Metallurgical Quarterly》1998,77(5):838-841
We describe doublex sequencing of human genomic PCR products using two differently labeled primers in a single reaction and analysis on two automated DNA sequencing devices. Feasibility of the methodology is demonstrated by isothermal and cycle sequencing for two different PCR products and by cycle sequencing on both strands of a single product. It was applied to analyze mutations in patient DNAs in routine sample screening. Because it has the advantage of increased throughput and cost reduction while retaining its accuracy and reading length, we found that doublex sequencing is an attractive option for molecular diagnosis of hereditary diseases. This approach would be even more beneficial if it used DNA sequencing devices with several lasers in a single instrument. 相似文献
75.
PURPOSE: The purpose of this study was to compare the fractional contributions of the three pathways of lactate transport (band 3 system, nonionic diffusion, and monocarboxylate pathway) into red blood cells (RBC) from trained and untrained humans. METHODS: Blood samples were obtained from 19 male subjects: 5 untrained, 5 aerobically-trained, 5 competitive collegiate cross-country runners, and 4 competitive collegiate sprinters. The influx of lactate into the RBC was measured by a radioactive tracer technique using [14C]lactate. Discrimination of each pathway of lactate transport was achieved by using PCMBS (1 mM) to block the monocarboxylate pathway and DIDS (0.2 mM) to block the band 3 system. Nonionic diffusion was calculated as the difference between total lactate influx and the sum of band 3 and monocarboxylate lactate influx. RESULTS: Total lactate influx into the RBC from the more aerobic individuals (trained subjects and cross-country runners) was significantly faster at 1.6 mM lactate concentration ([La]) as compared with the influx into RBC of the untrained subjects. Total influx of lactate was significantly higher (P < 0.05) in the RBC from the sprinters as compared with that in the RBC from the untrained subjects at 41 mM [La]. There were no significant differences among the four groups with regard to the total influx of lactate at 4.1, 8.1, and 20 mM [La]. In general, the percentage of total lactate influx accounted for by each of the three parallel pathways at 1.6, 8.1, and 41.0 mM [La] was not different among the four groups of subjects. CONCLUSIONS: Overall, the groups were more similar than different with regard to RBC lactate influx. 相似文献
76.
77.
LJ Shaw ED Peterson LK Shaw KL Kesler ER DeLong FE Harrell LH Muhlbaier DB Mark 《Canadian Metallurgical Quarterly》1998,98(16):1622-1630
BACKGROUND: Exercise testing is useful in the assessment of symptomatic patients for diagnosis of significant or extensive coronary disease and to predict their future risk of cardiac events. The Duke treadmill score (DTS) is a composite index that was designed to provide survival estimates based on results from the exercise test, including ST-segment depression, chest pain, and exercise duration. However, its usefulness for providing diagnostic estimates has yet to be determined. METHODS AND RESULTS: A logistic regression model was used to predict significant (>/=75% stenosis) and severe (3-vessel or left main) coronary artery disease, and a Cox regression analysis was used to predict cardiac survival. After adjustment for baseline clinical risk, the DTS was effectively diagnostic for significant (P<0.0001) and severe (P<0.0001) coronary artery disease. For low-risk patients (score >/=+5), 60% had no coronary stenosis >/=75% and 16% had single-vessel >/=75% stenosis. By comparison, 74% of high-risk patients (score <-11) had 3-vessel or left main coronary disease. Five-year mortality was 3%, 10%, and 35% for low-, moderate-, and high-risk DTS groups (P<0.0001). CONCLUSIONS: The composite DTS provides accurate diagnostic and prognostic information for the evaluation of symptomatic patients evaluated for clinically suspected ischemic heart disease. 相似文献
78.
This study examines long-term health and physical readiness trends in the U.S. Navy. We mailed lifestyle questionnaires to all participants in baseline studies between 1983 and 1989 who were still on active duty in 1994. Commands provided body composition and physical readiness test scores for the participants. Two longitudinal cohorts were created: an 8-year sample (N = 640) with matched data from 1986, 1989, and 1994; and an 11-year sample (N = 1,576), with data from 1983 and 1994. Analyses of both cohorts revealed significant improvements in cardiovascular fitness, muscle strength, exercise, lean body mass, dietary habits, and sleep, as well as significant decreases in tobacco and alcohol use and job stress. However, hypertension rates, percentage of body fat, and body mass index increased over time. Women's scores were significantly better than men's on a number of factors. Overall, these findings suggest that the Navy's health promotion efforts have had a significant positive effect on the fitness and health behaviors of career Navy men and women. 相似文献
79.
Nonproductive cough is a frequent and distressing symptom in patients with lung cancer, and it is not even relieved by palliative chemotherapy. A double-blind, randomized clinical trial regarding the treatment of nonproductive cough was performed in 140 adults with primary lung cancer or metastatic cancer of the lungs. The therapeutic efficacy and the tolerability of a 7-day treatment with levodropropizine drops (75 mg t.i.d.) were evaluated in comparison with dihydrocodeine drops (10 mg t.i.d.; 7 days). Efficacy was assessed on the basis of cough severity scores, number of night awakenings due to cough, and overall estimate of antitussive efficacy. Tolerability was evaluated by laboratory results, vital signs and any adverse event occurring during the clinical trial, including presence or absence of somnolence. Subjective cough severity was significantly reduced during treatment with either levodropropizine and dihydrocodeine, the antitussive effect and its time-profile being similar for both drugs. Also, according to the investigator's evaluation, both levodropropizine and dihydrocodeine produced a significant decrease in cough severity. Concurrently with the relief of cough, the number of night awakenings was decreased significantly by both drugs, with no difference between the two treatments. No change in laboratory test values was considered clinically relevant, and vital signs were not clinically affected. The number of patients reporting adverse events was similar in the levodropropizine (n=6) and dihydrocodeine (n=4) group. However, the percentage of patients experiencing somnolence in the group receiving levodropropizine (8%) was significantly lower as compared with that of the dihydrocodeine group (22%). These results confirm the antitussive effectiveness of levodropropizine and suggest a more favourable benefit/risk profile when compared to dihydrocodeine. 相似文献
80.
A 14-year-old boy presented acutely with occipital headache, nausea and vomiting. MRI showed obstructive hydrocephalus and marked bilateral cerebellar swelling with increased signal on T2-weighted imaging. Following treatment with oral corticosteroids, the clinical and radiological signs resolved. The clinical course and radiological appearances were consistent with cerebellitis associated with a significant mass effect and hydrocephalus. 相似文献