全文获取类型
收费全文 | 7492篇 |
免费 | 276篇 |
国内免费 | 97篇 |
专业分类
电工技术 | 250篇 |
综合类 | 114篇 |
化学工业 | 849篇 |
金属工艺 | 151篇 |
机械仪表 | 167篇 |
建筑科学 | 254篇 |
矿业工程 | 47篇 |
能源动力 | 111篇 |
轻工业 | 571篇 |
水利工程 | 76篇 |
石油天然气 | 65篇 |
武器工业 | 53篇 |
无线电 | 629篇 |
一般工业技术 | 718篇 |
冶金工业 | 2984篇 |
原子能技术 | 85篇 |
自动化技术 | 741篇 |
出版年
2022年 | 51篇 |
2021年 | 95篇 |
2020年 | 77篇 |
2019年 | 55篇 |
2018年 | 73篇 |
2017年 | 71篇 |
2016年 | 85篇 |
2015年 | 94篇 |
2014年 | 164篇 |
2013年 | 275篇 |
2012年 | 254篇 |
2011年 | 271篇 |
2010年 | 239篇 |
2009年 | 214篇 |
2008年 | 250篇 |
2007年 | 284篇 |
2006年 | 243篇 |
2005年 | 204篇 |
2004年 | 181篇 |
2003年 | 151篇 |
2002年 | 144篇 |
2001年 | 141篇 |
2000年 | 121篇 |
1999年 | 159篇 |
1998年 | 834篇 |
1997年 | 482篇 |
1996年 | 358篇 |
1995年 | 212篇 |
1994年 | 203篇 |
1993年 | 243篇 |
1992年 | 102篇 |
1991年 | 77篇 |
1990年 | 84篇 |
1989年 | 76篇 |
1988年 | 76篇 |
1987年 | 77篇 |
1986年 | 65篇 |
1985年 | 86篇 |
1984年 | 59篇 |
1983年 | 74篇 |
1982年 | 62篇 |
1981年 | 60篇 |
1980年 | 71篇 |
1979年 | 48篇 |
1978年 | 47篇 |
1977年 | 93篇 |
1976年 | 182篇 |
1975年 | 43篇 |
1973年 | 32篇 |
1972年 | 29篇 |
排序方式: 共有7865条查询结果,搜索用时 0 毫秒
31.
BACKGROUND: Despite recognition of the high prevalence of alcoholism among patients with head and neck cancer, the prognostic importance of alcoholism has not been evaluated adequately. Previous investigators have speculated that alcoholic patients may have a poorer prognosis than nonalcoholic patients because of more advanced stage of cancer, the immunosuppressive effects of alcohol, and an increased rate of death due to other alcohol-related diseases. PURPOSE: The goal of this population-based study was to identify the features of alcoholism that are associated with survival for patients with head and neck cancer and to develop an alcoholic severity staging system from a composite of the independent features of alcoholism. METHODS: This prospective study included 649 patients who were diagnosed with cancer of the oral cavity, oropharynx, hypopharynx, or larynx during the period from September 1, 1983, through February 28, 1987, in a three-county area of western Washington state that participates in the Surveillance, Epidemiology, and End Results Program of the U.S. National Cancer Institute. Details on lifetime alcohol consumption, treatment for alcoholism, abstinence from alcohol prior to the diagnosis of cancer, and alcohol-related health problems were ascertained through in-person interviews near the time of diagnosis. Patients were classified as either nonalcoholics or alcoholics according to their responses to questions from the Michigan Alcoholism Screening Test. The measures of alcohol consumption and abuse that were found to be independently associated with 5-year survival by logistic regression analysis were combined using conjunctive consolidation to create a final composite variable, called an alcoholic severity stage. Cox proportional hazards regression analysis was done to estimate the relative risk (R) of death within 5 years due to specific causes of death for each of the alcoholic severity stages. RESULTS: Alcoholism (RR = 2.06; 95% confidence interval [CI] = 1.43-2.98) and a history of alcohol-related systemic health problems (i.e., liver disease, pancreatitis, delirium tremens, or seizures) (RR = 2.76; 95% CI = 1.69-4.49) were associated with an increased risk of death, whereas abstinence (i.e., the consumption of fewer than one drink per week at 1 year prior to the diagnosis of cancer) (RR = 0.62; 95% CI = 0.39-0.97) was associated with a decreased risk of death. These associations were independent of age, site of cancer, anatomical stage, histopathologic grade, smoking, and type of antineoplastic treatment. Patients in the two worst alcoholic severity stages had an increased risk of dying not only of head and neck cancer but also of cardiovascular disease, pulmonary disease, and other alcohol-related causes. CONCLUSIONS: Alcohol abuse, measured by alcohol consumption, functional impairment, a history of alcohol-related health problems, or abstinence, can provide important prognostic information for patients with head and neck cancer. Our results suggest that sobriety among alcoholic patients can lead to prolonged survival. 相似文献
32.
Transient hot-electron effect and its impact on circuit reliability are investigated. The rate of device decay is monitored as a function of the gate pulse transient period. Simulation results reveal that excess charges during a fast turn off time may cause an increase in the maximum substrate current. This, along with our experimental data, identifies that transient excess carrier may cause the enhancement of device degradation under certain stress conditions. The enhancement factor of the degradation is a function of the gate pulse transient time. Correlation between the analysis based upon AC/DC measurement and calculations based upon transient simulation are shown in the paper. Better agreement with experimental data is obtained by using the transient analysis and on chip test/stress structures. The correlation between AC and DC stress data is also shown based on the impact ionization model. A hot-electron design guideline is proposed based on the circuit reliability analysis. This guideline can help improve the circuit reliability without adversely effecting the circuit performance. 相似文献
33.
This letter describes a technique for finding cyclic redundancy check polynomials for systems for transmission over symmetric channels which encode information in multiple voltage levels, so that the resulting redundancy check gives good error protection and is efficient to implement. The codes which we construct have a Hamming distance of 3 or 4. We discuss a way to reduce burst error in parallel transmissions and some tricks for efficient implementation of the shift register for these polynomials. We illustrate our techniques by discussing a particular example where the number of levels is 9, but they are applicable in general 相似文献
34.
35.
Ashmead Daniel H.; Davis DeFord L.; Northington Anna 《Canadian Metallurgical Quarterly》1995,21(2):239
Of the several sources of acoustic information for distance perception, those arising from motion of the listener or sound source have received little attention. This motion-related information (recently called acoustic tau) is described, and experiments evaluating its utilization are presented. Accuracy and consistency at walking to the locations of briefly presented sounds were better when people listened while walking than while standing still. Manipulations of the sound to simulate shorter or longer target distances produced appropriate undershooting but not overshooting. The results indicate that people use motion-related acoustic information about distance to guide their locomotor actions, although they do not take full advantage of this information. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
36.
Z Xu LE Reinstein G Yang S Pai G Gluckman PR Almond 《Canadian Metallurgical Quarterly》1997,24(11):1788-1792
The dose distribution from a 32P source has been measured and calculated in order to evaluate its application in endovascular irradiation. The source dimension was 27 mm in length and 0.3 mm in diameter and was embedded in the end of a Ni-Ti wire. Dose measurements were performed using radiochromic film in several specially designed tissue equivalent phantoms. Loevinger's point dose kernel was used for the calculation. The approximate dose rate at a radial distance of 1.5 mm from the center of the source was found to be 6.75 cGy/s per GBq (0.25 cGy/s per mCi), which allows the delivery of a therapeutic dose in a short time interval with a satisfactory homogeneity without stepping the source. However, the dose rate falls off almost exponentially along the radial distance. Therefore it may not be suitable for treating large diameter vessel from a centrally located source. The effect of a curved 32P wire source on the radial dose distribution was also investigated. The results showed that for a maximum bend of 180 degrees the dose rate was increased by as much as 20% along the inner radial distance but decreased by as much as 20% along the outer radial distance compared to the dose along a straight wire. However, for curvatures normally encountered in a clinical situation, the dose rate was changed less than 5%. 相似文献
37.
Administration of footshock (500-ms duration, 0.2–2.4 mA) increased the amplitude of the startle reflex for a long time after its presentation. The effect occurred with a single footshock, although its magnitude and consistency across animals were greater with 5 or 10 footshocks presented 1/s. The facilitatory effect came on within 2–4 min with a 0.6-mA shock, peaking in about 10 min and then dissipating over the next 40 min. Stronger shocks also increased startle, but with a more delayed onset of facilitation (8–20 min). Footshocks increased startle in rats not previously given startle-eliciting stimuli, indicating sensitization rather than dishabituation. The facilitatory effect may not be attributable to a rapid conditioning to the experimental context, because a change in lighting conditions from shock presentation to testing did not attenuate shock sensitization. This excitatory effect of shock on startle may represent the unconditioned effect of shock that can become associated with a neutral stimulus to support classical fear conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
38.
39.
Etchable thick-film multi-chip-module (MCM) technology has led to the possibility of fabricating microwave integrated circuits (MICs) with performance similar to MICs produced using more expensive conventional thin-film MCM-D techniques. However, little data is available on the loss characteristics of the technology at microwave frequencies. This paper describes an experimental investigation into the loss properties of high-definition etchable thick-film MCM microstrip lines formed on a variety of high dielectric constant (high-/spl epsiv//sub r/) ceramic substrates. Substrates investigated comprise 96% alumina (/spl epsiv//sub r/=9.5), (Zr,Sn)TiO/sub 4/(/spl epsiv//sub r/=36.6) and BaO-PbO-Nd/sub 2/O/sub 3/-TiO/sub 2/ (/spl epsiv//sub r/=90.9). Microstrip loss properties are determined by fabricating a series of loosely coupled half-wave resonators on each substrate, with a range of characteristic impedance values. Measurements to 6 GHz are compared to those for similar lines fabricated using conventional thin-film MCM-D technology. The results demonstrate that etchable thick-film MCM technology provides many of the advantages of thin-film MCM-D technology, such as low-loss and high-definition conductors, and is suitable for the cost-effective fabrication of miniaturised high-performance microstrip MICs in high volume. 相似文献
40.