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   3129篇
  免费   34篇
  国内免费   14篇
电工技术   41篇
综合类   28篇
化学工业   58篇
金属工艺   4篇
机械仪表   104篇
建筑科学   15篇
矿业工程   6篇
能源动力   6篇
轻工业   355篇
水利工程   2篇
无线电   15篇
一般工业技术   98篇
冶金工业   2325篇
原子能技术   4篇
自动化技术   116篇
  2022年   17篇
  2020年   18篇
  2019年   30篇
  2018年   5篇
  2014年   6篇
  2013年   155篇
  2012年   31篇
  2011年   22篇
  2010年   31篇
  2009年   28篇
  2008年   30篇
  2007年   34篇
  2006年   22篇
  2005年   23篇
  2004年   23篇
  2003年   21篇
  2002年   15篇
  2001年   4篇
  2000年   10篇
  1999年   80篇
  1998年   757篇
  1997年   435篇
  1996年   287篇
  1995年   156篇
  1994年   126篇
  1993年   150篇
  1992年   26篇
  1991年   31篇
  1990年   41篇
  1989年   40篇
  1988年   39篇
  1987年   55篇
  1986年   38篇
  1985年   38篇
  1984年   11篇
  1983年   6篇
  1982年   13篇
  1981年   22篇
  1980年   19篇
  1979年   8篇
  1978年   5篇
  1977年   60篇
  1976年   125篇
  1975年   6篇
  1974年   3篇
  1973年   5篇
  1969年   5篇
  1958年   4篇
  1955年   18篇
  1954年   16篇
排序方式: 共有3177条查询结果,搜索用时 9 毫秒
31.
High levels of lectin activity were found in sixty cultivated and ten wild tepary (Phaseolus acutifolius) accessions. No lectin deficient varieties were observed and all examples studied contained both the phytohemagglutinin-E and L-like lectins previously described (Pusztai et al. 1987). There appeared to be no obvious differences between the wild and cultivated forms of the tepary lectins and all teparies studied contained lectin-like proteins in addition to the tepary lectins. One of the lectin related proteins (40 Kdalton subunit) was present in all teparies and may be comparable to arcelin, a lectin found in certain wild accessions of Phaselus vulgaris. All wild teparies contained a lectin related polypeptide of about 34 Kdaltons which appears to distinguish the wild teparies from the cultivated forms. Three tepary-common bean hybrids were examined and one hybrid was found to be expressing both tepary and common bean lectin genes.  相似文献   
32.
针对现有的电缆温度监测方法主要存在无法检测电缆内部温度和测温时易受到应力影响等缺点。针对此本文研究采用低反射率光栅(wFBG)阵列智能复合电力电缆温度监测技术,用于实时监测电缆内部及外部的温度分布情况。通过免应力光栅阵列,解决了复合电缆和光缆的过程中因光纤受力而影响光栅波长准确测量从而造成对温度的影响问题。系统通过检测wFBG波长漂移得到温度信息从而实现高灵敏度高空间分辨率的智能电缆的分布式温度在线监测。搭建了基于wFBG阵列的复合电力电缆实验系统,结果表明,该系统可以实现对电缆内部及外部温度的高精度、高空间分辨率分布式测量,电缆沿线空间分辨率达到10 cm,测温精度达到0.1℃。该低反射率光栅阵列智能复合电力电缆温度监测系统可以满足智能电网运行的实际需要。  相似文献   
33.
Freezing Time Predictions for Brick and Cylindrical-Shaped Foods   总被引:1,自引:0,他引:1  
A simplified model previously developed for freezing time calculations in plate freezers is extended to systems with two or three dimensional heat flow. The model combines Plank's equation with the unsteady heat transfer solutions for bodies with constant properties, through the addition of pre-cooling, change of phase and tempering times. Average thermal properties, different for each period are used in order to take into account their change with the ice content along the freezing process. Freezing time predictions show a maximum difference of 10% with respect to freezing experiments performed with meat blocks shaped as cylinders or rectangular bricks. Processing times from 0.7–5 hr were compared with satisfactory agreement.  相似文献   
34.
A novel coupled-inductor commutation circuit is described. The circuit may also be used for inverters. It uses only two auxiliary thyristors, each operating in alternate cycles and eliminating the need for reversal of the commutating capacitor voltage. Trapped energy is fed back to the source. The output voltage is controllable from zero to the full input voltage. The commutation circuit can be connected to a voltage source other than that feeding the load. A DC chopper using this commutation scheme has been analysed and experimentally verified.  相似文献   
35.
Values for polyphenoloxidase (PPO), total phenols and ascorbic acid, browning intensity and pH in soursops (Annona muricata, L.) decreased during maturation. Optimum pH was 7.5 (fully developed immature, mature unripe and ripe fruits) or 7.0 (fully ripe fruits). High correlation was found between PPO activity and phenolic compounds. The enzymic browning of soursops occurred during maturation. The least browning intensity was in fully ripe fruits.  相似文献   
36.
37.
The orbital control of geostationary satellites requires the periodic execution of manoeuvres for gravitational and solar radiation perturbation compensation. To maintain a fine control of the satellite position, it is usually sufficient to perform orbit corrections every two weeks, firing chemical thrusters. The operational load for the manoeuvre's planning and execution is usually quite low. Innovative technologies for station-keeping are available now, based on high efficiency ion thrusters, which allow a significant propellant saving. They are included in Artemis, a telecommunications satellite developed by the European Space Agency. When low acceleration ion thrusters are adopted, a near continuous orbit control is required; the resulting long firing arcs can be in conflict with some operational or technical constraints. The station-keeping planning is a critical process, since it is not based on periodic impulsive manoeuvres, but rather on quasi-continuous firings. Furthermore the orbit control should be planned more on a seasonal or a yearly cycle rather than on short weekly cycles. Since each manoeuvre is defined by its start time and duration, the total number of parameters involved in a station-keeping optimization process amounts to several tens. The presence of many variables and logical constraints makes the use of classical optimization methods quite complex. The model proposed here is based on genetic algorithms, which allows the convergence to an optimum solution through successive iterations, in which a random set of solutions is progressively selected on the basis of a factor of merit tailored to the target. This paper presents some innovative concepts for station-keeping optimization when ion propulsion is adopted for orbital control; furthermore, the general properties of genetic algorithms are discussed, together with the results obtained in the specific station-keeping application.  相似文献   
38.
A method of determining qualitatively and quantitatively hexamidine, dibromohexamidine, dibromopropamidine and chlorhexidine in cosmetic products by means of paired ion, reverse-phase high-performance liquid chromatography (HPLC) and ultraviolet detection (UV) at 264 nm.
Détermination de l'hexamidine, du dibromohexamidine, du dibromopropamidine et de la chlorhexidine dans les produits cosmétiques  相似文献   
39.
A multivariate skimmilk extrusion process, designed to produce an acid coprecipitate was studied in terms of minimizing residual lactose, ash and fines. an experimental model system was utilized to simulate the extrusion process and evaluated using response surface methodology to assess the relationship between the responses (fines, residual lactose and minerals) and the process variables (concentration, pH, temperature, agitation, washing time and wash water ratio). Compromise optimum conditions were derived using the Generalized Distance Approach (GDA) and an Extended Response Surface Procedure (ERSP) which made use of the SAS RSREG procedure with and without constraints. the GDA procedure produced good results in terms of providing an optimum for a general acid casein process, while the ERSP allowed more extensive analysis of the data in terms of assessing specific processing conditions. Although more computing intensive, the ERSP conferred additional flexibility in determining optimal conditions for special situations such as extrusion processing. Both approaches are useful for process engineering, with the GDA being a more general tool while the ERSP is advantageous when the GDA procedure becomes limiting.  相似文献   
40.
CONTEXT: Many groups have developed guidelines to shorten hospital length of stay in pneumonia in order to decrease costs, but the length of time until a patient hospitalized with pneumonia becomes clinically stable has not been established. OBJECTIVE: To describe the time to resolution of abnormalities in vital signs, ability to eat, and mental status in patients with community-acquired pneumonia and assess clinical outcomes after achieving stability. DESIGN: Prospective, multicenter, observational cohort study. SETTING: Three university and 1 community teaching hospital in Boston, Mass, Pittsburgh, Pa, and Halifax, Nova Scotia. PATIENTS: Six hundred eighty-six adults hospitalized with community-acquired pneumonia. MAIN OUTCOME MEASURES: Time to resolution of vital signs, ability to eat, mental status, hospital length of stay, and admission to an intensive care, coronary care, or telemetry unit. RESULTS: The median time to stability was 2 days for heart rate (< or =100 beats/min) and systolic blood pressure (> or =90 mm Hg), and 3 days for respiratory rate (< or =24 breaths/min), oxygen saturation (> or =90%), and temperature (< or =37.2 degrees C [99 degrees F]). The median time to overall clinical stability was 3 days for the most lenient definition of stability and 7 days for the most conservative definition. Patients with more severe cases of pneumonia at presentation took longer to reach stability. Once stability was achieved, clinical deterioration requiring intensive care, coronary care, or telemetry monitoring occurred in 1% of cases or fewer. Between 65% to 86% of patients stayed in the hospital more than 1 day after reaching stability, and fewer than 29% to 46% were converted to oral antibiotics within 1 day of stability, depending on the definition of stability. CONCLUSIONS: Our estimates of time to stability in pneumonia and explicit criteria for defining stability can provide an evidence-based estimate of optimal length of stay, and outline a clinically sensible approach to improving the efficiency of inpatient management.  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