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   3246篇
  免费   183篇
  国内免费   22篇
电工技术   49篇
综合类   19篇
化学工业   803篇
金属工艺   62篇
机械仪表   59篇
建筑科学   120篇
矿业工程   8篇
能源动力   104篇
轻工业   489篇
水利工程   35篇
石油天然气   29篇
武器工业   2篇
无线电   303篇
一般工业技术   412篇
冶金工业   567篇
原子能技术   22篇
自动化技术   368篇
  2023年   25篇
  2022年   77篇
  2021年   118篇
  2020年   68篇
  2019年   67篇
  2018年   100篇
  2017年   99篇
  2016年   87篇
  2015年   88篇
  2014年   117篇
  2013年   191篇
  2012年   141篇
  2011年   181篇
  2010年   151篇
  2009年   150篇
  2008年   148篇
  2007年   151篇
  2006年   109篇
  2005年   100篇
  2004年   92篇
  2003年   99篇
  2002年   92篇
  2001年   61篇
  2000年   51篇
  1999年   41篇
  1998年   196篇
  1997年   127篇
  1996年   102篇
  1995年   66篇
  1994年   48篇
  1993年   53篇
  1992年   20篇
  1991年   19篇
  1990年   15篇
  1989年   16篇
  1988年   11篇
  1987年   15篇
  1986年   15篇
  1985年   14篇
  1984年   9篇
  1983年   10篇
  1982年   6篇
  1981年   7篇
  1980年   8篇
  1979年   12篇
  1978年   7篇
  1977年   13篇
  1976年   28篇
  1975年   3篇
  1971年   9篇
排序方式: 共有3451条查询结果,搜索用时 15 毫秒
11.
OBJECTIVE: To determine the distribution and amount of elastic fibers in the dermis of clinically normal dogs and dogs with dermatoses, particularly solar dermatitis. DESIGN: Skin specimens from 7 anatomic sites were obtained from 19 clinically normal dogs after euthanasia to evaluate the normal distribution of elastic fibers. Biopsy specimens also were obtained from 34 dogs with dermatoses, including 16 with solar dermatitis. Tissue sections were stained with H&E, Verhoeff-van Gieson, and periodic acid-Schiff. ANIMALS: 19 clinically normal dogs and 34 dogs with dermatoses. PROCEDURE: Numbers of elastic fibers were graded subjectively. Comparisons between clinically normal dogs and dogs with dermatoses were made. RESULTS: Normal elastic fibers were present in low numbers in the dermis of adult dogs, regardless of anatomic site or presence or severity of dermatitis. Condensed elastotic material was visualized in only 2 dogs with solar dermatitis. In both dogs, the elastotic material was Verhoeff-van Gieson and periodic acid-Schiff stain positive but was not visible with H&E stain. The most frequent histopathologic finding in the dermis of dogs with solar dermatitis was superficial dermal fibrosis. CONCLUSIONS: The dermis of clinically normal dogs does not contain abundant elastic fibers. Alterations of elastic fibers in dogs with solar dermatitis are rare. Superficial dermal fibrosis may be a better indicator of solar damage.  相似文献   
12.
Methodological issues in geropsychological assessment research are discussed and exemplified with recent investigations. Increased use of short-term longitudinal designs, more diverse and representative sampling, and routine inclusion of measures of health are among the recommendations made for future studies. Caution is urged in the application of new tests or screening batteries until sufficient independent validation studies are completed. Research contrasting neuropsychological performance of older persons from different racial and ethnic groups is briefly reviewed, and applications of geropsychological assessment to practical arenas such as competency determination are critiqued. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
13.
This paper presents the design and measured performance of a 1.8-GHz power amplifier featuring load mismatch protection and soft-slope power control. Load-mismatch-induced breakdown can be avoided by attenuating the RF power to the final stage during overvoltage conditions. This was accomplished by means of a feedback control system, which detects the peak voltage at the output collector node and clamps its value to a given threshold by varying the circuit gain. The issue of output power control has been addressed as well. To this end, a temperature-compensated bias network is proposed, which allows a moderate power control slope (dB/V) to be achieved by varying the circuit quiescent current according to an exponential law. The nonlinear power amplifier was fabricated using a low-cost silicon bipolar process with a 6.4-V breakdown voltage. It delivers a 33.5-dBm saturated output power with 46% maximum power-added efficiency and 36-dB gain at a nominal 3.5-V supply voltage. The device is able to tolerate a 10:1 load standing-wave ratio up to a 5.1-V supply voltage. Power control slope is lower than 80 dB/V between -15 dBm and the saturated output power level.  相似文献   
14.
结合工程实际介绍了异步电机计算机辅助测试(CAT)系统的硬件、软件组成、技术方案确定以及设计原则。现场运行表明,该系统稳定可靠,且具有较高的性能价格比。  相似文献   
15.
16.
The flow ice system including ozone (OFI condition) was tested for slaughtering and storage (up to 16 days) of farmed rainbow trout (Oncorhynchus mykiss). Lipid damage analyses were carried out and compared to sensory acceptance and instrumental colour changes. Comparison to individuals processed with the flow ice system in the absence of ozone (FI condition) was undertaken. Rainbow trout slaughtered and chilled under FI and OFI conditions showed a low lipid damage development, according to lipid oxidation and hydrolysis events and lipid composition (polyunsaturated fatty acids, phospholipids and endogenous antioxidants) changes. Additionally, both icing conditions led to largely good quality and shelf life times and to the absence of changes in colour properties. It is concluded that flow ice as such, or including the presence of ozone, can be considered as ideal strategy to be employed as slaughtering and storage system during the commercialisation of the actual farmed species. The ozone presence has shown some profitable effects as leading to an extended shelf life time by quality retention of several sensory parameters; in contrast, some negligible negative effects could be observed on the secondary and tertiary lipid oxidation development. However, the oxidation values reached by individuals kept under OFI conditions cannot be considered as particularly high.  相似文献   
17.
18.
19.
The purpose of this study is to clarify the volume effect of epidural saline injection 20 min after spinal anesthesia. Thirty patients undergoing combined spinal and epidural anesthesia for orthopedic surgery were randomly divided into two groups: a control group (n = 15) and a saline group (n = 15). In the control group, 2% lidocaine 3 ml with 0.4% tetracaine was injected into the subarachnoid space from L 4-5 interspace using Durasafe (Becton Dickinson, USA) and saline was not injected into the epidural space. In the saline group, saline 10 ml was injected through an epidural catheter 20 min after spinal anesthesia. The levels of analgesia 20 min after spinal anesthesia were not significantly different between the groups. However, the levels of analgesia 3, 5, 10, 40 and 100 min after epidural saline injection in the saline group were significantly higher than those in the control group (P < 0.05). The highest analgesic level was obtained 10 min after epidural saline injection and reached to T 4.3 +/- 1.1. In conclusion, epidural saline injection increases the analgesic level 20 min after spinal anesthesia because of the volume effect.  相似文献   
20.
BACKGROUND: The relationship between nonfatal acute myocardial infarction (AMI) and self-reported body weight and body mass index (BMI; Quetelet index, kg/m2) has been investigated. METHODS: A case-control study was conducted between 1983 and 1992 in northern Italy on 432 women with nonfatal AMI and 867 controls in hospital for acute, noncardiovascular, nonneoplastic, nondigestive, non-hormone-related conditions. Odds ratios (OR), with their 95% confidence intervals (CI), were computed by unconditional multiple logistic regression analysis, including terms for age, education, and smoking, plus history of selected diseases. RESULTS: Women with body weight and BMI in the highest quartile had an increased risk of AMI after allowance for age, education, and smoking status (OR 1.5, 95% CI 1.0 to 2.2, and OR 1.7, 95% CI 1.2 to 2.4, respectively). Compared with leaner women, the risk was higher among women with BMI above the median, in association with a history of diabetes (OR 5.2) or hyperlipidemia (OR 6.0). Hypertensive women had similar OR in the two strata of BMI (OR 5.1 and 4.8). The association of BMI with risk of AMI was apparently stronger among women younger than 50 years and among less educated women, but was similar among smokers and never smokers. CONCLUSIONS: The results of this study confirm that AMI among women is related to excess BMI, with a population attributable risk of 17%. The excess risk was substantial among overweight women with history of diabetes or hyperlipidemia, stressing the importance of controlling body weight among these women.  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