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   2392篇
  免费   111篇
  国内免费   2篇
电工技术   15篇
综合类   4篇
化学工业   484篇
金属工艺   18篇
机械仪表   21篇
建筑科学   107篇
矿业工程   3篇
能源动力   35篇
轻工业   348篇
水利工程   34篇
石油天然气   1篇
无线电   83篇
一般工业技术   323篇
冶金工业   844篇
原子能技术   4篇
自动化技术   181篇
  2024年   4篇
  2023年   29篇
  2022年   62篇
  2021年   90篇
  2020年   57篇
  2019年   47篇
  2018年   63篇
  2017年   50篇
  2016年   57篇
  2015年   45篇
  2014年   63篇
  2013年   117篇
  2012年   111篇
  2011年   162篇
  2010年   128篇
  2009年   129篇
  2008年   154篇
  2007年   126篇
  2006年   107篇
  2005年   95篇
  2004年   85篇
  2003年   69篇
  2002年   76篇
  2001年   42篇
  2000年   52篇
  1999年   44篇
  1998年   39篇
  1997年   48篇
  1996年   42篇
  1995年   45篇
  1994年   55篇
  1993年   36篇
  1992年   32篇
  1991年   9篇
  1990年   25篇
  1989年   16篇
  1988年   25篇
  1987年   13篇
  1986年   7篇
  1985年   11篇
  1984年   6篇
  1983年   7篇
  1982年   5篇
  1981年   6篇
  1979年   2篇
  1978年   3篇
  1977年   2篇
  1974年   2篇
  1970年   1篇
  1968年   2篇
排序方式: 共有2505条查询结果,搜索用时 468 毫秒
951.
Recent foodborne disease outbreaks involving minimally processed tree nuts have generated a need for improved sanitation procedures. Chemical sprays and dips have shown promise for reducing pathogens on fresh produce, but little research has been conducted for in‐shell hazelnuts. This study analyzed the effectiveness of 3 chemical sanitizers for reducing Salmonella on in‐shell hazelnuts. Treatments of water, sodium hypochlorite (NaOCl; 25 and 50 ppm), peroxyacetic acid (PAA; 80 and 120 ppm), and acidified sodium chlorite (ASC; 450, 830, and 1013 ppm) were sprayed onto hazelnut samples inoculated with Salmonella enterica serovar Panama. Hazelnut samples were immersed in liquid cultures of S. Panama for 24 h, air‐dried, and then sprayed with water and chemical treatments. Inoculation achieved S. Panama populations of approximately 8.04 log CFU/hazelnut. Surviving S. panama populations were evaluated using a nonselective medium (tryptic soy agar), incubated 3 h, and then overlaid with selective media (xylose lysine deoxycholate agar). All of the chemical treatments significantly reduced S. Panama populations (P ≤ 0.0001). The most effective concentrations of ASC, PAA, and NaOCl treatments reduced populations by 2.65, 1.46, and 0.66 log units, respectively. ASC showed the greatest potential for use as a postharvest sanitation treatment.  相似文献   
952.
Perfluoroalkyl contaminants (PFCs) were determined in Lake Ontario Lake Trout sampled annually between 1997 and 2008 in order to assess how current trends are responding to recent regulatory bans and voluntary phase-outs. We also combined our measurements with those of a previous study to provide an updated assessment of long-term trends. Concentrations of PFCs generally increased from the late 1970s until the mid-1980s to mid-1990s, after which concentrations either remained unchanged (perfluorooctane sulfonate (PFOS) and perfluorocarboxylates) or declined (perfluorodecanesulfonate (PFDS)). The temporal trends were assessed using three models, quadratic, exponential rise to maximum, and two-segment linear piecewise function, and then evaluated for best fit using Akaike Information Criteria. For PFOS and perfluorocarboxylates, the exponential rise to maximum function had the best fit. This is particularly interesting for PFOS as it suggests that although concentrations in Lake Ontario Lake Trout may have stopped increasing in response to voluntary phase-outs in 2000-2002, declines have yet to be observed. This may be due to continuing input of PFOS from products still in use and/or slow degradation of larger precursor molecules. A power analysis of PFOS suggested that 15 years of data with a within-year sample size of 10 is required to obtain sufficient power (80%) to detect a 5% decreasing trend. However, the length of the monitoring program had a greater influence on the ability to detect a trend compared to within-year sample size. This provides evidence that additional sampling years are required to detect a response to bans and phase-outs, given the variability in the fish data. The lack of observed declines of perfluorocarboxylate residues in fish may be expected as regulations for these compounds were only recently enacted. In contrast to the other compounds, the quadratic model had the best fit for PFDS. The results of this study emphasize the importance of long-term monitoring for assessing the effectiveness of bans and phase-outs on PFCs in the environment.  相似文献   
953.
A nationally consistent approach was used to assess the occurrence and potential sources of pyrethroid insecticides in stream bed sediments from seven metropolitan areas across the United States. One or more pyrethroids were detected in almost half of the samples, with bifenthrin detected the most frequently (41%) and in each metropolitan area. Cyhalothrin, cypermethrin, permethrin, and resmethrin were detected much less frequently. Pyrethroid concentrations and Hyalella azteca mortality in 28-d tests were lower than in most urban stream studies. Log-transformed total pyrethroid toxic units (TUs) were significantly correlated with survival and bifenthrin was likely responsible for the majority of the observed toxicity. Sampling sites spanned a wide range of urbanization and log-transformed total pyrethroid concentrations were significantly correlated with urban land use. Dallas/Fort Worth had the highest pyrethroid detection frequency (89%), the greatest number of pyrethroids (4), and some of the highest concentrations. Salt Lake City had a similar percentage of detections but only bifenthrin was detected and at lower concentrations. The variation in pyrethroid concentrations among metropolitan areas suggests regional differences in pyrethroid use and transport processes. This study shows that pyrethroids commonly occur in urban stream sediments and may be contributing to sediment toxicity across the country.  相似文献   
954.
While there is strong focus on eliminating pathogens from produce at a commercial level, consumers can employ simple methods to achieve additional pathogen reductions in the domestic kitchen. To determine the ability of antimicrobials to decontaminate peppers, samples of green bell pepper were inoculated with Salmonella enterica and Escherichia coli O157:H7 and then immersed in 3% (v/v) hydrogen peroxide (H?O?), 2.5% (v/v) acetic acid (AA), 70% (v/v) ethyl alcohol (EtOH), or sterile distilled water (SDW). The potential for transfer of pathogens from contaminated peppers to other non-contaminated produce items, and the effect of knife disinfection in preventing this cross contamination, were also tested. Knife disinfection procedures were evaluated by chopping inoculated peppers into 1 cm2 pieces with kitchen knives. Experimental knives were then treated by either no treatment (control), wiping with a dry sterile cotton towel, rinsing under running warm water for 5 or 10s, or applying a 1% (v/v) lauryl sulfate-based detergent solution followed by rinsing with warm running water for 10s. Following disinfection treatment, knives were used to slice cucumbers. Exposure to H?O? for 5 min and EtOH for 1 min resulted in reductions of 1.3±0.3 log?? CFU/cm2 for both pathogens. A 5 min exposure to AA resulted in a reduction of S. enterica of 1.0±0.7 log?? CFU/cm2 and E. coli of 0.7±0.8 log?? CFU/cm2. No differences (p ≥ 0.05) were found between numbers of pathogens on knives and numbers of pathogens transferred to cucumber slices, suggesting that organisms remaining on knife surfaces were transferred to cucumbers during slicing. Findings suggest that EtOH and H?O? may be effective antimicrobials for in-home decontamination of peppers, and that use of detergent and warm water is effective for decontamination of implements used during meal preparation.  相似文献   
955.
Evidence of the causal role of marketing in the tobacco epidemic and the advent of the WHO Framework Convention on Tobacco Control have inspired more than half the countries in the world to ban some forms of tobacco marketing. This paper briefly describes the ways in which cigarette marketing is restricted and the tobacco industry's efforts to subvert restrictions. It reviews what is known about the impact of marketing regulations on smoking by adults and adolescents. It also addresses what little is known about the impact of marketing bans in relation to concurrent population-level interventions, such as price controls, anti-tobacco media campaigns and smoke-free laws. Point of sale is the least regulated channel and research is needed to address the immediate and long-term consequences of policies to ban retail advertising and pack displays. Comprehensive marketing restrictions require a global ban on all forms of promotion, elimination of packaging and price as marketing tools, and limitations on the quantity, type and location of tobacco retailers.  相似文献   
956.
Blood-oxygenation-level-dependent (BOLD) contrast in magnetic resonance (MR) imaging of skeletal muscle mainly depends on changes of oxygen saturation in the microcirculation. In recent years, an increasing number of studies have evaluated the clinical relevance of skeletal muscle BOLD MR imaging in vascular diseases, such as peripheral arterial occlusive disease, diabetes mellitus, and chronic compartment syndrome. BOLD imaging combines the advantages of MR imaging, i.e., high spatial resolution, no exposure to ionizing radiation, with functional information of local microvascular perfusion. Due to intrinsic contrast provoked via changes in hemoglobin oxygen saturation, it is a safe and easy applicable procedure on standard whole-body MR devices. Therefore, BOLD MR imaging of skeletal muscle is a potential new diagnostic tool in the clinical evaluation of vascular, inflammatory, and muscular pathologies. Our review focuses on the current evidence concerning the use of BOLD MR imaging of skeletal muscle under pathological conditions and highlights ways for future clinical and scientific applications.  相似文献   
957.
International practice shows an increasing use of web-based Environmental Assessment (EA) registries or project websites to provide public access to information. In support of statutory public participation, registries inform when to participate and provide relevant documents and the continued availability of information. However, what constitutes good practice in this field? This article proposes a framework for assessing good practice in providing access to information via registries. Based on performance indicators for an ‘ideal’ registry, the framework allows evaluation in the review categories: framework conditions, notice of projects, provision of documents, accessibility of information and ongoing information regarding the status of the environmental assessment. An explorative case study analysis applies the framework and presents results on the performance of international practice examples.  相似文献   
958.
One of the assumptions of the theory of accident proneness is that drivers’ accident liability is stable over time, which was tested in the present paper. Previous investigations of this problem (or rather the conclusions) were found to be deficient because they did not take into account the statistical problem of low variance in the accident variable. However, by correlating the between time periods association coefficient and the mean number of accidents across several samples, this problem can be overcome. Therefore, the stability of accident record over time was investigated in five samples of British bus drivers. It was found that the size of the correlations between time periods increased with the increase in mean accident frequency. Furthermore, this increase could be described by a linear regression line, which fit the various points extremely well. Also, the size of correlations of ‘at fault’ accidents increased faster with the mean than did ‘all accidents’, although the latter had a higher initial value. It was therefore concluded, in contrast to previous authors, that the accident record of drivers is quite stable over time and that the very low correlations that have often been found were due to the samples and methods used (low-risk drivers and short time periods equalling low crash means) and not of any inherent instability in drivers’ behaviour and/or accident record. It was also concluded that only culpable accidents should be used for this type of calculation. No evidence was found for a decrease in correlation size between single years' accidents when time periods between the years were lengthened, i.e. accidents in one year predicted accidents in several other years equally well. However, the period used was rather short. The results are discussed with reference to training intervention for accident-involved drivers, especially for organisations with major fleets, such as bus companies.  相似文献   
959.
The thermostamping of prepreg woven fabrics shows promise as a low-cost high-volume manufacturing process for composite parts. One concern associated with the process is the unwanted formation of defects in the form of fabric wrinkling. This wrinkling can be prevented during the thermostamping process by inducing in-plane forces through the use of one or more metal binder rings. However, if the in-plane forces are too low, then the fabric may wrinkle as the fabric conforms to the shape of the punch, and conversely, if the in-plane forces are too high, then the yarns in the fabric can separate and the fabric may tear and yarns can break. The in-plane forces are a result of the friction between the fabric and the metal binder rings. As the fabric slides over the surfaces of the punch and die, further friction is induced between the metal tooling and the fabric part. In addition, most composite parts consist of multiple layers, and therefore as the fabric is drawn into the die adjacent layers of fabric may slide relative to one another. Thus, the friction at the tool/fabric interface and the interlaminar friction must be understood and quantified to predict part quality as a function of the processing parameters. In this paper, the design and implementation of a load-control test apparatus used to measure the friction between the tool and the fabric and between adjacent layers of fabric during a composite forming process is presented.  相似文献   
960.
An integrated approach for determining the stability of flavonoids and ascorbic acid in powders was developed. A rapid analyte solubility procedure was used to decide whether HPLC, GLC, or NMR would best serve to measure the stability of each flavonoid. Both the flavonoid under study and ascorbic acid were measured in one analysis with any of the determinative methods. Seventeen flavonoids of differing types, as well as ascorbic acid, were evaluated to establish the approach. The effects of storage relative humidity (RH) on the stability of several flavonoid powders with and without ascorbic acid were then determined. Quercetin, luteolin, taxifolin, naringenin, and naringin were stable for up to 8 weeks in RHs up to 98%, and ascorbic acid was significantly destroyed in the presence of quercetin, luteolin, and taxifolin at 98% RH. Both grape anthocyanins and ascorbic acid were unstable in the presence of each other at 98% RH. The integrated approach could be used for shelf-life testing of powdered flavonoids under a variety of storage conditions.  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