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   562篇
  免费   20篇
电工技术   2篇
化学工业   111篇
金属工艺   4篇
机械仪表   10篇
建筑科学   23篇
能源动力   13篇
轻工业   88篇
水利工程   7篇
石油天然气   2篇
无线电   20篇
一般工业技术   75篇
冶金工业   168篇
原子能技术   1篇
自动化技术   58篇
  2023年   2篇
  2022年   10篇
  2021年   21篇
  2020年   10篇
  2019年   6篇
  2018年   9篇
  2017年   13篇
  2016年   7篇
  2015年   7篇
  2014年   14篇
  2013年   37篇
  2012年   12篇
  2011年   33篇
  2010年   32篇
  2009年   24篇
  2008年   25篇
  2007年   19篇
  2006年   20篇
  2005年   15篇
  2004年   18篇
  2003年   12篇
  2002年   10篇
  2001年   16篇
  2000年   12篇
  1999年   12篇
  1998年   26篇
  1997年   8篇
  1996年   12篇
  1995年   12篇
  1994年   8篇
  1993年   7篇
  1992年   7篇
  1991年   3篇
  1990年   9篇
  1989年   7篇
  1988年   6篇
  1987年   6篇
  1986年   15篇
  1985年   6篇
  1984年   4篇
  1983年   6篇
  1982年   4篇
  1981年   4篇
  1980年   5篇
  1979年   6篇
  1978年   2篇
  1977年   5篇
  1976年   2篇
  1974年   4篇
  1922年   1篇
排序方式: 共有582条查询结果,搜索用时 15 毫秒
1.
2.
Tropical corn silage was compared with sorghum silage as a basal forage in the diets of high producing dairy cows. Sorghum and tropical corn silages were each included in place of ground corn at incremental concentrations in the experimental diets. Eight separate diets were fed, four diets containing each silage ranging in forage neutral detergent fiber (NDF) from approximately 25 to 31% and ranging in total NDF from approximately 41 to 45%. Diets were arranged in a 2 x 4 factorial design and were fed to lactating cows (n = 24; pretrial mean milk production = 39 kg/d; body weight = 656 kg; and days in milk = 81). As concentrations of dietary NDF increased, intake and milk production decreased linearly. The impact of dietary NDF on intake was greater for diets based on tropical corn silage than for diets based on sorghum silage. Energy intake and milk production were reduced, but cows consumed more fiber when challenged with higher dietary concentrations of fiber. The in vitro rate and extent of digestion of dietary samples were correlated with intake response. The rate of in vitro fiber digestion was slower for samples that contained tropical corn silage than for samples that contained sorghum silage. In vivo digestibility measurements were influenced by intake and dietary composition. Results of this trial indicated that sorghum silage can have equal or slightly greater nutritional value than tropical corn silage when these forages are fed at equal concentrations of dietary fiber.  相似文献   
3.
According to hopelessness theory, hopelessness expectancy is the proximal, sufficient cause of hopelessness depression. Consequently, hopelessness expectancy is viewed as mediating the influence of all other factors on hopelessness depression. Using a longitudinal research design, hopelessness expectancy was examined as a mediator of the relation between illness attributions and hopelessness depression in a sample of 57 adults with rheumatoid arthritis. Although hopelessness expectancy was a strong predictor of hopelessness depression, it moderated rather than mediated the relation between attributions and depression. Finding support for a moderating rather than a mediating model is inconsistent with theory but is consistent with the findings of J. H. Riskind et al (1987). (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
4.
Chromium is a redox active 3d transition metal with a wide range of valences (−2 to +6) that control the geochemistry and toxicity of the element. Therefore, techniques that measure Cr valence are important bio/geochemical tools. Until now, all established methods to determine Cr valence were bulk techniques with many specific to a single, or at best, only a few oxidation state(s). We report an electron energy loss spectroscopy (EELS) technique along with an extensive suite of affined reference spectra that together, unlike other methods, can determine Cr valence (or at least constrain the possible valences) at high-spatial resolution (tens-of-nanometer scale) across a wide valence range, Cr(0)–Cr(VI). Fine structure of Cr-L2,3 edges was parametrized by measurement of the chemical shift of the L3 edge and the ratio of integrated intensity under the L3 and L2 edges. These two parameterizations were correlated to Cr valence and also the dn orbital configuration which has a large influence on L-edge fine structure. We demonstrate that it is not possible to unambiguously determine Cr valence from only one fine-structure parameterization which is the method employed to determine metal valence by nearly all previous EELS studies. Rather, multiple fine-structure parameterizations must be used together if the full range of possible Cr valences is considered. However even with two parameterizations, there are limitations. For example, distinguishing Cr(IV) from Cr(III) is problematic and it may be difficult to distinguish low-spin Cr(II) from Cr(III). Nevertheless, when Cr is known to be divalent, low- and high-spin dn orbital configurations can be readily distinguished.  相似文献   
5.
After suffering a rupture of a left-sided aneurysm on the middle cerebral artery with prolonged arterial spasms, the prognosis for recovery of speech and motor functions of a 27-yr-old male patient was considered to be poor. Based on a Luria neuropsychological investigation that revealed many areas of preserved brain functions, rehabilitation efforts were directed toward training of specific abilities as well as compensatory skills using intact functions. This case demonstrates the interaction between the training program and the levels of motivation and persistence of the S and his family necessary to produce optimal rehabilitation outcomes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  相似文献   
6.
About 50% of patients with arrhythmogenic cardiomyopathy (ACM) carry a pathogenic or likely pathogenic mutation in the desmosomal genes. However, there is a significant number of patients without positive familial anamnesis. Therefore, the molecular reasons for ACM in these patients are frequently unknown and a genetic contribution might be underestimated. Here, we used a next-generation sequencing (NGS) approach and in addition single nucleotide polymor-phism (SNP) arrays for the genetic analysis of two independent index patients without familial medical history. Of note, this genetic strategy revealed a homozygous splice site mutation (DSG2–c.378+1G>T) in the first patient and a nonsense mutation (DSG2–p.L772X) in combination with a large deletion in DSG2 in the second one. In conclusion, a recessive inheritance pattern is likely for both cases, which might contribute to the hidden medical history in both families. This is the first report about these novel loss-of-function mutations in DSG2 that have not been previously identi-fied. Therefore, we suggest performing deep genetic analyses using NGS in combination with SNP arrays also for ACM index patients without obvious familial medical history. In the future, this finding might has relevance for the genetic counseling of similar cases.  相似文献   
7.
8.
Changes in the crustacean zooplankton community composition and abundance in Lake Winnipeg (1969–2006) provide a rare opportunity to examine their response to environmental changes in the largest naturally eutrophic lake on the Canadian prairies. Since 1929, zooplankton species composition in Lake Winnipeg has changed little except for the addition of the invasive cladoceran, Eubosmina coregoni in 1994. The dominant taxa in the lake in summer include: Leptodiaptomus ashlandi, Acanthocyclops vernalis, Diacyclops thomasi, Daphnia retrocurva, Daphnia mendotae, Diaphanosoma birgei, Eubosmina coregoni, and Bosmina longirostris. Climate-accelerated nutrient loading to southern Lake Winnipeg over the last two decades has led to increased phytoplankton abundance and higher frequency of cyanobacterial blooms especially in its northern basin. Crustacean zooplankton have likewise increased especially in the North Basin, but less so in the more nutrient rich South Basin, possibly as a consequence of higher densities of pelagic planktivorous fish and light-limited primary production compared with the more transparent North basin (Brunskill et al., 1979, 1980). Calanoid copepods play a larger role in the South basin food web in contrast to cyclopoid copepods and Cladocera in the North basin. The study begins to fill the recognized gap in understanding of Lake Winnipeg's food web structure and provides a baseline for evaluating ongoing changes in the zooplankton community with the arrival of new non-indigenous taxa, e.g. Bythotrephes longimanus and Dreissena polymorpha. It reinforces previous work demonstrating that zooplankton provide valuable indices toward evaluating the health of an ecosystem.  相似文献   
9.
Relationships between large-scale environmental factors and the incidence of type E avian botulism outbreaks in Lake Michigan were examined from 1963 to 2008. Avian botulism outbreaks most frequently occurred in years with low mean annual water levels, and lake levels were significantly lower in outbreak years than in non-outbreak years. Mean surface water temperatures in northern Lake Michigan during the period when type E outbreaks tend to occur (July through September) were significantly higher in outbreak years than in non-outbreak years. Trends in fish populations did not strongly correlate with botulism outbreaks, although botulism outbreaks in the 1960s coincided with high alewife abundance, and recent botulism outbreaks coincided with rapidly increasing round goby abundance. Botulism outbreaks occurred cyclically, and the frequency of outbreaks did not increase over the period of record. Climate change scenarios for the Great Lakes predict lower water levels and warmer water temperatures. As a consequence, the frequency and magnitude of type E botulism outbreaks in the Great Lakes may increase.  相似文献   
10.
In this paper we propose a nonlinear control approach for the path‐tracking of an autonomous underactuated airship. A backstepping controller is designed from the airship nonlinear dynamic model including wind disturbances, and further enhanced to consider actuators saturation. Control implementation issues related to airship underactuation are also addressed, namely control allocation and an attitude reference shaping to obtain a faster error correction with smoother input requests. The results obtained demonstrate the capacity of an underactuated unmanned airship to execute a realistic mission including vertical take‐off and landing, stabilization and path‐tracking, in the presence of wind disturbances, with a single robust control law. Copyright © 2008 John Wiley & Sons, Ltd.  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