全文获取类型
收费全文 | 6555篇 |
免费 | 18篇 |
国内免费 | 3篇 |
专业分类
电工技术 | 5篇 |
综合类 | 3篇 |
化学工业 | 264篇 |
金属工艺 | 21篇 |
机械仪表 | 26篇 |
建筑科学 | 30篇 |
矿业工程 | 1篇 |
能源动力 | 17篇 |
轻工业 | 152篇 |
水利工程 | 8篇 |
无线电 | 71篇 |
一般工业技术 | 142篇 |
冶金工业 | 5720篇 |
原子能技术 | 14篇 |
自动化技术 | 102篇 |
出版年
2022年 | 8篇 |
2021年 | 17篇 |
2020年 | 11篇 |
2019年 | 9篇 |
2018年 | 13篇 |
2017年 | 12篇 |
2016年 | 19篇 |
2015年 | 8篇 |
2014年 | 21篇 |
2013年 | 31篇 |
2012年 | 33篇 |
2011年 | 52篇 |
2010年 | 29篇 |
2009年 | 37篇 |
2008年 | 48篇 |
2007年 | 34篇 |
2006年 | 49篇 |
2005年 | 35篇 |
2004年 | 22篇 |
2003年 | 33篇 |
2002年 | 19篇 |
2001年 | 21篇 |
2000年 | 16篇 |
1999年 | 183篇 |
1998年 | 1746篇 |
1997年 | 943篇 |
1996年 | 678篇 |
1995年 | 350篇 |
1994年 | 337篇 |
1993年 | 346篇 |
1992年 | 52篇 |
1991年 | 75篇 |
1990年 | 95篇 |
1989年 | 83篇 |
1988年 | 80篇 |
1987年 | 106篇 |
1986年 | 79篇 |
1985年 | 81篇 |
1984年 | 15篇 |
1983年 | 19篇 |
1982年 | 36篇 |
1981年 | 42篇 |
1980年 | 50篇 |
1978年 | 15篇 |
1977年 | 152篇 |
1976年 | 357篇 |
1975年 | 13篇 |
1974年 | 7篇 |
1973年 | 7篇 |
1968年 | 8篇 |
排序方式: 共有6576条查询结果,搜索用时 15 毫秒
51.
Three cases of haematemesis associated with alcohol abuse are described. Early fibreoptic endoscopical examination in each showed a focal, well demarcated area of gastric mucosal haemorrhage, close to the oesophagogastric junction. Two patients showed prolapse of the lesion into the lower part of the oesophagus, and the third had coexistent Mallory-Weiss tears. Our observations suggest that forceful vomiting is responsible for this lesion, by causing abrupt retrograde gastrooesophageal prolapse. The prognosis of the lesion appears good. 相似文献
52.
Optimal conditions for using high-performance liquid chromatography (HPLC) in the size exclusion mode have been determined for measuring the molecular-weight (MW) distribution of chitosan samples. Physical separation according to molecular size was accomplished on the stationary phase of glass supports having controlled pore sizes ranging from 2500 to 40 A. Selection of column combinations was based on the requirements to resolve the higher MW fraction of chitosan and to give a linear calibration curve within the required MW range. The best combination of glass pore sizes and column lengths in two foot sections joined sequentially was: 2500 A (2 ft.), 1500 A (4 ft.), 550 A (6 ft.), 250 A (2 ft), 100 A (2 ft.), and 40 A (2 ft.). A loading study showed that an injection load of 500 mug, i.e. 100 mul at 5 g/l or 50 mul at 10 g/l (w/v), was the optimal load to give reproducible elution volumes, precision in quantitation, and minimum viscosity effects. The best calibration curve using defined dextran standards was obtained from the geometric mean of Mw (weight average MW) and Mn (number average MW) values and peak elution volumes. Precision in determining MW distribution of chitosan as well as dextran standards was better than 5% relative standard deviation, and the differences between these results and the manufacturer's data on the dextran standards were 6 to -17%. The MW distribution of a selected chitosan samples in 2% acetic acid thus determined was Mw = 2,055,000, Mn = 936,000, dispersity = 2.16, and the most abundant species was around 1,103,000. Analysis time for the HPLC separation was less than 20 min per sample. Chitosan is an effective coagulating agent for the treatment of food processing wastes and activated sludge from biological treatment systems. It is manufactured from chitin in shrimp and crab wastes. The rapid methods developed here for determining the MW distribution of chitosan preparations will be used to optimize the manufacturing process and guide the selection of more effective chitosan products. 相似文献
53.
Seven Holtzman rats were kept on a polydipsia-induced schedule of alcohol consumption for 3 months in a replication of a 1972 study by Falk and colleagues. Contrary to their results, there was no evidence of alcoholism. 相似文献
54.
The importance of extrapyramidal and mesolimbic function for circling behaviour was investigated by placing 6-hydroxydopamine (6-OHDA) and electrolesions in the cell bodies, axons and terminals of each system. Circling behaviour was weak when 6-OHDA was placed at the centre of the substantia nigra (SN), but the characteristic contralateral/ipsilateral turning to apomorphine/amphetamine were recorded. Circling was more marked when 6-OHDA was placed anterior to the SN but was generally absent following injections posterior to the SN. However, 6-OHDA placed in the medial forebrain bundle in the lateral hypothalamus resulted in intense contralateral/ipsilateral turning to apomorphine/amphetamine. Generally, the intensity of circling responses was related to the degree of striatal dopamine (DA) depletion but the more effective lesions also caused reductions in mesolimbic DA content. However, circling was not observed following any 6-OHDA injection into the mesolimbic DA system and it is concluded that mesolimbic DA function is not essential for the initiation of circling. In contrast to the 6-OHDA lesions, rats circled ipsilateral to both apomorphine and amphetamine when the SN was damaged by electrocoagulation to cause marked depletion of striatal dopamine. Lesser depletions of striatal dopamine after electrocoagulation in different regions of the medial forebrain bundle were associated with a lower intensity of ipsilateral circling to both drugs. In general, the differences between 6-OHDA and electrolesions could not be explained by additional damage to ascending noradrenaline or 5-hydroxytryptamine pathways. Lower doses of apomorphine were effective in the 6-OHDA circling rats, and the ipsilateral striatum of such rats was more sensitive to directly applied DA. Higher doses of apomorphine were required to produce circling after chronic electrolesions which rendered the ipsilateral striatum insensitive to DA. The contralateral circling to apomorphine after 6-OHDA lesions was abolished by chronic but not by acute electrolesion of the SN. It is suggested that electrolesions of the SN cause different effects to 6-OHDA because they destroy neuronal pathways in addition to the dopaminergic nigrostriatal tract. These appear to be required for the expression of circling behaviour caused by stimulation of the denervated striatum. Whereas 6-OHDA lesions result in super-sensitivity of the denervated strital DA receptors, electrolesions may cause a hypo-sensitivity of the same receptor sites. 相似文献
55.
Commercially available lots of plasma derivatives prepared between 1957 and 1975 were tested for hepatitis B surface antigen (HBsAg) by radioimmunoassay. In all, 69 per cent of lots of plasma protein fraction, 40 per cent of factor IX concentrate, 20 per cent of normal serum albumin, 13 per cent of antihemophilic factor, 3 per cent of fibrinogen, and 0.7 per cent of immune serum globulin lots tested were HBsAg-positive. There was great variation in the prevalence of HBsAg-positive lots of each product among the different manufacturers, reflecting not only differences in methods of processing plasma, but also differences in donor populations. Those manufacturers relying upon volunteer donor plasma or placental source material demonstrated lower rates of HBsAg-positive lots of final products than those relying upon commercial donor plasma. There was a marked decrease in the prevalence of positive lots during the period 1971 to 1973, coincident with the onset of routine plasma donor screening for HBsAg. However, current requirements for plasma screening have not resulted in totally HBsAg-free plasma products. Use of more sensitive and more reliable tests for HBsAg will probably reduce contamination of plasma pools with HBsAg to undetectable levels. Despite HBsAg-status, however, the "high-risk" plasma products (fibrinogen, antihemophilic factor, factor IX concentrate) must still be considered capable of transmitting hepatitis and used only with the strictest indications. 相似文献
56.
57.
58.
P Ahlers RJ Sullivan WE Hammond EL Walter HD Tolley 《Canadian Metallurgical Quarterly》1976,69(10):1328-1330
When patient records are to be auditied in a program designed to assess the quality of medical care rendered, careful consideration must be given to the cost of the system implemented. Structured data collection and a defined treatment plan are advantageous in facilitating the use of nonphysicians for the majority of the audit. A system using checklists to ensure adequate recording of subjective and objective data and a defined treatment plan for a common symptom complex were implemented by physicians and physician's assistants with a patient load averaging more than 1,000 patient contacts per week in a general medical clinic at Duke University Medical Center. Audit was subsequently accomplished at a cost of 96 cents per record. To reduce this cost, more efficient methods of selecting records for audit should be developed. 相似文献
59.
60.
A tract in the chick is described that runs from the posterior nucleus paraventricularis magnocellularis (PVM) through the lateral hypothalamus to a small area of the anterior tegmentum, which is adjacent to the ventral tegmental area. Bilateral lesions of the posterior PVM or of the PVM tract or of its tegmental destination abolish the facilitation of copulation in male chicks by testosterone. The effect is specific in that the facilitation of attack is either unaffected or only slightly depressed by such lesions, while being itself specifically depressed by other hypothalamic lesions. No other hypothalamic tracts were found that were necessary for the facilitation of copulation. An equivalent copulatory system probably exists in the mammal, with a more diffuse distribution in the preoptic area but with similar connections back to the tegmentum. 相似文献