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   14539篇
  免费   412篇
  国内免费   36篇
电工技术   206篇
综合类   21篇
化学工业   2201篇
金属工艺   314篇
机械仪表   330篇
建筑科学   526篇
矿业工程   28篇
能源动力   305篇
轻工业   1767篇
水利工程   112篇
石油天然气   37篇
无线电   1674篇
一般工业技术   2167篇
冶金工业   3292篇
原子能技术   179篇
自动化技术   1828篇
  2023年   68篇
  2022年   131篇
  2021年   273篇
  2020年   170篇
  2019年   244篇
  2018年   273篇
  2017年   263篇
  2016年   333篇
  2015年   194篇
  2014年   346篇
  2013年   697篇
  2012年   542篇
  2011年   665篇
  2010年   537篇
  2009年   513篇
  2008年   575篇
  2007年   546篇
  2006年   441篇
  2005年   448篇
  2004年   346篇
  2003年   343篇
  2002年   320篇
  2001年   320篇
  2000年   308篇
  1999年   344篇
  1998年   1074篇
  1997年   669篇
  1996年   492篇
  1995年   366篇
  1994年   297篇
  1993年   309篇
  1992年   176篇
  1991年   152篇
  1990年   157篇
  1989年   151篇
  1988年   145篇
  1987年   116篇
  1986年   124篇
  1985年   160篇
  1984年   112篇
  1983年   101篇
  1982年   85篇
  1981年   101篇
  1980年   93篇
  1979年   98篇
  1978年   53篇
  1977年   117篇
  1976年   162篇
  1975年   55篇
  1973年   61篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Excision of perianal fistulas using a 1.064 micron wavelength neodymium:yttrium aluminum garnet (ND:YAG) contact tipped laser with primary wound closure was used to treat 20 dogs with perianal fistulas. Overall, 19 of 20 (95%) dogs had resolution of fistulas after one or more ND:YAG treatments. The period of resolution ranged from 10 to 42 months with a mean of 22.9 months. Sixteen of 20 (80%) dogs had resolved fistulas after one laser excision. Three of the four recurrences underwent additional laser treatments with successful results. The total number of laser procedures ranged from one to three with a mean of 1.2 procedures. Postoperatively, anal tone as judged by digital rectal examination was reduced in about 60% of the cases, but clinical evidence of fecal incontinence only occurred in four of 20 cases. This was managed effectively with diet modification. The tendency toward loss of anal tone or fecal incontinence depended on the severity of preexisting anal stenosis. On a client survey, 19 of 20 owners believed that their animals experienced less pain during defecation after surgery and rated the results as excellent or good. The overall success rate using ND:YAG laser excision compared very favorably with previously reported studies of other methods of treatment for perianal fistulas in dogs.  相似文献   
92.
The present study aims at the role of ferritin in the regulation of syncytiotrophoblast free iron levels. The differentiated cytotrophoblast cell in culture is used as a model for this maternal-fetal interface. Cytotrophoblast cells isolated from term placentae are cultured in iron-poor (Medium 199), iron-depleted [desferrioxamine(DFO)] and iron-supplemented [diferric transferrin (hTF-2Fe), ferric ammonium citrate (FAC)] medium. Distribution and de novo synthesis of isoferritins is studied, together with the cellular iron concentration and the ferritin iron saturation. Compared to ferritin isolated from total placenta, ferritin obtained from villous tissue is enriched with acidic isoforms. This observation is in agreement with measured light (L) to heavy (H) subunit ratios < 1 of de novo synthesized ferritin in cultured cytotrophoblast cells. Neither iron-poor culture medium, nor hTf-2Fe supplemented medium affects the cellular iron or ferritin concentration. FAC increased the cellular ferritin iron saturation and (by synthesis) the acidic isoferritin concentrations. The results strongly suggest, that the term syncytiotrophoblast is able to balance transferrin-mediated iron uptake and iron release. In case of FAC supplementation, the syncytiotrophoblast is unable to keep intracellular iron low, and ferritin synthesis is stimulated. The predominance of acidic ferritins and the preferential synthesis of H subunits can be functionally explained by the established fact that iron incorporation in acidic ferritins is faster due to the presence of ferroxidase centres. Damage by free iron catalysed hydroxyl radical formation is therefore minimized.  相似文献   
93.
94.
The bone response to different calcium phosphate (Ca-P) coated implants was evaluated in a goat animal model. Two types of plasma spray coatings were applied to a commercially pure titanium (cpTi) tapered, conical screw-design implant (BioComp®); hydroxyapatite (HA-PS) and a dual coating, consisting of FA and HA (FA/HA-PS). In addition an amorphous RF magnetron sputter coating (Ca-P-a) and uncoated implants were investigated. Forty-eight implants were inserted in the maxilla of 12 adult female goats. After implantation periods of 3 and 6 months, the bone implant interface was evaluated histologically and histomorphometrically. After both implantation periods all plasma spray coated implants were maintained. On the other hand three Ca-P-a and two cpTi implants were lost. Histological examination revealed a better bone response to both plasma spray coated implants. Histomorphometrical evaluation confirmed this finding. At 3 and 6 months significantly higher percentages of bone contact (p<0.001, ANOVA) were measured for both plasma spray coated implants than for the cpTi and Ca-P-a implants, while no significant difference (p<0.05) existed between both implantation periods. Degradation of both plasma spray coatings was observed. Supported by the results, it is concluded that, although Ca-P coatings can improve the performance of dental implants, the presence of a Ca-P coating is not the only important factor for bone healing around implants placed in low density trabecular bone.  相似文献   
95.
This study was undertaken with two objectives: 1) to determine whether the effect of excess dietary protein on intrauterine pH in cattle is specific to the uterus or manifested in other bodily fluids and 2) to determine whether the effect of excess ruminally degradable protein on uterine pH can be ameliorated by substitution with a less-degradable protein source. Thirty-six Holstein cows in early lactation were fed isoenergetic total mixed rations that either 1) met undegradable intake protein (UIP) and degradable intake protein (DIP) requirements (Balanced), 2) met DIP requirements and exceeded UIP requirements by 25% (High UIP), or 3) met UIP requirements and exceeded DIP requirements by 25% (High DIP). After diets had been fed > or = 2 wk, uterine, blood, salivary, and urinary pH and plasma urea nitrogen were determined at estrus (d 0) and d 7. Plasma urea nitrogen (mg/dL) was not different between estrus and d 7 but was significantly affected by diet (Balanced, 16.1 +/- 2.3; High UIP, 19.2 +/- 1.6; High DIP, 22.3 +/- 2.6; P < .05). There was no effect of treatment on the pH of any fluid measured at estrus: intrauterine, blood, salivary, and urinary pH averaged 6.84 +/- .05, 7.39 +/- .01, 8.30 +/- .05, and 8.15 +/- .05, respectively. In contrast, on d 7, uterine pH was significantly lower in both high-protein groups, regardless of protein degradability (Balanced, 7.13 +/- .05; UIP, 6.95 +/- .04; DIP, 6.85 +/- .05; P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)  相似文献   
96.
97.
The specific surface areas (A) of different clay mineral powders were measured by both the BET method and by thin layer wicking. The values of A for the BET and the wicking experiments coincided within a few percent. Thus, the simple and inexpensive thin layer wicking approach may well suffice to obtain reliable specific surface area values for most powders. From the wicking data it is also possible to obtain a rough estimate of the average particle size.  相似文献   
98.
We report a previously undescribed ultrastructural calcification of glomerular immune complex depositions in a patient with lupus nephritis. Such processes give rise to a distinctive ultrastructural calcified microspheriol in electron-dense immune deposits, and were associated with hypercalcemia and nephrocalcinosis. The cause of hypercalcemia in this patient was uncertain, but related to a granulomatous inflammation and abscess of the breast. Whereas ultrastructural microcalcification was associated with hypercalcemia, its distribution in the segmental glomerular lesions suggests that an inflammatory process was contributory. The clinical follow-up indicates that the microcalcifications of immune complexes have not affected the outcome of nephritis and renal function.  相似文献   
99.
100.
The finite element analysis method is used directly in optimisation algorithms to optimise in multidimensions the design of the cageless reluctance synchronous machine. Two optimisation methods are evaluated to minimise or maximise the function value. These are the direction set method of Powell and the quasi-Newton algorithm. Both methods proved to be successful, with some advantages and disadvantages. Using these methods at a power level below 10 kW, results are given of structures of the reluctance synchronous machine which have been optimised according to specific criteria. Calculated and measured results show that the maximum torque optimum designed reluctance synchronous machine has the advantages of high power density and high efficiency  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