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   1833篇
  免费   78篇
  国内免费   8篇
电工技术   104篇
综合类   3篇
化学工业   406篇
金属工艺   50篇
机械仪表   35篇
建筑科学   45篇
矿业工程   1篇
能源动力   64篇
轻工业   200篇
水利工程   9篇
石油天然气   3篇
无线电   197篇
一般工业技术   371篇
冶金工业   226篇
原子能技术   48篇
自动化技术   157篇
  2023年   8篇
  2022年   36篇
  2021年   59篇
  2020年   25篇
  2019年   46篇
  2018年   38篇
  2017年   33篇
  2016年   57篇
  2015年   47篇
  2014年   71篇
  2013年   95篇
  2012年   102篇
  2011年   142篇
  2010年   99篇
  2009年   90篇
  2008年   102篇
  2007年   88篇
  2006年   65篇
  2005年   70篇
  2004年   61篇
  2003年   53篇
  2002年   40篇
  2001年   34篇
  2000年   22篇
  1999年   30篇
  1998年   102篇
  1997年   52篇
  1996年   36篇
  1995年   20篇
  1994年   24篇
  1993年   26篇
  1992年   10篇
  1991年   8篇
  1990年   11篇
  1989年   10篇
  1988年   15篇
  1987年   8篇
  1986年   13篇
  1985年   11篇
  1984年   10篇
  1983年   8篇
  1982年   5篇
  1981年   2篇
  1980年   5篇
  1979年   3篇
  1978年   7篇
  1977年   8篇
  1976年   6篇
  1975年   3篇
  1966年   1篇
排序方式: 共有1919条查询结果,搜索用时 31 毫秒
971.
Human exposure to methylating agents appears to be widespread, as indicated by the frequent occurrence of methylated DNA adducts in human DNA. The high incidence of methylated DNA adducts even in humans thought not to have suffered extensive exposure to environmental methylating agents implies that chemicals of endogenous origin, probably N-nitroso compounds such as the strongly carcinogenic N-nitrosodimethylamine (NDMA), may be primarily responsible for their formation and raises the question of the carcinogenic risks associated with such exposure. In addition to accumulation of DNA damage, other factors (such as induced cell proliferation) appear to be important in determining the probability of induction of mutation or cancer by NDMA, implying that high to low dose risk extrapolations should not be based on the assumption of dose- or even adduct-linearity. Comparative studies of the accumulation and repair of methylated adducts in humans and animals treated with methylating cytostatic drugs do not reveal significant species differences. Based on this and the dosimetry of adduct accumulation in rats chronically exposed to very low doses of NDMA, it is suggested that the exposure needed to account for the levels of adducts found in human DNA may be of the order of hundreds of micrograms NDMA (or equivalent) per day, a level of exposure which may well represent a significant carcinogenic hazard for man.  相似文献   
972.
We herein report a case of multiple colonic liver metastases treated with repeated percutaneous isolated liver chemoperfusion of cisplatin under hepatic venous isolation and charcoal hemoperfusion (HVI.CHP) as an inductive treatment. The patient was a 70-year-old man who underwent curative resections for metachronous rectal and descending colonic cancer in 1979 and 1995, respectively. During outpatient follow-up, he showed increases of serum CEA level. An abdominal CT study demonstrated multiple low-density lesions in the bilateral lobes of the liver, indicating colonic liver metastases. The patient had a total of 2 treatments with percutaneous isolated liver chemoperfusion of cisplatin (total dose, 500 mg) under HVI.CHP. His post-treatment course was uneventful without side effects, including leukopenia and renal dysfunction. Serum CEA levels showed a sharp decrease to 160 ng/ml 3 months after treatment from the pretreatment value of 1,064 ng/ml. CT scan 3 months after treatment revealed remarkable tumor regression along with liquefaction of liver tumors, showing a partial response. These results suggested that repeated percutaneous isolated liver chemoperfusion of cisplatin under HVI.CHP is a potent inductive treatment for patients with multiple colonic liver metastases.  相似文献   
973.
The crystal transformation method using ethanol as a dehydration medium is a new method which can create a porous crystal. In this study α-cyclodextrin (α-CD) hexahydrate was transformed by this method and porous ethanol dihydrate crystal was obtained. The pore volume measured was 0.25 mL/g and the median pore diameter was 0.11 μm. Dissolution rate of this crystal was several times higher than that of (α-CD) hexahydrate or anhydrate obtained by drying.  相似文献   
974.
Fine oxide–metal composite powders have been prepared under an H2–Ar gas environment using an arc-plasma irradiation technique. Initial Cu particles with grain sizes between 10 and 30 μm were mixed with TiO2 powder of submicron size at different weight ratios to form targets by sintering. Mixtures with varying Cu/TiO2 weight ratios were prepared, but mostly a mixture with the weight ratio at 95:5 was used. The targets were exposed to arc-plasma irradiation to form a fine composite powder. Particles prepared are Cu-fine core particles each coated with a thin film of TiO2. The thickness of the oxide film ranges from a few nanometers to some tens of nanometers, and is independent of the ambient gas composition and irradiation. The size distribution of prepared composite particles is broad, and most of them range from 20 to 100 nm. H2 gas in the chamber plays a significant role in forming the composite powder.  相似文献   
975.
The cold-rolled and recrystallization textures of Fe-Cr-Co-Mo permanent magnet alloys are described. The studied composition is Fe-30%Cr-15%Co-3%Mo (in wt.%). The cold-rolled texture can be considered to be {111}<110>, {111}<112>, {100}<110>, and {211}<110>, while the recrystallization texture can be considered to be {111}<100>, {110}<112>, {211}<110>, and {110}<110>. The secondary recrystallization is caused by heat-treating the alloys in the sequence of α, α+γ, α+γ+σ, α phase region. This results in a favorable texture of {110}<110> and <100> direction, aligning along the transverse direction (TD) of the strips. The best magnetic properties obtained in this study were 1.2 T (12.0 kG), iH c=82.0 kAm-1 (1025 Oe), and (BH)max= 60.8 kJm-3 (7.6 MGOe) with TD alloys  相似文献   
976.
OBJECTIVE: This study is based on promising results using the ratio of free/ total (F/T) prostatic-specific antigen (PSA) for discrimination between benign prostatic hypertrophy and prostate cancer. We tried to determine the value of F/T PSA in different clinical situations at a certain time point during follow-up of luteinizing-hormone-releasing hormone (LH-RH) agonist treatment and to correlate it to T-PSA. PATIENTS AND METHODS: 182 patients followed-up for different periods in the last 3 years were routinely monitored for serum T-PSA. During the last 11 months, F-PSA was also measured together with T-PSA, and the ratio of F/T PSA was calculated. In 26 patients, the ratio of F/T PSA was monitored sequentially in several samples. RESULTS: Although 5 patterns of clinical response to LH-RH agonists were identified according to previous T-PSA, the F/T ratio could significantly (p < 0.05) discriminate between patients responding to treatment in contrast to patients escaping, fluctuating or not responding to hormonal ablation. Those patients responding to hormones showed a higher F/T PSA ratio (36.5 +/- 33.1%) compared to the nonresponding group (12.0 +/- 10.1%). CONCLUSIONS: During individual follow-ups, the pattern of response to LH-RH treatment is reflected by the F/T PSA ratio: while successful treatment causes an F/T PSA increase, relapse is accompanied by a decrease in this ratio. However, the changes in the F/T PSA ratio did not precede the indicative changes in T-PSA. It seems that increased values of F/T PSA ratios are intrinsic features of 'benign' prostatic disease, and the molecular events resulting in different PSA molecules in various clinical situations have to be elucidated.  相似文献   
977.
978.
Hydrogen-plasma treatment of the polychlorotrifluoroethylene (PCTFE) film produced by radio-frequency sputtering induces considerable elimination of halogen (fluorine and chlorine) atoms. This dehalogenation produces some noteworthy structural features: surface densification (crosslinks), trapped radicals, and incorporation of oxygen in the carbon networks. These structural changes modify the vapor-sorption properties of the sputtered PCTFE film, producing a molecular-sieving effect and enhancing the polar character of solubility properties of organic vapors.  相似文献   
979.
We have reported a inhibitor of acid lipases in liver lysosomes and erythrocytes from chickens [M. Fujii et al., Int. J. Biochem., 22, 895-898 (1990)]. In this paper, the properties of the inhibitor were described in comparison with those of apo A-I of chicken. The purified inhibitor migrated with the same mobility on SDS-PAGE as apo A-I, and had a molecular weight of 27,000. The peptide map from the lipase inhibitor was similar to that of apo A-I. Antibodies to the acid lipase inhibitor also reacted with apo A-I. Apo A-I inhibited the acid lipase activities of liver lysosomes and erythrocytes from chickens as strongly as the lipase inhibitor. The N-terminal amino acid sequence of lipase inhibitor was identical to that of apo A-I as far as residue 20. The amino acid sequence of peptides obtained from the inhibitor by cleavage with CNBr corresponded to internal sequence of apo A-I, and so the CNBr-peptides were derived by cleavage after the methionine residues in apo A-I. The findings showed that the inhibitor of the acid lipases in liver lysosomes and erythrocytes from chickens was identical to apo A-I.  相似文献   
980.
Electronic-optical conversion efficiency of 11.4% in a vertical-to-surface-transmission electrophotonic device with a vertical cavity is reported. Reduction of the electrical resistance by the double mesa structure, efficient confinement of carriers in the active region by the proton implanted structure, and photon recycling by sidewall reflectors lead to this high conversion efficiency. Efficiency over 10% is achieved for the first time in surface-emitting devices  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