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   1056篇
  免费   18篇
  国内免费   1篇
电工技术   8篇
综合类   1篇
化学工业   102篇
金属工艺   20篇
机械仪表   20篇
建筑科学   16篇
能源动力   15篇
轻工业   28篇
水利工程   5篇
石油天然气   1篇
无线电   72篇
一般工业技术   108篇
冶金工业   540篇
原子能技术   7篇
自动化技术   132篇
  2024年   3篇
  2023年   9篇
  2022年   20篇
  2021年   22篇
  2020年   30篇
  2019年   26篇
  2018年   20篇
  2017年   16篇
  2016年   27篇
  2015年   14篇
  2014年   16篇
  2013年   21篇
  2012年   29篇
  2011年   28篇
  2010年   13篇
  2009年   28篇
  2008年   23篇
  2007年   26篇
  2006年   9篇
  2005年   20篇
  2004年   13篇
  2003年   13篇
  2002年   5篇
  2001年   3篇
  2000年   4篇
  1999年   11篇
  1998年   171篇
  1997年   82篇
  1996年   75篇
  1995年   42篇
  1994年   38篇
  1993年   47篇
  1992年   11篇
  1991年   13篇
  1990年   12篇
  1989年   13篇
  1988年   13篇
  1987年   10篇
  1986年   6篇
  1985年   5篇
  1983年   3篇
  1982年   9篇
  1981年   5篇
  1980年   6篇
  1978年   2篇
  1977年   13篇
  1976年   40篇
  1975年   2篇
  1974年   3篇
  1973年   2篇
排序方式: 共有1075条查询结果,搜索用时 15 毫秒
1.
2.
Upconversion phosphors are known as a material system that can convert near-infrared light into visible/ultraviolet emissions by sequentially absorbing multiple photons. The studies on upconversion materials often use two rare earth (RE) ions as a sensitizer-activator pair. We investigated the influences on luminescence intensity depending on Cr-doping content (x) of hexagonal NaLu0.98–xCrxF4Er0.02 (x = 0–0.9) upconversion material by substituting Lu3+ ions with Cr3+in the absence of Gd3+. The change in upconversion luminescence intensity appears with saddle-like shape. We suggest that Cr3+ ions play the dual role as a constituent in host lattice and a sensitizer in the upconversion process. Optimal conditions for gaining the strongest upconversion emission correspond to x = 0.3–0.5, where there are effective energy transfers between Cr3+ and Er3+ ions and CrEr dimers. Apart from these values, the emission intensity decreases rapidly which can be ascribed to the absence of multiple-photon absorption for the case of low Cr3+ contents, and to the coupling between Cr3+ and/or Er3+ ions for the case of high Cr3+ contents. Magnetization and electron-spin-resonant measurements were performed to understand the correlation between the optical and magnetic properties.  相似文献   
3.
4.
In summary, recent literature indicates that the complications of chronic otitis media have been decreasing. However, even with the advent of modern and more powerful antimicrobials and aggressive surgical eradication of disease, the morbidity and mortality are still high. Some complications may initially be quite obvious and some complications may be quite subtle. Therefore, the most important tools in making early diagnosis are careful history and physical examination, and a high index of suspicion for impending complications.  相似文献   
5.
Intracellular and whole-cell patch-clamp recordings were used to evaluate the actions of different metabotropic glutamate receptor (mGluR) agonists on the synaptic inputs evoked on principal cells of the rat mesencephalon. Bath application of the group III mGluR agonists L-2-amino-4-phosphonobutyric acid (L-AP4) and L-serine-O-phosphonobutanoate (L-SOP) did not change the holding current of the cells held at resting potential (-60 mV) but produced a dose-dependent inhibition of the amplitude of the excitatory and inhibitory events. L-AP4 and L-SOP were more effective at inhibiting the excitatory postsynaptic currents (EPSCs) than the GABA(A) and GABA(B) inhibitory postsynaptic currents (IPSCs). The suppressing effects of L-AP4 and L-SOP were antagonized by (S)-2-amino-2-methyl-4-phosphonobutanoic acid (MAP-4) but not by +/- -alpha-methyl-4-carboxyphenylglycine (MCPG). Moreover, the group II agonist (2S,1'S,2'S)-(carboxycyclopropyl)glycine (L-CCG1) and the group I agonist (RS)-3,5-dihydrophenylglycine (3,5-DHPG) depressed in a dose-related manner the EPSC, the GABA(A) IPSC and the GABA(B) IPSC. The suppressing effect of the two mGluRs agonists was partially antagonized by MCPG but not by MAP-4. In addition, both L-CCG1 and 3,5-DHPG caused an inward shift of the holding current. To characterize the site of action of the metabotropic receptor agonists, experiments were performed to examine the amplitude and ratio of EPSC and GABA(A) IPSC pairs. The increase of the s2/s1 ratio caused by the agonists suggests that the location of the inhibitory mGluRs was presynaptic. These results indicate that the activation of presynaptic mGluRs controls the release of excitatory and inhibitory transmitters on presumed dopaminergic cells within the ventral mesencephalon.  相似文献   
6.
Segmental colonic transit has been measured in 101 patients. Two MBq of 111Indium absorbed on resin pellets and encapsulated in an enteric coated capsule was given at 7 00 am. Hourly images during the first day, and three images during each subsequent day were acquired for up to three days. Using all scan and patient data the scans were categorised in one of the five patterns of colonic transit: normal, rapid, right delay, left delay, or generalised delay. The geometric centres and per cent activity at each time point was compared between the five groups of colonic transit patients to find the best time for imaging and so to distinguish the five groups. During the first day, early images did not help in diagnosis of patterns of transit, however, in the later images (six hours onwards after the ingestion of the activity) the rapid transit groups could be identified. Images at 27 and 51 hours were both required to distinguish all five groups of patients from each other. Only in the 'normal' transit patients was there some excretion of the activity during the course of the second day, otherwise there was no difference in the images taken in the course of a day (second or third day). A simplified protocol requires a minimum of three images to distinguish all five patterns of colonic transit. The activity should be ingested in the morning (7 00 am) and the first image taken at the end of the working day (8-10 hours after ingestion), the second image on the morning of the second day, and the third image during the course of the third day. This simple protocol would provide all the clinically relevant information necessary for correct classification of the colonic transit.  相似文献   
7.
To be efficient, the control of alumina feeding of the electrolytic cell must be based on cell resistance, alumina concentration, and cell state. Most control schemes now in use are based on cell resistance only, and, thus, constitute an open-loop control that lacks robustness because their decision criteria are not explicitly tied to concentration nor to cell state. This results in the cell operating at nonoptimal concentrations, and cell efficiency is diminished. An optimal operation requires a knowledge of concentration and an adjustment of the decision criteria as a function of concentration. A learning vector quantization (LVQ) type of neural network was built and trained to recognize the cell state. Knowing the state of the cell and its resistance, concentration can be estimated using predetermined regression functions. The decision criteria for the control logic are then consequently adapted. A closed-loop control scheme is thus obtained. Results show that, with its control so structured, the cell can operate at or near optimal concentrations independently of its state. This flexible and intelligent character of the neural control can provide a considerable advantage as compared to the standard control.  相似文献   
8.
We examined 59 breast cancers for p53 and bcl-2 protein expression by immunohistochemistry. The results were correlated with Ki-67 immunostaining. p53-negativity was noted in 40 cases and the remaining 19 tumours were p53-positive. Thirty-six tumours showed strong expression of bcl-2 and in 23 no staining for this protein was observed. We found statistically significant reverse correlation between expression of p53 and bcl-2 in majority of carcinomas: 31 cases were bcl-2 positive and p53-negative, and 14 tumours were bcl-2-negative and p53-positive. Six carcinomas showed no nuclear staining for Ki-67 and in the remaining 53 the percent of cancer cells positive for Ki-67 ranged from 1 to 60 (mean: 14.6). In these 53 cases we found that bcl-2-positive tumours were characterized by lower proliferation than bcl-2-negative tumours, the mean value of Ki-67 immunostaining being 10.7% and 23.0%, respectively. p53-negative tumours showed lower proliferation than p53-positive tumours: mean Ki-67 index was 10.2% and 23.9%, respectively. We conclude that immunohistochemically detected p53 and bcl-2 proteins show a significant inverse relationship in majority of breast carcinomas and their expression correlates with tumour proliferation (Ki-67 immunostaining).  相似文献   
9.
HBM ESD tests on two types of 0.6 μm DRAM devices showed that internal circuit or output driver failures would occur after the input or I/O pins were ESD stressed negative with respect to Vcc at ground. These failures occurred at lower than expected ESD stress voltages due to power-up circuit interactions that either turned-on unique internal parasitic ESD current paths or disrupted the normal operation of the output pin’s ESD protection circuit. ESD analysis found there exists a set of power-up sensitive circuits and if placed near a Vcc bond pad can result in low voltage ESD failures.  相似文献   
10.
A patient who developed a mixed neuroendocrine carcinoma and adenocarcinoma at the site of a previous long-standing ileostomy is reported. The neuroendocrine features are documented by both ultrastructural and immunocytochemical findings. Carcinoma arising in an ileostomy site is rare but has been recorded in patients with long-standing ileostomies after colectomy for chronic inflammatory bowel disease, as in this patient. Neuroendocrine carcinoma developing in this setting apparently has not been described before, however.  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