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   2675篇
  免费   35篇
  国内免费   1篇
电工技术   129篇
化学工业   508篇
金属工艺   132篇
机械仪表   88篇
建筑科学   28篇
矿业工程   14篇
能源动力   93篇
轻工业   128篇
水利工程   6篇
无线电   249篇
一般工业技术   361篇
冶金工业   742篇
原子能技术   49篇
自动化技术   184篇
  2023年   8篇
  2022年   29篇
  2021年   48篇
  2020年   19篇
  2019年   19篇
  2018年   27篇
  2017年   19篇
  2016年   34篇
  2015年   24篇
  2014年   51篇
  2013年   111篇
  2012年   70篇
  2011年   87篇
  2010年   57篇
  2009年   80篇
  2008年   98篇
  2007年   103篇
  2006年   72篇
  2005年   74篇
  2004年   53篇
  2003年   59篇
  2002年   76篇
  2001年   47篇
  2000年   49篇
  1999年   85篇
  1998年   287篇
  1997年   199篇
  1996年   103篇
  1995年   95篇
  1994年   76篇
  1993年   74篇
  1992年   32篇
  1991年   45篇
  1990年   38篇
  1989年   31篇
  1988年   24篇
  1987年   26篇
  1986年   19篇
  1985年   31篇
  1984年   25篇
  1983年   16篇
  1982年   19篇
  1981年   17篇
  1980年   19篇
  1979年   22篇
  1978年   16篇
  1977年   28篇
  1976年   29篇
  1975年   6篇
  1973年   8篇
排序方式: 共有2711条查询结果,搜索用时 31 毫秒
991.
BACKGROUND: The incidence of abdominal aortic aneurysm (AAA) has increased steadily during the past 30 years. METHODS: Trends in the incidence and surgical intervention for AAA in Western Australia were reviewed for the interval 1985-1994. A population-based health database was used to link morbidity and mortality records of all patients aged 55 years or more who died from rupture or were admitted and treated surgically for AAA. Three groups were separated for analysis: patients with a ruptured AAA, those admitted for elective repair and those admitted as an emergency with an acute (non-ruptured) aneurysm. RESULTS: There was a decline in the incidence of both emergency and elective procedures for AAA after 1992. While the mortality rate from ruptured AAA has also fallen since 1991, the overall case fatality rate for ruptured AAA has fallen by only 1.3 per cent (from 80.7 to 79.3 per cent). CONCLUSION: The decline in mortality rate and emergency procedures may result from a fall in the incidence of ruptured AAA, due to an increasing rate of elective surgery before 1992. The decline in elective procedures from 1992 may be due to a fall in the prevalence of AAA owing to high rates of elective surgery, or to a fall in the incidence of the disease itself.  相似文献   
992.
We focus here upon regulation by the notochord of myocardial cell fate in zebrafish. Myocardial precursors, defined by lineage tracing in the living embryo, are in the lateral plate mesoderm adjacent to the notochord-prechordal plate junction. Interestingly, the anterior end of the notochord corresponds to the posterior extent of the heart progenitor field, defined by this lineage analysis. This suggested that the notochord might suppress, or the prechordal plate might enhance, the cardiogenic fate. Nkx2.5 expression is, in the zebrafish embryo, closely correlated with the position of myocardial precursors, which reside adjacent to the notochord-prechordal plate junction. This expression, however, is extinguished in the region posterior to this junction, a region normally not contributing cells to the heart. Laser ablation of the notochord tip between the 4-somite and 12-somite stage causes posterior expansion of the Nkx2. 5-expressing region. The ntl mutation of the notochord is associated with posterior extension of Nkx2.5 expression. Lineage tracking, by laser activation of caged fluoresceinated dextran, confirms that, normally, lateral plate cells next to the notochord do not contribute progeny to the heart. After anterior notochord ablation, these cells are redirected to a heart cell fate. These data suggest that the anterior notochord delimits the posterior extent of the heart field by suppressing the heart cell fate.  相似文献   
993.
We established a co-culture system with a monolayer of the murine bone marrow (BM) stroma cell line, MS-5, in which human cord blood CD34+ cells differentiated to CD19+ cells. The addition of stem cell factor (SCF) and granulocyte colony-stimulating factor (G-CSF) highly enhanced the production of CD19+ cells. The expansion of the cell numbers was over 10(3)-fold. Furthermore, a significant proportion (<45%) of the cells expressed surface IgM (sIgM) after 5 weeks of co-culture. CD34+CD19- cells also showed a similar development of CD19+ cells and CD19+sigM+ cells. Filter separation of MS-5 cells and CD34+ cells did not inhibit the growth of CD19+ cells. However, when further purified CD34+CD19-CD13- CD33- cells were cultured in the presence of MS-5 cells with or without a separation filter, CD19+ cells did not appear in the non-contact setting. This result suggested that the highly purified CD34+CD19-CD13-CD33- progenitors require the cell-cell contact for the development of CD19+ cells, whereas other CD34+ fractions contain progenitors that do not require the contact. This co-culture system should be useful for the study of early human B-lymphopoiesis.  相似文献   
994.
Both high and low affinity sulfonylurea receptors (SURs) reside on glucose responsive neurons where they influence cell firing and neurotransmitter release via the adenosinetriphosphate (ATP)-sensitive K+ (katp) channel. Here, the effect of diabetes on [3H] glyburide binding to SURs was assessed in male obesity-resistant Sprague-Dawley rats rendered diabetic with streptozotocin (65 mg/kg, i.p.). Additional streptozotocin-treated rats were supplemented with insulin (1.5 U/kg/ day). Streptozotocin reduced plasma insulin to 13% of control associated with hyperglycemia (25.3 +/- 1.7 mmol/l), while insulin lowered plasma glucose (9.56 +/- 1.78 mmol/l) to near control levels (7.65 +/- 0.22 mmol/l). Over 7 days, all streptozotocin-treated rats lost 12% of their initial body wt. while controls gained 1%. Despite equivalent wt. loss, streptozotocin-induced diabetes selectively increased high affinity [3H] glyburide binding in the hypothalamic dorsomedial nuclei (DMN) and ventromedial nuclei (VMN) and lateral area (LH). This was prevented by insulin injections. Low affinity binding was similarly increased in the DMN and VMN, as well as two amygdalar subnuclei but decreased in the substantia nigra, pars compacta. Insulin fully prevented these changes only in the DMN and one amygdalar nucleus and the substantia nigra. Therefore, binding to (SURs) appears to be generally upregulated in the face of hypoinsulinemia with hyperglycemia and this is prevented by insulin treatment. These and other data suggest that this combination of abnormalities in diabetes should have an adverse effect on the glucose sensing capacity of the brain.  相似文献   
995.
996.
A thin 3 m diameter × 5 m, 1.5 T superconducting solenoid for the Fermilab collider detector facility (CDF solenoid) was constructed. Cool-down and excitation tests of the solenoid were carried out. The design current is 5000 A and the stored magnetic energy is 30 × 106 J. The solenoid utilizes the forced flow cooling method of two-phase helium and does not have a permanent inner bobbin. The material thickness of the solenoid is 0.85 radiation length in the radial direction. An aluminum-stabilized NbTi/Cu superconductor fabricated with the EFT method was used. Radially outward magnetic forces must be supported with an outer support cylinder shrink-fitted outside the coil. The helium cooling tube of 20 mm in inner diameter and about 140 m in length was welded to the outer support cylinder.The maximum excitation current was limited to 2800 A in the present tests without an iron return yoke. Thermal response of the solenoid during the cool-down and excitation tests was very steady. A series of heater quench tests was attempted by using a heater installed at the outer support cylinder. The solenoid did not quench even for a heater input of about 10 kJ. In a warm-up test the liquid helium supply was shut off. The coil stayed superconducting for about 90 min and then the entire coil became normal very uniformly. This result is consistent with the measured heat load of the solenoid of about 35 W. The results of the present tests indicate the excellent thermal stability of the solenoid.  相似文献   
997.
Upper and lower bounds for the trace of the solution of the Lyapunov matrix differential equation are derived. It is shown that they are obtained as solutions to simple scalar differential equations. As a special case, the bounds for the stationary solution give ones for the solution to the Lyapunov algebraic equation.  相似文献   
998.
The results of specific heat and susceptibility measurements on PrNi2 over the temperature region from 12 mK to 80 K are reported. Anomalies in the susceptibility and the magnetic relaxation time are found at 0.25 K. The specific heat starts to increase around 0.5 K, much higher than 0.25 K, and it continues increasing at the lowest measured temperature of 30 mK. The entropy estimation shows that the specific heat anomaly below 1 K is predominantly nuclear in origin. Data are discussed in terms of a mixed nuclear-electronic transition at 0.25 K.  相似文献   
999.
Recently, a controller parametrization method without coprime factorizability was presented by Mori. In this note, we present this method using elementary matrix operations. Unlike in the earlier paper, good knowledge of commutative algebra and module theory is not required.  相似文献   
1000.
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