全文获取类型
收费全文 | 6629篇 |
免费 | 239篇 |
国内免费 | 5篇 |
专业分类
电工技术 | 75篇 |
综合类 | 5篇 |
化学工业 | 1169篇 |
金属工艺 | 94篇 |
机械仪表 | 121篇 |
建筑科学 | 203篇 |
矿业工程 | 17篇 |
能源动力 | 154篇 |
轻工业 | 745篇 |
水利工程 | 75篇 |
石油天然气 | 19篇 |
无线电 | 427篇 |
一般工业技术 | 1147篇 |
冶金工业 | 1795篇 |
原子能技术 | 59篇 |
自动化技术 | 768篇 |
出版年
2023年 | 31篇 |
2022年 | 58篇 |
2021年 | 122篇 |
2020年 | 90篇 |
2019年 | 95篇 |
2018年 | 115篇 |
2017年 | 114篇 |
2016年 | 137篇 |
2015年 | 99篇 |
2014年 | 151篇 |
2013年 | 329篇 |
2012年 | 269篇 |
2011年 | 348篇 |
2010年 | 273篇 |
2009年 | 272篇 |
2008年 | 316篇 |
2007年 | 313篇 |
2006年 | 231篇 |
2005年 | 235篇 |
2004年 | 210篇 |
2003年 | 191篇 |
2002年 | 192篇 |
2001年 | 127篇 |
2000年 | 113篇 |
1999年 | 125篇 |
1998年 | 252篇 |
1997年 | 188篇 |
1996年 | 167篇 |
1995年 | 141篇 |
1994年 | 127篇 |
1993年 | 130篇 |
1992年 | 92篇 |
1991年 | 56篇 |
1990年 | 70篇 |
1989年 | 72篇 |
1988年 | 63篇 |
1987年 | 60篇 |
1986年 | 70篇 |
1985年 | 83篇 |
1984年 | 59篇 |
1983年 | 47篇 |
1982年 | 37篇 |
1981年 | 47篇 |
1980年 | 31篇 |
1979年 | 40篇 |
1978年 | 35篇 |
1977年 | 54篇 |
1976年 | 48篇 |
1975年 | 31篇 |
1972年 | 34篇 |
排序方式: 共有6873条查询结果,搜索用时 15 毫秒
81.
BACKGROUND: The authors described unusual pathologic features in a left frontal lobe malignant glioma in a 31/2-year-old boy. The pathology was similar in the initial excision and two subsequent recurrences at 9 and 11 months and at autopsy, when extensive subarachnoid spread was noted. METHODS: The tumor was studied by conventional histology, immunohistochemistry, flow cytometry, transmission electron microscopy (TEM), immune electron microscopy (IEM), and cytogenetic and Southern blot analysis. RESULTS: The tumor revealed two different histologic patterns. One component showed large cells with eosinophilic cytoplasm, vesicular nuclei with prominent nucleoli, eosinophilic perinuclear inclusions, and immunoreactivity for glial fibrillary acidic protein (GFAP) and vimentin. The other component consisted of undifferentiated cells with hyperchromatic nuclei and scanty cytoplasm. By TEM, the perinuclear aggregates were composed of tubuloreticular inclusions, which were also observed in endothelial cells within the tumor vasculature. By IEM, the intermediate filaments in the tumor cell cytoplasm were decorated with GFAP. Flow cytometric results revealed a marked increase in the S-phase (48%), whereas cytogenetic analysis of short-term cultures showed an abnormal karyotype containing marker chromosomes and double minutes. In the second resection, additional karyotypic abnormalities were noted, including 1p- and several additional markers. The first and second resections showed MYCN amplification by Southern Blot analysis in the 60- to 80-fold range. CONCLUSIONS: This tumor presents unique histologic, ultrastructural, and cytogenetic findings as well as MYCN amplification that is notable for a pediatric malignant glioma. Tubuloreticular inclusions were a prominent feature in this tumor, which again is unique for a glioma. 相似文献
82.
WH Newman LM Zhang MR Castresana MG Currie BF Becker 《Canadian Metallurgical Quarterly》1994,54(18):1351-1356
Atrial natriuretic peptide (ANP) and sodium nitroprusside (SNP) increase cGMP in vascular smooth muscle cells and act as vasodilators in some, but not all, blood vessels. In this present study, we attempted to correlate the ability of these two agents to dilate blood vessels with the ability to increase cGMP in cultured vascular smooth muscle cells. In the isolated guinea pig heart, SNP dose-dependently increased coronary flow while ANP was ineffective. In smooth muscle cells cultured from the coronary system, SNP increased intracellular cGMP in a dose-dependent manner while ANP had no effect on cGMP in these cells. In isolated guinea-pig thoracic aorta, precontracted with K+, both ANP and SNP produced relaxation and ANP was the more potent. In smooth muscle cells cultured from the aorta, ANP and SNP increased cGMP and the potency relationship was similar to the intact vessel. These results support the view that phenotypic properties of vascular smooth muscle cells can account for differences in the response of blood vessels to vasodilators. 相似文献
83.
BACKGROUND: Deaths exhibit a seasonal pattern in most parts of the world. Analyses of deaths for the years 1972-1974 from the vital registration system of Matlab, Bangladesh, published in this journal 17 years ago, showed sinusoidal seasonal patterns. As death rates have declined in other nations, the seasonal pattern is attenuated. Death rates have declined substantially in Bangladesh in the past two decades. Thus, the present study examines monthly counts of deaths from Matlab data for a period 15 years later and tests the hypothesis of a decrease or shift in seasonality over time. METHODS: Trigonometric regression models were fit to monthly data by age and cause of death from the Matlab vital registration system for the years 1982-1990. A total of 20,328 death records were available for analyses. RESULTS: In the recent period significant sinusoidal seasonal patterns are found in all but one of the age and cause of death groups. Total deaths peak in the winter as do neonatal deaths but post-neonatal and child deaths are maximum in April and July respectively. Among cause groups, injury deaths (mostly attributed to drowning) show the greatest seasonal swing. The time of peak has only shifted for one age group--neonates--since the 1972-1974 period. The magnitude of the seasonal swing has declined significantly only for the neonatal age group and injury cause of death group. CONCLUSION: Marked seasonal patterns of deaths persist in the Matlab area of Bangladesh even as the level of mortality has declined. 相似文献
84.
Hospitals are increasingly attempting to integrate their operations with physician constituents through the development of hospital-physician joint ventures and acquisition of physician practices. This article provides an overview of valuation-related issues that result with respect to such affiliation efforts and guidance to assist the health care financial and legal professional in resolving such issues consistent with the business objectives of the transaction parties. 相似文献
85.
VL Beggs SB Willis EL Maislen TM Stokes D White M Sanford A Becker S Barber PC Pawlow C Downs 《Canadian Metallurgical Quarterly》1998,12(4):72-86
Short hospitalizations for patients undergoing coronary artery bypass grafting (CABG) require continuous nursing evaluation of patients' discharge education. Six institutions collaborated in surveying 300 postoperative patients with CABG to identify learning priorities and patients' perceptions of the effectiveness of discharge education. Data analysis from the self-administered questionnaire demonstrated consistent patient priorities across institutions. Differences in teaching methods and content did not affect perceived preparedness or importance scores. Regional experience demonstrates that variable teaching efforts meet patients' priorities and provide high overall patient preparedness for discharge. Patients with the shortest hospitalizations had higher preparedness scores. 相似文献
86.
Rosalie Bruel J. Ellen Marsden Bernie Pientka Nick Staats Timothy Mihuc Jason D. Stockwell 《Journal of Great Lakes research》2021,47(4):1171-1181
Invasive species can cause major disruptions in native food webs, yet the impact of species introductions and whether they will become invasive appears to be context-dependent. Rainbow smelt and alewife co-exist as invasive species in the Laurentian Great Lakes and as native species on the Atlantic coast of North America, but in Lake Champlain rainbow smelt is the dominant native forage fish and alewife are invasive. Alewife became abundant by 2007, providing an opportunity to explore the dynamics of these two species in a system where only one is invasive. We used data from a 28-year forage fish survey to compare demographics of rainbow smelt populations in three basins of Lake Champlain with different volumes, nutrient levels, and predator abundances. Rainbow smelt catch-per-unit-effort (CPUE) remained constant in the large, deep Main Lake before and after alewife invaded, but decreased in the two smaller basins. Declines were primarily a result of increased age-0 and age-1 mortality. Predation by top piscivores, system productivity, and competition for resources alone could not explain the patterns in CPUE across the basins. The mechanisms that allow alewife and rainbow smelt to co-exist could be related to system volume and oxythermal habitat availability, and may explain why the two species do not negatively affect each other in the Great Lakes. Summer hypoxia in the smaller basins could force individuals into smaller habitat volumes with higher densities of competitors and cannibalistic adult rainbow smelt. Habitat availability may mediate the impact of invasive alewife on native rainbow smelt. 相似文献
87.
Stream habitat restoration is an important tool for fisheries management in impaired lotic systems. Although small‐scale benefits of stream habitat restoration are commonly investigated, it is difficult to demonstrate population effects. The Pahsimeroi River Chinook salmon Oncorhynchus tshawytscha population was previously restricted to the lower portion of the river by multiple irrigation structures. To address fish passage issues, a combination of restoration projects was initiated including barrier removals, instream flow enhancements and installation of fish screens on diversions. The largest barrier was removed in 2009, more than doubling the amount of accessible linear habitat. We hypothesized restoration efforts would expand the distribution of spawning salmon in the Pahsimeroi River watershed, leading to a broader distribution of juveniles. We also hypothesized a broader juvenile distribution would have population effects by reducing the prevalence of density‐dependent growth and survival. Redds were documented in newly accessible habitat immediately following barrier removal and accounted for a median of 42% of all redds in the Pahsimeroi River watershed during 2009–2015. Snorkel surveys also documented juvenile rearing in newly accessible habitat. Juvenile productivity increased from a median of 64 smolts/female spawner for brood years 2002–2008 to 99 smolts/female spawner for brood years 2009–2014. Overall, results suggested increased habitat accessibility in the Pahsimeroi River broadened the distribution of spawning adult and rearing juvenile salmon and reduced the effects of density‐dependent survival. Large‐scale stream restoration efforts can have a population effect. Despite the large‐scale effort and response, habitat restoration alone is likely not sufficient to restore this population. 相似文献
88.
Ly Thi Thanh Le Lukasz Kotula Kadambot H. M. Siddique Timothy D. Colmer 《International journal of molecular sciences》2021,22(4)
Grain legumes are important crops, but they are salt sensitive. This research dissected the responses of four (sub)tropical grain legumes to ionic components (Na+ and/or Cl−) of salt stress. Soybean, mungbean, cowpea, and common bean were subjected to NaCl, Na+ salts (without Cl−), Cl− salts (without Na+), and a “high cation” negative control for 57 days. Growth, leaf gas exchange, and tissue ion concentrations were assessed at different growing stages. For soybean, NaCl and Na+ salts impaired seed dry mass (30% of control), more so than Cl− salts (60% of control). All treatments impaired mungbean growth, with NaCl and Cl− salt treatments affecting seed dry mass the most (2% of control). For cowpea, NaCl had the greatest adverse impact on seed dry mass (20% of control), while Na+ salts and Cl− salts had similar intermediate effects (~45% of control). For common bean, NaCl had the greatest adverse effect on seed dry mass (4% of control), while Na+ salts and Cl− salts impaired seed dry mass to a lesser extent (~45% of control). NaCl and Na+ salts (without Cl−) affected the photosynthesis (Pn) of soybean more than Cl− salts (without Na+) (50% of control), while the reverse was true for mungbean. Na+ salts (without Cl−), Cl− salts (without Na+), and NaCl had similar adverse effects on Pn of cowpea and common bean (~70% of control). In conclusion, salt sensitivity is predominantly determined by Na+ toxicity in soybean, Cl− toxicity in mungbean, and both Na+ and Cl− toxicity in cowpea and common bean. 相似文献
89.
Asher Ornoy Maria Becker Liza Weinstein-Fudim Zivanit Ergaz 《International journal of molecular sciences》2021,22(6)
In spite of the huge progress in the treatment of diabetes mellitus, we are still in the situation that both pregestational (PGDM) and gestational diabetes (GDM) impose an additional risk to the embryo, fetus, and course of pregnancy. PGDM may increase the rate of congenital malformations, especially cardiac, nervous system, musculoskeletal system, and limbs. PGDM may interfere with fetal growth, often causing macrosomia, but in the presence of severe maternal complications, especially nephropathy, it may inhibit fetal growth. PGDM may also induce a variety of perinatal complications such as stillbirth and perinatal death, cardiomyopathy, respiratory morbidity, and perinatal asphyxia. GDM that generally develops in the second half of pregnancy induces similar but generally less severe complications. Their severity is higher with earlier onset of GDM and inversely correlated with the degree of glycemic control. Early initiation of GDM might even cause some increase in the rate of congenital malformations. Both PGDM and GDM may cause various motor and behavioral neurodevelopmental problems, including an increased incidence of attention deficit hyperactivity disorder (ADHD) and autism spectrum disorder (ASD). Most complications are reduced in incidence and severity with the improvement in diabetic control. Mechanisms of diabetic-induced damage in pregnancy are related to maternal and fetal hyperglycemia, enhanced oxidative stress, epigenetic changes, and other, less defined, pathogenic mechanisms. 相似文献