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   2103篇
  免费   0篇
化学工业   16篇
机械仪表   1篇
轻工业   4篇
石油天然气   1篇
无线电   28篇
一般工业技术   1篇
冶金工业   2028篇
自动化技术   24篇
  2015年   1篇
  2014年   1篇
  2013年   2篇
  2011年   1篇
  2010年   2篇
  2009年   2篇
  2008年   1篇
  2007年   4篇
  2006年   2篇
  2005年   6篇
  2004年   2篇
  2003年   6篇
  2002年   1篇
  2001年   1篇
  2000年   1篇
  1999年   84篇
  1998年   653篇
  1997年   356篇
  1996年   226篇
  1995年   143篇
  1994年   117篇
  1993年   113篇
  1992年   16篇
  1991年   16篇
  1990年   14篇
  1989年   27篇
  1988年   24篇
  1987年   25篇
  1986年   21篇
  1985年   19篇
  1983年   5篇
  1982年   8篇
  1981年   15篇
  1980年   13篇
  1979年   1篇
  1978年   5篇
  1977年   62篇
  1976年   92篇
  1975年   3篇
  1969年   1篇
  1968年   1篇
  1967年   4篇
  1966年   1篇
  1965年   4篇
  1964年   1篇
排序方式: 共有2103条查询结果,搜索用时 15 毫秒
111.
BACKGROUND: Controversy continues regarding the optimal extent of primary thyroid resection in most patients with papillary thyroid carcinoma (PTC), who are at minimal risk of cause-specific mortality (CSM). This study was designed to compare CSM and recurrence rates after either unilateral lobectomy (UL) or bilateral lobar resection (BLR) in patients with PTC considered low risk by AMES criteria. METHODS: Outcome was studied in 1685 patients initially treated during 1940 through 1991 and followed for up to 54 postoperative years (mean, 18 years). One thousand six hundred fifty-six patients (98%) had complete primary tumor resection; 634 (38%) had involvement of regional nodes. One hundred ninety-five patients (12%) had UL; BLR accounted for 1468 (near-total 60%; total thyroidectomy 18%). RESULTS: Thirty-year rates for CSM and distant metastasis were 2% and 3%, respectively. Twenty-year rates for local recurrence and nodal metastasis were 4% and 8%, respectively. There were no significant differences in CSM or distant metastasis rates between UL and BLR (P > .2). After UL, 20-year rates for local recurrence and nodal metastasis were 14% and 19%, significantly higher (P = .0001) than the 2% and 6% rates seen after BLR. CONCLUSIONS: UL was not associated with higher CSM rates, but it was associated with a significantly higher risk of locoregional recurrence. Thus BLR probably represents a preferable initial surgical approach to patients with low-risk PTC.  相似文献   
112.
Previous studies of the vitamin D receptor (VDR) polymorphisms and bone mineral density (BMD) have suggested that there may be differences in calcium absorption among groups of women with different VDR genotypes, and that the association may be stronger in younger women. To investigate the association between the VDR polymorphisms and BMD, this study was undertaken in the Framingham Study Cohort and a group of younger volunteers. Subjects from the Framingham Study (ages 69-90 years) included those who underwent BMD testing and who had genotyping for the VDR alleles (n = 328) using polymerase chain reaction methods and restriction fragment length polymorphisms with BsmI (B absence, b presence of cut site). A group of younger volunteer subjects (ages 18-68) also underwent BMD testing and VDR genotyping (n = 94). In Framingham Cohort subjects with the bb genotype, but not the Bb or BB genotypes, there were significant associations between calcium intake and BMD at five of six skeletal sites, such that BMD was 7-12% higher in those with dietary calcium intakes greater than 800 mg/day compared with those with intakes < 500 mg/day. The data also suggested that BMD was higher in persons with the bb genotype only in the group with calcium intakes above 800 mg/day. No significant differences were found in the Framingham Cohort for age-, sex-, and weight-adjusted BMD at any skeletal site between those with the BB genotype and those with the bb genotype regardless of 25-hydroxyvitamin D levels or country of origin. In the younger volunteers, BMD of the femoral neck was 5.4% higher (p < 0.05) in the bb genotype group compared with the BB group and 11% higher (p < 0.05) in males with the bb genotype compared with the BB group. There were no significant differences at the lumbar spine. In this study, the association between calcium intake and BMD appeared to be dependent upon VDR genotype. The findings of an association between dietary calcium intake and BMD only in the bb genotype group suggests that the VDR genotype may play a role in the absorption of dietary calcium. Studies that do not consider calcium intake may not detect associations between VDR genotype and BMD. In addition, the association between VDR alleles and BMD may become less evident in older subjects.  相似文献   
113.
Since the inception of mobile coronary care units (MCCU), patients with sudden cardiac death (SCD) saved by advanced emergency medical technicians (EMT-A) can be studied retrospectively and prospectively. Forty-eight cases of SCD found in ventricular fibrillation (VF) were successfully resuscitated. Only 32% had a myocardial infarction. Most survivors were New York Heart Association (NYHA) class I or II. All class IV survivors with severe congestive heart failure died within 45 days. All class II survivors had angina as the limiting factor. Of all patients with VF, 23% survived. Eighty percent of survivors were class I or II and have resumed previous lifestyles. No clear cut symptom complex was identified. Rescue response time was generally less than five minutes. Intracardiac medications were administered without complications. Empirical administration of sodium bicarbonate correlated poorly with arterial blood gas determinations.  相似文献   
114.
The effect of nitric oxide (NO) donors on high-voltage-activated Ca2+ channels in insulin-secreting RINm5F cells was investigated using the patch-clamp technique in the whole-cell configuration. Sodium nitroprusside (SNP, 2-400 microM) induced a dose-dependent reduction in Ba2+ currents with maximal inhibition of 58%. The IC50 for SNP was 45 microM. A different NO donor, (+/-)S-nitroso-N-acetylpenicillamine (SNAP, 500 microM), also produced a 50% decrease in current amplitude. When 200 microM SNP was administered together with the NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidozoline-1-oxyl-3-oxide (carboxy-PTIO, 300 microM), the Ba2+ current inhibition was lowered to 7%. Administration of 500 microM 8-bromoguanosine 3':5'-cyclic monophosphate sodium salt (8-Br-cGMP) mimicked the effects of SNP, causing a comparable decrease (56%) in peak-current amplitude. When soluble guanylyl cyclase was blocked by 10 microM 1H-[1,2, 4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ), the inhibitory effect of 200 microM SNP was reduced from 39% to 15%. The SNP-induced current decrease was 36% of controls after the blockade of L-type Ca2+ channels and 30% in the presence of 2.5 microM omega-conotoxin-MVIIC. These data indicate that NO inhibits both L-type and P/Q-type Ca2+ channels in RINm5F cells, probably by an increase in the intracellular levels of cGMP. NO may then significantly influence the Ca2+-dependent release of hormones from secretory cells as well as that of neurotransmitters from nerve terminals.  相似文献   
115.
116.
117.
During October 1992 an increase in the number of isolates of Salmonella mikawasima, a rare serotype, was noted including a cluster of nine cases in the South West Thames region. A case control study was conducted and univariate analysis showed a statistical association between illness and eating at take-away A for cases compared with household controls (P = 0.003) and with neighbourhood controls (P = 0.0245). Cases were also more likely to have eaten kebabs than were controls or average take-away A customers, implicating doner kebabs as the most likely vehicle of infection. Plasmid profile analysis of the nine cases' isolates showed them to be indistinguishable and to be characterized by a single plasmid of approximately 60 MDa. The original source of the Salmonella mikawasima contamination was not determined, but food preparation practices for kebabs at take-away A were insufficient to protect against illness if contaminated. This outbreak was only recognized because of the unusual serotype, but could be an indication of a more widespread problem with doner kebabs.  相似文献   
118.
While many cell types express receptors for the Fc domain of IgG (Fc gamma R), only primate polymorphonuclear neutrophils (PMN) express an Fc gamma R linked to the membrane via a glycan phosphoinositol (GPI) anchor. Previous studies have demonstrated that this GPI-linked Fc gamma R (Fc gamma RIIIB) cooperates with the transmembrane Fc gamma R (Fc gamma RIIA) to mediate many of the functional effects of immune complex binding. To determine the role of the GPI anchor in Fc gamma receptor synergy, we have developed a model system in Jurkat T cells, which lack endogenously expressed Fc gamma receptors. Jurkat T cells were stably transfected with cDNA encoding Fc gamma RIIA and/or Fc gamma RIIIB. Cocrosslinking the two receptors produced a synergistic rise in intracytoplasmic calcium ([Ca2+]i) to levels not reached by stimulation of either Fc gamma RIIA or Fc gamma RIIIB alone. Synergy was achieved by prolonged entry of extracellular Ca2+. Cocrosslinking Fc gamma RIIA with CD59 or CD48, two other GPI-linked proteins on Jurkat T cells also led to a synergistic [Ca2+]i rise, as did crosslinking CD59 with Fc gamma RIIA on PMN, suggesting that interactions between the extracellular domains of the two Fc gamma receptors are not required for synergy. Replacement of the GPI anchor of Fc gamma RIIIB with a transmembrane anchor abolished synergy. In addition, tyrosine to phenylalanine substitutions in the immunoreceptor tyrosine-based activation motif (ITAM) of the Fc gamma RIIA cytoplasmic tail abolished synergy. While the ITAM of Fc gamma RIIA was required for the increase in [Ca2+]i, tyrosine phosphorylation of crosslinked Fc gamma RIIA was diminished when cocrosslinked with Fc gamma RIIIB. These data demonstrate that Fc gamma RIIA association with GPI-linked proteins facilitates Fc gamma R signal transduction and suggest that this may be a physiologically significant role for the unusual GPI-anchored Fc gamma R of human PMN.  相似文献   
119.
PURPOSE: We determine the presence of an open bladder neck during video urodynamic studies and relate that finding to the presence of stress urinary incontinence. MATERIALS AND METHODS: Patients presenting with urinary incontinence, voiding dysfunction or pelvic floor prolapse underwent video urodynamics. With the patient upright and after 200 ml. contrast material had been instilled into the bladder the bladder neck was viewed to determine if it was open or closed. At that point the abdominal leak point pressure was measured. RESULTS: Of 102 women, average age 56.5 years (range 31 to 82), 13% had an open bladder neck and demonstrable stress incontinence on video urodynamics with an average abdominal leak point pressure of 45 cm. water (range 26 to 90). Of those with stress incontinence on urodynamics 23% had an open bladder neck. No continent patient had an open bladder neck. CONCLUSIONS: The presence of an open bladder neck with the bladder filled to 200 ml. correlates strongly with the presence of stress incontinence.  相似文献   
120.
The properties of enhancement-mode InP metal-insulator-semiconductor field-effect transistors fabricated on semi-insulating InP substrates are reported. The epitaxial layers of the device structure have been grown by chloride vapor-phase epitaxy. Short-circuit current gain cutoff frequencies of 29.6 GHz were measured for 1-μm-gate-length devices. For devices with submicrometer gate lengths, extrinsic transconductance values up to 300 mS/mm and short-circuit current-gain cutoff frequencies of 38.1 GHz were measured. SiO2 deposited by electron beam evaporation and plasma-enhanced CVD Si3N4 have been utilized as gate insulators, and a drain current drift of 30% within the first 50 h of operation has been observed. The high-speed performance of these devices represent to the authors' knowledge the fastest InP-based MIS field-effect transistor demonstrated  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