全文获取类型
收费全文 | 4514篇 |
免费 | 1篇 |
专业分类
综合类 | 1篇 |
化学工业 | 15篇 |
建筑科学 | 4篇 |
轻工业 | 3篇 |
无线电 | 3篇 |
一般工业技术 | 4篇 |
冶金工业 | 4473篇 |
自动化技术 | 12篇 |
出版年
2019年 | 1篇 |
2014年 | 6篇 |
2013年 | 2篇 |
2012年 | 3篇 |
2011年 | 1篇 |
2009年 | 2篇 |
2006年 | 3篇 |
2004年 | 1篇 |
2003年 | 9篇 |
2000年 | 2篇 |
1999年 | 127篇 |
1998年 | 1388篇 |
1997年 | 848篇 |
1996年 | 490篇 |
1995年 | 268篇 |
1994年 | 250篇 |
1993年 | 278篇 |
1992年 | 35篇 |
1991年 | 49篇 |
1990年 | 73篇 |
1989年 | 61篇 |
1988年 | 68篇 |
1987年 | 63篇 |
1986年 | 45篇 |
1985年 | 35篇 |
1984年 | 1篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 29篇 |
1980年 | 44篇 |
1979年 | 2篇 |
1978年 | 7篇 |
1977年 | 92篇 |
1976年 | 203篇 |
1975年 | 5篇 |
1964年 | 1篇 |
1955年 | 3篇 |
排序方式: 共有4515条查询结果,搜索用时 265 毫秒
941.
PJ Schwartz DL Murphy TA Wehr D Garcia-Borreguero DA Oren DE Moul N Ozaki AJ Snelbaker NE Rosenthal 《Canadian Metallurgical Quarterly》1997,54(4):375-385
BACKGROUND: Multiple lines of evidence suggest that brain serotonergic systems may be disturbed in seasonal affective disorder (SAD). Previously, we found that the serotonergic agent meta-chlorophenylpiperazine (m-CPP) produced increases in activation and euphoria in depressed patients with SAD, but not in patients with SAD following light treatment or in the summer, nor in healthy control subjects in any condition. In the present study, we attempted to replicate and extend this finding using better methods. METHODS: Seventeen outpatients with SAD and 15 control subjects underwent successive 3-week periods of bright light treatment and light avoidance in a randomized order. During the third week of each condition, on 2 different occasions, subjects were admitted to the hospital for a night of sleep (core temperatures were recorded), followed by infusions of m-CPP (0.08 mg/kg) or placebo the next morning. Dependent measures included the 24-item National Institute of Mental Health Self-Rating Scale, plasma corticotropin, cortisol, prolactin, growth hormone, and norepinephrine concentrations, and core temperatures. RESULTS: Meta-chlorophenylpiperazine produced (1) significant increases in "activation-euphoria" ratings only in depressed patients with SAD in the untreated condition and (2) blunted corticotropin and norepinephrine responses in patients with SAD compared with controls across both light treatment conditions. In both groups, light treatment was associated with significant reductions in nocturnal core temperatures, which were correlated with similarly significant reductions in mean diurnal growth hormone concentrations. In patients with SAD, (1) the reductions in nocturnal core temperatures also were correlated with the reductions in baseline depression ratings and (2) the reductions in mean growth hormone concentrations were significantly smaller compared with controls. CONCLUSIONS: The abnormal m-CPP-induced activation-euphoria responses represent a replicated state marker of winter depression in patients with SAD. The blunted m-CPP-induced responsiveness of the hypothalamic-pituitary-adrenal axis and the sympathetic nervous system may represent traitlike abnormalities. The improvements in mood following light treatment in patients with SAD seem to be associated with the lowering of nocturnal core temperatures. The findings, although not easily explained based on a uniform abnormality of serotonin receptors, are nonetheless compatible with the notion that selected regions of the central nervous system are deficient in serotonin transmission during winter depression. 相似文献
942.
M Barnes DJ Clouthier PG Hajigeorgiou G Huang CT Kingston AJ Merer GF Metha JRD Peers SJ Rixon 《Canadian Metallurgical Quarterly》1997,186(2):374-402
Laser excitation spectra of gaseous CoO have been recorded using Doppler-limited intracavity spectroscopy and supersonic jet-cooled molecular beam methods. As seen in the molecular beam spectra there are nearly 100 bands arising from the X4Delta7/2 spin component of the ground state in the wavelength region 430-720 nm. All of them are very strongly red-degraded and most of them are perturbed. 59Co hyperfine broadenings or splittings have been observed in many. The bands can be arranged into five electronic transitions, together with a considerable number of "extra" bands induced by perturbations; two of the excited electronic states are 4Phi, while the other three, which lie within 1700 cm-1, are 4Delta. Semi-empirical calculations have been carried out for the 4Delta excited states, using exchange integrals transferred from the TiO and VO spectra; the predicted energies, spin-orbit structures, and relative intensities in absorption are consistent with the assignment of all three 4Delta states to the same electron configuration, (4ssigma)1(3ddelta)3(3dpi)2(3dsigma)1. It seems that there are extensive interactions between the excited 4Delta states, because only one of them gives rise to a recognizable, though irregular, long vibrational progression in absorption. The other two progressions die out abruptly and unexpectedly after three or four members, presumably as a result of interference effects; however, wavelength-resolved fluorescence studies, together with calculations of the Franck-Condon overlap integrals to the various vibrational levels of the ground state, show that the upper state progressions reappear following a region of confused absorption. All of the excited states show highly irregular variations of their vibrational intervals and rotational constants with the vibrational quantum number, v. The wavelength-resolved fluorescence spectra give evidence for new low-lying electronic states of CoO at 3038, 5989, and 9105 cm-1; based on its vibrational frequency the 3038 cm-1 state is possibly the sigmadelta4pi2 B4Sigma- state. Copyright 1997 Academic Press. Copyright 1997Academic Press 相似文献
943.
RA Lafayette G Paré CH Schmid AJ King RJ Rohrer SA Nasraway 《Canadian Metallurgical Quarterly》1997,48(3):159-164
BACKGROUND: Numerous clinicians and researchers have expressed concern about the necessity and potential adverse consequences of many cesarean births in the United States. The purpose of this study was to explore college students' attitudes and beliefs about cesarean section. METHODS: One hundred two college students (66% women) completed a 20-item questionnaire that asked if they viewed cesarean delivery as a potentially negative experience, as a normal or acceptable method of childbirth, and as medically necessary, and asked about their beliefs concerning risk and prevention of cesarean birth. RESULTS: The number of "undecided" responses in the study was striking (7.8% to 69.6% across the 20 items). In general, women and men responded similarly, although women were significantly more likely than men to say they would be profoundly disappointed if their babies had to be delivered by cesarean section. Despite expressing cynicism about the cesarean birth rate (40% agreed that many unnecessary cesarean births occurred) and not viewing the procedure as a normal way of giving birth (47%), most respondents (over 70%) disagreed that giving birth by cesarean would be a negative experience or would make a woman feel like a failure. CONCLUSION: A high level of uncertainty exists about certain aspects of cesarean birth among young women and men, highlighting the need for information for prospective parents. Most college students did not view the cesarean birth experience as either potentially negative or normal. Future research should explore coverage of cesarean birth in childbirth education classes and the roles physicians, nurses, and midwives play in preparing expectant parents for the possibility of cesarean delivery. 相似文献
944.
E de Haan P van Oppen AJ van Balkom P Spinhoven KA Hoogduin R Van Dyck 《Canadian Metallurgical Quarterly》1997,96(5):354-361
Several factors must be taken into account when deciding which specimen preparation technique(s) to use. These factors include the amount of material available, ease of preparing this material due to its properties and familiarity, location and size of the region of interest, amount of information sought, facilities accessible, and time available by the researcher to devote to the preparation of the specimen. The more popular specimen preparations for thin films, namely, electropolishing, cleaving, crushing, mechanical thinning followed by ion milling, and ultramicrotomy are discussed and the more unusual techniques such as extraction/replication, photochemical etching, lithography and reactive ion etching (RIE), chemically assisted ion beam etching (CAIBE), and precision polishing-based techniques are described. Their advantages and disadvantages in the context of the above factors are discussed. Suggestions for increasing one's success rate in preparing specimens are given. The role of transmission electron microscopy (TEM) analysis is considered since it rarely stands in isolation from other physical analytical techniques, nor is it often used as a quick diagnostic tool. Conservation of material by the minimization of the amount of material used (or destroyed) by TEM specimen preparation, and conservation of one's time by performing TEM analysis only on "worthy" samples should be given maximum consideration. 相似文献
945.
KL Dreher RH Jaskot JR Lehmann JH Richards JK McGee AJ Ghio DL Costa 《Canadian Metallurgical Quarterly》1997,50(3):285-305
A theoretical framework is proposed for understanding how the innovative use of behavioral science evidence is both produced and diffused among members of the global legal community. Using case law analyses and interviews with key individuals involved in selected cases, we examine how battered woman syndrome (BWS) is produced and diffused between and among Australia, Canada, England, and the United States. The following diffusion mechanisms are proposed: (1) The availability and accessibility of credible dissemination sources; (2) characteristics of the overall practice environment operating in each legal culture; (3) the attitudes and knowledge of attorneys and judges about the use of scientific evidence; (4) political and social support for the use of the evidence in the legal culture; and (5) the level of structural equivalence, communication, and "neighbor effects" between and among legal cultures. Each mechanism is discussed and supplemented with information from interviews with individuals involved in key cases involving BWS evidence. 相似文献
946.
TM Wolever S Hamad J Gittelsohn J Gao AJ Hanley SB Harris B Zinman 《Canadian Metallurgical Quarterly》1997,66(6):1470-1474
The high prevalence of diabetes mellitus in North American aboriginal populations may be due to recent changes in lifestyle, including the adoption of a high-fat, low-fiber diet. To determine whether fat or fiber intakes were associated with new cases of diabetes, we studied 72% (728/1018) of residents aged > 9 y from a remote aboriginal community in northern Ontario using the 75-g oral-glucose-tolerance test and 24-h dietary recall. The mean fat intake of this population (36% of energy) was typical for North America, but fiber intake (1.2 g/MJ) was very low. Logistic-regression analysis, adjusted for age, sex, and body mass index, showed that a 1-SD increase in fiber intake reduced the risk of having diabetes by 39% (P = 0.026) whereas the same increase in protein intake increased the risk by 38% (P = 0.027). There was no significant effect of energy, fat, starch, or simple sugars. These data support Trowell's original dietary-fiber hypothesis that "... dietary fiber depleted starchy foods are conducive to the development of diabetes mellitus in susceptible human genotypes." 相似文献
947.
AJ Ferreira WP Elias JS Pelayo R Giraldi MZ Pedroso IC Scaletsky 《Canadian Metallurgical Quarterly》1997,19(4):285-288
In the present study, in order to get a better insight into the mechanism of action of cyclophosphamide (CY) in rheumatoid arthritis (RA), we monitored the changes in lymphocytes' expression of leukocyte function associated antigen 1 (LFA-1). A group of 28 patients with refractory severe RA were treated with CY and methylprednisolone (MO) intravenously. Using flow cytometry we evaluated the changes in LFA-1 molecule expression on peripheral lymphocytes. In the analyzed group of patients the proportion of LFA-1 "dim" cells was reduced. After the treatment the ratio was partly normalized. Twelve months after cessation of the therapy high proportion of LFA-1 "dim" was observed only among CY/MP treated patients. The changes were related to clinical improvement. Based on the obtained data, it seems, that the treatment affecting the expression of LFA-1 may slow down lymphocyte migration and by that limit chronic inflammation within the synovium. 相似文献
948.
The role of the blood-brain barrier transporter PTS-1 in regulating concentrations of methionine enkephalin in blood and brain 总被引:1,自引:0,他引:1
The entrainment of the circadian rhythm of locomotor activity was studied in the field mouse Mus booduga in order to examine the relationship between the free-running period (tau) and minimum tolerable light pulse interval of the skeleton photoperiods. The animals were entrained under three different light/dark (LD) schedules, each out of phase with the other. They were then subjected to various skeleton photoperiods created by two repeated light pulses (LPs) interrupting darkness. Animals that selected the shorter interval between the LPs as their "subjective night" had significantly shorter tau (23.13 +/- 0.38 h) as compared to those that selected the longer dark interval as subjective night (tau = 23.87 +/- 0.18 h). When the longer dark interval was 12 h, animals selecting that interval as their subjective night included both long-tau and short-tau individuals. When both intervals of darkness were of equal duration, no difference in the selection of subjective night was seen between short and long-tau animals. When the "dusk" LP for the animals that selected the longer dark interval as subjective night was advanced by 2 h to create a new skeleton photoperiod, the number of transient cycles appearing before steady-state entrainment was found to depend on the duration of the photoperiods. When the night defined by the two LPs was reduced below 6h, a dramatic "phase jump" in the activity rhythm was observed, and the initial phase relationship was restored after a relaxation in the night duration. We observed considerable interindividual variation in the "minimum tolerable light pulse interval of skeleton photoperiods," which we suggest may be due to the observed variation in tau among individuals. 相似文献
949.
OBJECTIVES: To use existing data sources to refine prior estimates of the U.S. emergency medicine (EM) workforce and to estimate effects of proposed changes in the U.S. health care system on the EM workforce. METHODS: Relevant data were extracted from the American College of Emergency Physicians (ACEP) 1995 Membership Activity Report, the American Medical Association (AMA) publication "1995/96 Physician Characteristics and Distribution in the U.S.," the American Hospital Association (AHA) 1994 hospital directory, a written survey of each state's medical licensing board and state medical society, and the American Board of Emergency Medicine (ABEM) annual activity report for 1995. These data were used to project workforce supply and demand estimates applicable to workforce models. RESULTS: None of the available information sources had complete data on the number and distribution of emergency physicians (EPs) currently practicing in the United States. Extrapolating the limited reliable statewide EP numbers to make nationwide projections reveals a shortage of EPs needed to fully staff the nation's existing EDs. At least 22 states had an average ratio of < 5 EPs per existing ED. Additional national projections incorporating a decreasing number of U.S. EDs indicate that the current annual number of EM residency graduates will not eliminate the deficit of EPs for at least several decades, given that projected numbers of retiring EPs annually will soon equal the total annual EM residency graduate production. CONCLUSIONS: Although the current data on EPs in practice in the United States are incomplete, the authors project a relative shortage of EPs. More accurate and complete information on the numbers and distribution of EPs in America is needed to improve workforce projections. 相似文献