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   30篇
  免费   0篇
电工技术   1篇
化学工业   5篇
机械仪表   1篇
无线电   4篇
一般工业技术   3篇
冶金工业   12篇
原子能技术   1篇
自动化技术   3篇
  2023年   1篇
  2022年   1篇
  2019年   1篇
  2013年   2篇
  2012年   2篇
  2008年   1篇
  2005年   3篇
  2001年   1篇
  2000年   1篇
  1999年   2篇
  1998年   4篇
  1997年   3篇
  1996年   1篇
  1995年   1篇
  1993年   1篇
  1987年   2篇
  1984年   1篇
  1976年   1篇
  1974年   1篇
排序方式: 共有30条查询结果,搜索用时 46 毫秒
1.
Avian biodiversity is threatened, and in order to prioritize limited conservation resources and conduct effective conservation planning a better understanding of avian species richness patterns is needed. The use of image texture measures, as a proxy for the spatial structure of land cover and vegetation, has proven useful in explaining patterns of avian abundance and species richness. However, prior studies that modeled habitat with texture measures were conducted over small geographical extents and typically focused on a single habitat type. Our goal was to evaluate the performance of texture measures over broad spatial extents and across multiple habitat types with varying levels of vertical habitat structure. We calculated a suite of texture measures from 114 Landsat images over a study area of 1,498,000 km2 in the Midwestern United States, which included habitats ranging from grassland to forest. Avian species richness was modeled for several functional guilds as a function of image texture. We subsequently compared the explanatory power of texture-only models with models fitted using landscape composition metrics derived from the National Land Cover Dataset, as well as models fitted using both texture and composition metrics. Measures of image texture were effective in modeling spatial patterns of avian species richness in multiple habitat types, explaining up to 51% of the variability in species richness of permanent resident birds. In comparison, landscape composition metrics explained up to 56% of the variability in permanent resident species richness. In the most heavily forested ecoregion, texture-measures outperformed landscape metrics, and the two types of measurements were complementary in multivariate models. However, in two out of three ecoregions examined, landscape composition metrics consistently performed slightly better than texture measures, and the variance explained by the two types of measures overlapped considerably. These results show that image texture measures derived from satellite imagery can be an important tool for modeling patterns of avian species richness at broad spatial extents, and thus assist in conservation planning. However, texture measures were slightly inferior to landscape composition metrics in about three-fourths of our models. Therefore texture measures are best considered in conjunction with landscape metrics (if available) and are best used when they show explanatory ability that is complementarity to landscape metrics.  相似文献   
2.
High solids loading silicon carbide (SiC)-based aqueous slurries containing only .5 wt. % organic additives were utilized to create specimens of various geometries via an extrusion-based additive manufacturing (AM) technique. Pressureless electric field-assisted sintering was performed to densify each specimen without deformation. The combination of these techniques produced parts with >98% relative density despite containing only 5 wt.% oxide sintering additives. After sintering, specimens contained only the α-SiC and yttrium aluminum perovskite phases. This suggests the evolution of a nonequilibrium yttrium aluminate phase, as well as transformation from β-SiC to α-SiC. The fabrication method presented in this work has advantages over other AM techniques commonly used with SiC, because it does not require significant organic additives nor additional postprocessing steps such as chemical vapor infiltration or polymer impregnation and pyrolysis.  相似文献   
3.
OBJECTIVES: Treatment failure after radical prostatectomy is most commonly heralded by an increase in serum prostate-specific antigen (PSA) to detectable levels. We evaluated the clinical utility of an ultrasensitive chemiluminescent PSA assay. METHODS: We evaluated the assay in banked sera obtained from 170 men after radical prostatectomy. Controls consisted of 142 females, 29 men who had undergone cystoprostatectomy without evidence of prostate cancer, and 25 men without evidence of recurrent disease at least 5 years after prostatectomy for organ-confined disease. Lead time to diagnosis of recurrence was based on comparisons with the IMx or Tandem E assays using a cutoff of 0.1 ng/mL (100 pg/mL). RESULTS: The biologic level of detection of this assay is 8 pg/mL. Serum PSA levels were undetectable in 82.4% of females, 86.2% of the cystoprostatectomy patients, and 96% of the radical prostatectomy controls. After radical prostatectomy, PSA levels were undetectable at last check in 104 of 168 (61.9%) men. In the 24 men with prostate cancer recurrence, the enhanced sensitivity of 8 pg/mL provided a mean lead time based on conservative calculations of 12.7 to 22.5 months over conventional assays. Thirty-four of the 41 men with detectable PSA levels and no evidence of disease recurrence had PSA levels of 30 pg/mL or less. CONCLUSIONS: PSA levels are undetectable in most men who do not have recurrence of disease after radical prostatectomy. Low but detectable serum PSA levels less than or equal to 30 pg/mL can be produced by nonmalignant sources of PSA. PSA assays with enhanced sensitivity can detect recurrent prostate cancer with significant lead time over conventional assays.  相似文献   
4.
BACKGROUND: Many studies have shown the efficacy of patient-controlled analgesia (PCA). However, it is not clear whether PCA has clinical or economic benefits in addition to efficient analgesia. The current study was designed to evaluate these issues by comparing PCA with regularly administered intramuscular injections of opioids after hysterectomy. METHODS: This prospective study included 126 patients who underwent abdominal hysterectomy and were randomly assigned to receive PCA or regularly timed intramuscular injections of morphine during a period of 48 h. Doses were adjusted to provide satisfactory analgesia in both treatment groups. Pain at rest and with movement, functional recovery, drug side effects, and patient satisfaction were measured using rating scales and questionnaires. The costs of PCA and intramuscular therapy were calculated based on personnel time and drug and material requirements. RESULTS: Comparable analgesia was observed with the two treatment methods, with no significant differences in the incidence of side effects or patient satisfaction. The medication dosage had to be adjusted significantly more frequently in the intramuscular group than in the PCA patients. The PCA did not favor a faster recuperation time compared with intramuscular therapy in terms of times to ambulation, resumption of liquid and solid diet, passage of bowel gas, or hospital discharge. The results of the economic evaluation, which used a cost-minimization model and sensitivity analyses, showed that PCA was more costly than regular intramuscular injections despite the fact that no costs for the pump were included in the analyses. Cost differences in nursing time favoring PCA were offset by drug and material costs associated with this type of treatment. CONCLUSIONS: Compared with regularly scheduled intramuscular dosing, PCA is more costly and does not have clinical advantages for pain management after hysterectomy. Because of the comparable outcomes, the general use of PCA in similar patients should be questioned.  相似文献   
5.
Administration of streptococcal cell wall (SCW) preparation induces an inflammatory response in susceptible animals that is a model frequently used for rheumatoid arthritis. The degree of inflammation produced by SCW is greatly enhanced by low endogenous levels of glucocorticoids due to diminished hypothalamic-pituitary-adrenal activity. Because decreased glucocorticoid production is known to occur in the hypothyroid state, we tested whether hypothyroidism would increase, and conversely, whether hyperthyroidism would decrease, the inflammatory responses to SCW. Adult female Sprague Dawley rats were fed a regular diet (control), L-T4 (T4; hyperthyroid), or 6-propyl-thiouracil (hypothyroid) in drinking water for 7 weeks. Hypothyroidism resulted in elevated plasma levels of TSH and hypothalamic preproTRH messenger RNA (mRNA) while reducing anterior pituitary POMC mRNA and plasma ACTH and corticosterone levels. In contrast, hyperthyroid rats produced opposite results: decreased measures of central thyroid function but increased pituitary-adrenal function. Three days after administration of SCW, macrophage inflammatory protein-1alpha and interleukin-1beta mRNA expression increased dramatically in controls and even further in hypothyroid animals, as measured by Northern blot analysis. In contrast, T4-treated rats showed significant inhibition of these inflammatory markers. Thus, the hyperthyroid state combined with increased endogenous glucocorticoid levels is protective against inflammatory challenges. The inverse relationship between preproTRH expression and pituitary-adrenal function suggests the possibility of a direct inhibitory link connecting the hypothalamic-pituitary-adrenal and thyroid axes, and suggests alternative sites of therapeutic intervention for rheumatoid arthritis and other inflammatory associated disorders.  相似文献   
6.
We report the experimental results of the first MOSFET's ever fabricated using a laser plasma-source X-ray stepper. The minimum gate length of these transistors is 0.12 μm with an effective channel length of 0.075 μm. These transistors were patterned using a mix-and-match lithography scheme where the gate level was printed using a 1.4 nm plasma-source X-ray stepper while the other layers were patterned using optical lithography  相似文献   
7.
In the published version of the above abstract (from the conferenceProtein Engineering '87), the enzyme was incorrectly printedas ‘fructose-2,6-bisphosphatase’. The correct versionappears above.  相似文献   
8.
This study proposes a novel method to mechanically characterize the performance of individual bonds in low‐density, thermomechanically bonded nonwoven fabrics. Commercial bicomponent, polyethylene/polypropylene (PE/PP), nonwoven fabric was laser cut into bowtie‐shaped specimens for uniaxial tensile testing so that the central region of each specimen contained an individual bond. Three groups, each composed of 20 specimens, were tested with their longitudinal axes oriented along the machine direction (MD), the cross direction (CD), and at 45° between these two directions (DD). Prior to testing, the intrinsic variation in areal density and fiber orientation in the region surrounding the individual bond were quantified via orientation and relative basis weight parameters. During testing, images of the specimens were acquired to determine the occurrence of fiber breakage, bond deformation, and bond cohesive failure. Maximum force, stiffness, and orientation parameters were found to be significantly different among the three specimen groups (p < 0.01) but the relative basis weight was not (p > 0.01). The stiffness and maximum load were linearly correlated with both the areal density and fiber orientation. Pre‐existing voids or windows within the bond lowered the maximum force for specimens with the longitudinal axes aligned with the MD. These voids had no influence on the maximum force achieved by the specimens aligned with the CD and DD. The bonds in these specimens were observed to deform less than the bonds in the specimens with the longitudinal axes aligned with the MD. The results indicate the importance of the fiber structure surrounding the bond on the tensile properties, deformation and failure mode of individual bonds within the nonwoven fabric. POLYM. ENG. SCI., 59:311–322, 2019. © 2018 Society of Plastics Engineers  相似文献   
9.
Recent studies on human kallikrein 2 (hK2) have revealed striking similarities and significant differences with the closely related kallikrein PSA. Both PSA and hK2 are primarily localized to the prostate and share close structural similarities. Although both kallikreins are produced by the same secretory epithelial cells in the prostate, hK2 is associated more with prostate tumors than PSA and is highly expressed in poorly differentiated cancer cells. The potent trypsin-like activity of hK2 contrasts with the weak chymotrypsin-like activity of PSA. The inactive precursor form of PSA, proPSA, is converted rapidly to active PSA by hK2, suggesting an important in vivo regulatory function by hK2 on PSA activity. The high homology between hK2 and PSA results in significant cross-reactivity to hK2 by polyclonal and some monoclonal antibodies to PSA. Future studies on both PSA and hK2 need to take into account this potential for cross-reactivity. Specific monoclonal antibodies to hK2 have now demonstrated that serum levels of hK2, like PSA, are correlated with prostate cancer. The production of hK2 protein in active protease form and specific monoclonal antibodies to the hK2 antigen will allow extensive future studies delineating the physiological and clinical utility of this new prostate antigen.  相似文献   
10.
This analysis of drug use in family dyads draws on data from a series of nationwide studies in which interviews were conducted separately with teenagers and with older members of their families (i.e., their mothers, fathers, older siblings). The interview schedule for these studies examines each individual's personal experience with a broad range of psychoactive substances. Thus, to the extent that behavioral similarities do occur in family dyads, "same drug links" can be compared to "cross drug links," providing a basis for differentiating evidence of direct imitation from less specific patterns of behavioral similarity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)  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