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- Leijon, Mats, et al.
(författare)
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An electrical approach to wave energy conversion
- 2006
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Ingår i: Renewable energy. - : Elsevier BV. - 0960-1481 .- 1879-0682. ; :31, s. 1309-1319
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Tidskriftsartikel (refereegranskat)abstract
- Motions in nature, for example ocean waves, can play a significant role in tomorrow's electricity production, but the constructions require adaptations to its media. Engineers planning hydropower plants have always taken natural conditions, such as fall height, speed of flow, and geometry, as basic design parameters and constraints in the design. The present paper describes a novel approach for electric power conversion of the vast ocean wave energy. The suggested linear electric energy converter is adapted to the natural wave motion using straightforward technology. Extensive simulations of the wave energy concept are presented, along with results from the experimental setup of a multisided permanent magnet linear generator. The prototype is designed through systematic electromagnetic field calculations. The experimental results are used for the verification of measurements in the design process of future full-scale direct wave energy converters. The present paper, describes the energy conversion concept from a system perspective, and also discusses the economical and some environmental considerations for the project.
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- Majdi, Saman, 1977-, et al.
(författare)
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Enhanced Hall mobility in graphene-on-electronic-grade diamond
- 2023
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Ingår i: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 123:1
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Tidskriftsartikel (refereegranskat)abstract
- The outstanding electronic properties of graphene make this material a candidate for many applications, for instance, ultra-fast transistors. However, self-heating and especially the detrimental influence of available supporting substrates have impeded progress in this field. In this study, we fabricate graphene-diamond heterostructures by transferring graphene to an ultra-pure single-crystalline diamond substrate. Hall-effect measurements were conducted at 80 to 300 K on graphene Hall bars to investigate the charge transport properties in these devices. Enhanced hole mobility of 2750 cm(2) V-1 s(-1) could be observed at room-temperature when using diamond with reduced nitrogen (N-s(0)) impurity concentration. In addition, by electrostatically varying the carrier concentration, an upper limit for mobility is determined in the devices. The results are promising for enabling carbon-carbon (C-C) devices for room-temperature applications.
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- Ahlqvist, Jan, et al.
(författare)
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Bone demarcation of the temporomandibular joint. Validity of clinical assessment of bone thickness by means of CT.
- 1998
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Ingår i: Acta Radiologica. - 0284-1851 .- 1600-0455. ; 39:6, s. 649-55
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: To study the CT depiction of bone demarcations in the temporomandibular joint, using conventional window level and window width; and to evaluate observer performance in estimating bone thickness in these images.MATERIAL AND METHODS: Seven joint specimens were imaged by CT and then cryosectioned. The measurements of bone wall thickness in the images were compared to the true bone thickness at each cutting level. In addition, 4 experienced radiologists estimated the thickness of the bone walls in the images.RESULTS: The relative difference between the CT reproduction and the true bone thickness was small for bone walls thicker than 2 mm. This difference increased with the decrease in bone thickness and the increase in the inclination of the bone wall from the perpendicular to the image plane. Bone walls thinner than 1 mm were reproduced as considerably thicker than their true thickness. This resulted in a clinical overestimation of bone thickness.CONCLUSION: Both the CT representation and the interpretation of bone demarcation in the temporomandibular joint may constitute a problem. Partial volume averaging effects can result in an overestimation of bone dimensions amounting to 200% for thin bones. The central white zone in images of thin bone walls obtained with the parameters described here could serve as an indicator that could help to reduce the risk of overestimating bone thickness.
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- Ahlqvist, Jan, et al.
(författare)
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Sources of radiographic distortion in conventional and computed tomography of the temporal bone.
- 1998
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Ingår i: Dento-Maxillo-Facial Radiology. - 0250-832X .- 1476-542X. ; 27:6, s. 351-7
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVES: To identify those bony regions of the glenoid fossa where, due to the inclination, there is an increased risk of radiographic distortion in conventional and computed tomography (CT).METHODS: The inclination of the roof and posterior wall of the glenoid fossa was determined relative to established imaging planes. Measurements were performed on 50 corrected coronal MR and 50 axial CT images and 200 sagittal cryosections of 50 temporomandibular joints (TMJs). The location of regions with unfavourable bone wall inclination was identified using the condyle as a reference.RESULTS: The inclination of parts of the fossa roof exceeded the limit for reliable depiction in corrected sagittal and coronal planes in 40% and 8% of the joints respectively. The inclination of parts of the posterior wall of the fossa exceeded the limit for reliable depiction in corrected sagittal and in true sagittal planes in 100% and 84% of the joints respectively. In 84% of the joints the inclination exceeded the limit for reliable depiction in the axial plane. For both bone walls the regions with unfavourable inclination were in the medial part of the joint.CONCLUSIONS: The angulation of parts of the roof and posterior wall of the glenoid fossa in relation to established imaging planes makes them highly susceptible to distortion. The oblique coronal projection is well suited for depiction of the roof of the fossa and preferable to a sagittal projection. An oblique axial projection is required for the posterior wall.
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- Ahlqvist, Jan, 1952-
(författare)
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The temporomandibular joint : Tomopraphic and CT assesment of its bone demarcations with reference to adjacent organs
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The wall of the temporal bone separating the temporomandibular joint (TMJ) from surroundings organs, can be very thin and also have development defects. Distortion effects in the radiographic reproduction of these bone walls can result in misinterpretations when exanimating suspected pathologic changes in the region. These areas need to be radiographic identified prior to taking any invasive measures. Incorrect assessment of bone thickness may lead to serious sequelae due to the risk of penetration injury during invasive investigation or treatment of the TMJ or ear. The purpose of this project was to gain more detailed knowledge about the anatomy and topography of the TMJ with special reference to its bone demarcations regarding adjacent organs and to evaluate the tomographic and computed tomographic (CT) depiction of these bone walls. To obtain a basic analysis of the tissue anatomy and tomographic and CT reproduction of the TMJ region, autopsy specimens were studied. After CT and conventional tomography, the specimens were sectioned in a microtom. Three- dimensional orientation systems allowed identification of section depth in the radiograms and in the histologic sections, allowing the radiograms in turn to be correlated with the true anatomy. The angle of inclination relative to the perpendicular to established imaging planes the bone walls studied was examined in three projections in order to identify regions where the bone demarcation showed an unfavorable inclination regarding the possibility of valid radiographic representation. The thickness of the bone wall between the TMJ and the middle cranial fossa, measured in the thinnest part, varied between 0.08 and 3.62 mm, averaging 1.14 mm. The bone wall between the TMJ and the middle ear showed less variation in thickness ranging from 0.00 to 1.80 mm. The thickness of the bone wall separating the TMJ from the external auditory canal varied between 1.50 mm (lateral part) and 1.21 mm (central part), with a range of between 0.21 and 4.10 mm. Development defects of this bone wall were found in 5.2 % of the examined joints. The validity in tomographic depiction of these walls was highly dependent on an optimal orientation of the bone wall in relation to the image plane. The variations in the anatomy and sagittal dimension of the external auditory canal led to variations in tomographic blurring, and suggested the need for examinations after patient repositioning in cases of suspected bone resorbing lesions so that image aberration due to unfavorable inclination of the bone wall relative to the image plane may be excluded. CT of these bone walls was valid (± 10 %) for walls thicker than approximately 1 mm, forming an angle of less than 35® with the perpendicular to the scan plane when the bone wall thickness was determined as the full-width-at-half-maximum (FWHM). For bone walls thinner than 1 mm, and for those thicker than 1 mm and at an angle exceeding 35®, partial volume averaging effects resulted in a progressively increasing magnification of bone dimensions. Observer estimations of bone thickness from images obtained using conventional bone window settings (c=400, W=2000) showed good agreement for bone walls thicker than 1 mm and with an angle of inclination relative to the perpendicular to the image plane of less than approximately 25®. For bone walls thinner than 1 mm and for thicker than 1 mm with an inclination exceeding approximately 25®, the estimations resulted in a progressively increasing overestimation amounting 200% for gracile bone walls with an inclination of 45® to 50®. Determination of width or absence of the central white zone in images obtained with the described parameters could help to reduce the risk of overestimation of bone thickness. A considerable part of the bone walls separating the TMJ from the middle cranial fossa and the external auditory canal/middle ear, respectively, have dimensions and inclinations to established imaging planes used at TMJ examinations that make the depiction of these walls highly susceptible to image distortion.
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- Ahlqvist, Jan, et al.
(författare)
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Validity of computed tomography in imaging thin walls of the temporal bone
- 1999
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Ingår i: Dento-Maxillo-Facial Radiology. - : British Institute of Radiology. - 0250-832X .- 1476-542X. ; 28:1, s. 13-19
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Tidskriftsartikel (refereegranskat)abstract
- OBJECTIVES: To evaluate the validity of computed tomography (CT) for reproduction of the bone margins of the temporomandibular joint (TMJ).METHODS: Seven TMJ specimens were examined with a CT and then cryosectioned. The bone separating the TMJ from the middle cranial fossa, middle ear and external auditory canal was measured as the full width at half maximum (FWHM). Measurements were compared with the true thickness of the bone wall.RESULTS: There was good agreement when the bone walls were thicker than 1 mm: accuracy was influenced only by the angle of the bone wall to the scanning plane. Conversely, bone walls thinner than 1 mm were reproduced with a magnification that increased with decreasing bone thickness. The difference increased further as the inclination of the bone wall became greater.CONCLUSION: Measurements performed at FWHM are reliable within +/- 10% for bone walls more than approximately 1 mm thick which form an angle of less than 35 degrees to the perpendicular of the scanning plane. For bone walls thinner than 1 mm and for those thicker than 1 mm with an inclination exceeding approximately 35 degrees, partial volume effects result in a progressively increasing magnification of bone thickness.
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