| 1. |
- Zhang, Guobin, et al.
(författare)
-
Thermal Analysis of AlGaN/GaN High-Electron-Mobility Transistors with Graphene
- 2018
-
Ingår i: Journal of Nanoscience and Nanotechnology. - : American Scientific Publishers. - 1533-4880 .- 1533-4899. ; 18:11, s. 7578-7583
-
Tidskriftsartikel (refereegranskat)abstract
- A thermal analysis of AlGaN/GaN high electron mobility transistors (HEMTs) with Graphene is investigated using Silvaco and Finite Element Method. Two thermal management solutions are adopted; first of all, graphene is used as dissipation material between SiC substrate and GaN buffer layer to reduce thermal boundary resistance of the device. At the same time, graphene is also used as a thermal spread material on the top of the source contacts to reduce thermal resistance of the device. The thermal analysis results show that the temperature rise of device adopting graphene decreases by 46.5% in transistors operating at 13.86 W/mm. Meanwhile, the thermal resistance of GaN HEMTs with graphene is 6.8 K/W, which is much lower than the device without graphene, which is 18.5 K/W. The thermal management solutions are useful for integration of large-scale graphene into practical devices for effective heat spreading in AlGaN/GaN HEMT.
|
|
| 2. |
- Zäll, Erik, et al.
(författare)
-
Aerosol-based deposition of broadband antireflective silica coating with closed mesoporous structure
- 2023
-
Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 250, s. 112078-112078
-
Tidskriftsartikel (refereegranskat)abstract
- Solar energy will be a crucial part of the sustainable, fossil free energy production of the future. A majority of this will be produced by solar collectors and photovoltaics. Important for the efficient utilization of the incident solar energy for both technologies are a cover glass with antireflective coatings giving it a high solar transmittance. In the current paper we describe the development of antireflective mesoporous silica coatings on low-iron float glass using the aerosol-based nFOG™ deposition technique. The coatings exhibit a hexagonal and closed pore structure, high smoothness, superhydrophilic properties (contact angle <5°) and consistent thicknesses of approximately 110 nm. This is in line with optimal thickness determined from simulations of the antireflective behavior. Low-iron float glass coated on both sides show a highly reproducible solar weighted transmittance of 95% in the wavelength range 300–2500 nm and an antireflective effect increasing with incident angle. The smoothness, closed pores and low contact angle indicate a high cleanability, which in combination with the high transmittance render a competitive broadband antireflective coating well adapted for solar glass applications.
|
|
| 3. |
- Zäll, Erik, et al.
(författare)
-
Durability of antireflective SiO2 coatings with closed pore structure
- 2023
-
Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier. - 0927-0248 .- 1879-3398. ; 261
-
Tidskriftsartikel (refereegranskat)abstract
- The use of antireflective coatings to increase the transmittance of the cover glass is a central aspect of achieving high efficiencies for solar collectors and photovoltaics alike. Considering an expected lifetime of 20–30 years for solar energy installations, the durability of the antireflective surfaces is essential. Here, a novel antireflective SiO2 coating with a hexagonally ordered closed pore structure, produced with an aerosol-based sol-gel method is benchmarked against two commercial coatings; produced with acid etching and sol-gel roll coating. The optical and mechanical properties together with contact angle characteristics were evaluated before and after various durability tests, including climate chamber tests, outdoor exposure, and abrasion. Compared to the commercial antireflective coatings with open pore structures, the novel coating performed in parity, or better, in all tests. Based on the results of humidity freeze and industrial climate chamber tests, it appears that the coating with closed pore structure has a better ability to prevent water adsorption. Additionally, the closed pore structure of the coating seems to minimize the accumulation of dirt and deposits. The abrasion and cleanability test further confirm the advantages of a closed pore structure, showcasing the coating's mechanical durability. While the coatings exhibit similar hardness and reduced elastic modulus, the closed pore coating proves to be even harder after undergoing the industrial climate chamber test, but also slightly more brittle, as indicated by the probability of crack initiation. In summary the closed pore structure is well suited for tempered and arid climates, making it a truly competitive alternative to existing antireflective coatings.
|
|
| 4. |
- Liu, X., et al.
(författare)
-
Biomimetic Photonic Multiform Composite for High-Performance Radiative Cooling
- 2021
-
Ingår i: Advanced Optical Materials. - : Wiley. - 2162-7568 .- 2195-1071. ; 9:22
-
Tidskriftsartikel (refereegranskat)abstract
- Nanostructures on bodies of biological inhabitants in severe environments can exhibit excellent thermoregulation, which provide inspirations for artificial radiative cooling materials. However, achieving both large-scale manufacturing and flexible form-compatibility to various applications needs remains as a formidable challenge. Here a biomimetic strategy is adopted to design a thermal photonic composite inspired by the previously unexplored golden cicada's evolutionarily optimized thermoregulatory ability. A microimprint combined with phase separation method is developed for fabricating a biomimetic photonic material made of porous polymer–ceramic composite profiled in microhumps. The composite demonstrates high solar reflectance (97.6%) and infrared emissivity (95.5%) in atmospheric window, which results in a cooling power of 78 W m−2 and a maximum subambient temperature drop of 6.6 °C at noon. Moreover, the technique facilitates multiform manufacturing of the composites beyond films, as demonstrated by additive printing into general 3D structures. This work offers biomimetic approach for developing high-performance thermal regulation materials and devices.
|
|
| 5. |
- dos Reis, Glaydson Simões, et al.
(författare)
-
Flexible supercapacitors of biomass-based activated carbon-polypyrrole on eggshell membranes
- 2021
-
Ingår i: Journal of Environmental Chemical Engineering. - : Elsevier. - 2213-3437. ; 9:5
-
Tidskriftsartikel (refereegranskat)abstract
- The quest to develop flexible membrane-like supercapacitors to be applied in advanced electronic devices with a flexible structure is important for the modern world. In this study, we developed biomass-based supercapacitors by depositing activated carbon on an eggshell membrane and subsequently coating these with polypyrrole in a two-step procedure. The competition between the electrical double layer capacitance (EDLC) from activated carbon and the pseudocapacitance (PC) for the hybrid device is controlled by varying the amount of polypyrrole (PC component) in a time-dependent polymerization process. An areal capacitance of 172.5 mF cm−2, a corresponding energy density of 4.73 W h kg−1, and power density of 320.8 W kg−1, with a 60% retention even after 1000 cycles were obtained for samples prepared with the polymerization of polypyrrole on the activated carbon (incorporation of an active layer of 3.18 mg cm−2).
|
|
| 6. |
- Iakunkov, Artem, et al.
(författare)
-
Activated graphene as a material for supercapacitor electrodes : effects of surface area, pore size distribution and hydrophilicity
- 2019
-
Ingår i: Physical Chemistry, Chemical Physics - PCCP. - : RSC Publishing. - 1463-9076 .- 1463-9084. ; 21:32, s. 17901-17912
-
Tidskriftsartikel (refereegranskat)abstract
- Activated reduced graphene oxide (a-rGO) is a material with a rigid 3D porous structure and high specific surface area (SSA). Using variation of activation parameters and post-synthesis mechanical treatment we prepared two sets of materials with a broad range of BET (N2) SSA ∼1000–3000 m2 g−1, and significant differences in pore size distribution and oxygen content. The performance of activated graphene as an electrode in a supercapacitor with KOH electrolyte was correlated with the structural parameters of the materials and water sorption properties. a-rGO is a hydrophobic material as evidenced by the negligibly small BET (H2O) SSA determined using analysis of water vapor sorption isotherms. However, the total pore volume determined using water vapor sorption and sorption of liquid water is almost the same as the one found by analysis of nitrogen sorption isotherms. Ball milling is found to provide an improved bulk density of activated graphene and collapse of all pores except the smallest ones (<2 nm). A decrease in the activation temperature from 850 °C to 550 °C is found to result in materials with a narrow micropore size distribution and increased oxygen content. Elimination of mesopores using ball milling or a lower activation temperature provided materials with better specific capacitance despite a significant decrease (by ∼30%) of the BET (N2) SSA. The best gravimetric and volumetric capacitances in KOH electrolyte were achieved not for samples with the highest value of the BET (N2) SSA but for materials with 80–90% of the total pore volume in micropores and an increased BET (H2O) SSA. Comparing the performance of electrodes prepared using rGO and a-rGO shows that a more hydrophilic surface is favorable for charge storage in supercapacitors with KOH electrolyte.
|
|
| 7. |
- Kumar Wagri, Naresh, et al.
(författare)
-
An overview of the machinability of alloy steel
- 2022
-
Ingår i: Materials Today. - : Elsevier. - 2214-7853. ; 62, s. 3771-3781
-
Tidskriftsartikel (refereegranskat)abstract
- Machinability is an important aspect for the machining of any materials. In general, it is controlled by means of tool life, surface roughness, power consumption, chip morphology, cutting forces, cutting fluid and material removal rate. Furthermore, it is also affected the microstructure, grain size, hardness, yield, tensile strength and chemical composition of the work piece material and tool materials. Advanced materials such as alloy steel, structural ceramics, and hardened steel are difficult to machine using conventional tool materials. The machining parameters such as depth of cut, cutting velocity and feed rate play a very important role to achieve high quality of surface finish on the machined surface. High interface temperatures at the tool tip and work piece material resulted poor surface finish and increases cutting forces and tool wear. A high cutting speeds with high feed and depth of cut causes high heat generation which reduces the tool substrates bonding strength resulting in accelerated tool wear and reducing tool life. In this article influence of heat treatment to enhance machinability alloy steel reviewed in terms of tool life, surface roughness, cutting forces and chip morphology. This article will be very useful for the students, research scholar and industrialist working in the area of machining.
|
|
| 8. |
- Kumar Wagri, Naresh, et al.
(författare)
-
Investigation on the performance of coated carbide tool during dry turning of AISI 4340 alloy steel
- 2023
-
Ingår i: Materials. - : MDPI. - 1996-1944 .- 1996-1944. ; 16:2
-
Tidskriftsartikel (refereegranskat)abstract
- The machinability of materials is highly affected by their hardness, and it affects power consumption, cutting tool life as well as surface quality while machining the component. This work deals with machining of annealed AISI 4340 alloy steel using a coated carbide tool under a dry environment. The microhardness of annealed and non-annealed workpieces was compared and a significant reduction was found in the microhardness of annealed samples. Microstructure examination of the annealed sample revealed the formation of coarse pearlite which indicated a reduction of hardness and improved ductility. A commercially CVD multilayer (TiN/TiCN/Al2O3/ZrCN) coated cemented carbide cutting tool was employed for turning quenched and tempered structural AISI 4340 alloy steel by varying machining speed, rate of feed, and depth of cut to evaluate the surface quality, machining forces, flank wear, and chip morphology. According to the findings of experiments, the feed rate possesses a high impact on surface finish, followed by cutting speed. The prominent shape of the serrated saw tooth chip was noticed at a higher cutting speed. Machined surface finish and cutting forces during turning is a function of the wear profile of the coated carbide insert. This study proves that annealing is a low-cost and economical process to enhance the machinability of alloy steel.
|
|
| 9. |
- Moradi, Mahmoud, et al.
(författare)
-
Mathematical modelling of fused deposition modeling (FDM) 3D printing of poly vinyl alcohol parts through statistical design of experiments approach
- 2023
-
Ingår i: Mathematics. - : MDPI. - 2227-7390. ; 11:13
-
Tidskriftsartikel (refereegranskat)abstract
- This paper explores the 3D printing of poly vinyl alcohol (PVA) using the fused deposition modeling (FDM) process by conducting statistical modeling and optimization. This study focuses on varying the infill percentage (10–50%) and patterns (Cubic, Gyroid, tri-hexagon and triangle, Grid) as input parameters for the response surface methodology (DOE) while measuring modulus, elongation at break, and weight as experimental responses. To determine the optimal parameters, a regression equation analysis was conducted to identify the most significant parameters. The results indicate that both input parameters significantly impact the output responses. The Design Expert software was utilized to create surface and residual plots, and the interaction between the two input parameters shows that increasing the infill percentage (IP) leads to printing heavier samples, while the patterns do not affect the weight of the parts due to close printing structures. On the contrary, the discrepancy between the predicted and actual responses for the optimal samples is below 15%. This level of error is deemed acceptable for the DOE experiments.
|
|
| 10. |
- Näsström, Gunilla, 1968-
(författare)
-
Measurement of alignment between standards and assessment
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many educational systems of today are standards-based and aim at for alignment, i.e. consistency, among the components of the educational system: standards, teaching and assessment. To conclude whether the alignment is sufficiently high, analyses with a useful model are needed. This thesis investigates the usefulness of models for analyzing alignment between standards and assessments, with emphasis on one method: Bloom’s revised taxonomy. The thesis comprises an introduction and five articles that empirically investigate the usefulness of methods for alignment analyses.In the first article, the usefulness of different models for analyzing alignment between standards and assessment is theoretically and empirically compared based on a number of criteria. The results show that Bloom’s revised taxonomy is the most useful model. The second article investigates the usefulness of Bloom’s revised taxonomy for interpretation of standards in mathematics with two differently composed panels of judges. One panel consisted of teachers and the other panel of assessment experts. The results show that Bloom’s revised taxonomy is useful for interpretation of standards, but that many standards are multi-categorized (placed in more than one category). The results also show higher levels of intra- and inter-judge consistency for assessment experts than for teachers. The third article further investigates the usefulness of Bloom’s revised taxonomy for analyses of alignment between standards and assessment. The results show that Bloom’s revised taxonomy is useful for analyses of both standards and assessments. The fourth article studies whether vague and general standards can explain the large proportion of multi-categorized standards in mathematics. The strategy was to divide a set of standards into smaller substandards and then compare the usefulness and inter-judge consistency for categorization with Bloom’s revised taxonomy for undivided and divided standards. The results show that vague and general standards do not explain the large proportion of multi-categorized standards. Another explanation is related to the nature of mathematics that often intertwines conceptual and procedural knowledge. This was also studied in the article and the results indicate that this is a probable explanation. The fifth article focuses on another aspect of alignment between standards and assessment, namely the alignment between performance standards and cut-scores for a specific assessment. The validity of two standard-setting methods, the Angoff method and the borderline-group method, was investigated. The results show that both methods derived reasonable and trustworthy cut-scores, but also that there are potential problems with these methods.In the introductory part of the thesis, the empirical studies are summarized, contextualized and discussed. The discussion relates alignment to validity issues for assessments and relates the obtained empirical results to theoretical assumptions and applied implications. One conclusion of the thesis is that Bloom’s revised taxonomy is useful for analyses of alignment between standards and assessments. Another conclusion is that the two standard setting methods derive reasonable and trustworthy results. It is preferable if an alignment model can be used both for alignment analyses and in ongoing practice for increasing alignment. Bloom’s revised taxonomy has the potential for being such an alignment model. This thesis has found this taxonomy useful for alignment analyses, but its’ usefulness for increasing alignment in ongoing practice has to be investigated.
|
|
