| 1. |
- Ahmed, Taha, et al.
(författare)
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Phonon–phonon and electron–phonon coupling in nano-dimensional ZnO
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Thermal losses through vibrational coupling are critical bottlenecks limiting several materials classes from reaching their full potential. Altering the phonon–phonon and electron–phonon coupling by controlled suppression of vibrational degrees of freedom through low-dimensionality are promising but still largely unexplored approaches. Here we report a detailed study of the first- and second-order Raman processes as a function of size for low-dimensional ZnO. Wurtzite ZnO nanoparticles were synthesised into 3D frameworks of ZnO crystallites, with tailored crystallite diameters from 10 nm to 150 nm and characterised by electron microscopy, X-ray diffraction and non-resonant and resonant Raman spectroscopy.We present a short derivation of how resonance Raman and the relation between the longitudinal optical (LO) phonons can be utilised to quantify the electron–phonon coupling, its merits, and limitations. Theoretical Raman response using density functional theory is corroborating the experimental data in assigning first- and second-order Raman modes. The Lyddane-Sachs-Teller equation was applied to the measured LO–TO split and revealed no change in the ratio between the static and high-frequency dielectric constant with changing ZnO dimension from 10 nm to 150 nm. The second-order Raman revealed a phonon–phonon coupling that generally increased with particle size and markedly so for differential modes. Resonance Raman showed the fundamental LO mode and the 2nd, 3rd, and 4th overtones. The intensity relation between the fundamental LO mode and its overtones enabled the extraction of the change in electron–phonon coupling via the Huang-Rhys parameter as a function of particle size, which showed an increase with particle size.
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| 2. |
- Gruber, Paul H., et al.
(författare)
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WC-Ni cemented carbides prepared from Ni nano-dot coated powders
- 2023
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Ingår i: International journal of refractory metals & hard materials. - : Elsevier Ltd. - 0263-4368. ; 117
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Tidskriftsartikel (refereegranskat)abstract
- This study presents a novel approach for the synthesis of WC-Ni cemented carbides with enhanced mechanical properties. A low-cost solution-based route was used to coat WC powders with well-distributed metallic nickel dots measuring between 17 nm and 39 nm in diameter. Varying compositions with loadings of 2, 6, and 14 vol% Ni were consolidated using spark plasma sintering (SPS) at 1350 °C under 50 MPa of uniaxial pressure giving relative densities of 99 ± 1 %. The sintered WC-Ni cemented carbides had an even distribution of the Ni binder phase in all compositions, with retained ultrafine WC grain sizes of 0.5 ± 0.1 μm from the starting powder. The enhanced sinterability of the coated powders allowed for consolidation to near theoretical densities, with a binder content as low as 2 vol%. This is attributed to the uniform distribution of nickel and an extensive Ni-WC interface existing prior to sintering. The small size of the Ni dots likely also contributed to the solid-state sintering starting temperatures of as low as 800 °C. The mechanical performance of the resulting cemented carbides was evaluated by measuring the hardness at temperatures between 20 °C and 700 °C and estimating toughness at room temperature using Vickers indentations. These results showed that the mechanical properties of the WC-Ni cemented carbides synthesised by our method were comparable to conventionally prepared WC-Co cemented carbides with similar grain sizes and binder contents and superior to conventionally prepared WC-Ni cemented carbides. In particular, the 2 vol% Ni composition had excellent hardness at room temperature of up to 2210HV10, while still having an indentation fracture toughness of 7 MPa·m0.5. Therefore, WC-Ni cemented carbides processed by this novel approach are a promising alternative to conventional WC-Co cemented carbides for a wide range of applications.
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| 3. |
- Katea, Sarmad Naim
(författare)
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Carbothermal synthesis of nano-structured metal, carbides and nitrides, using solution processed precursors; from precursor to product
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis describes how solution chemical based carbothermal syntheses can be used for the synthesis of nanostructured zirconium carbide and nitride nanophase powders as well as a nickel-metal coating on NbC for use as a sintering agent. The focus is on the development of novel synthesis routes using atomic or small nanoscale level mixing of the metal oxide and carbon source in the starting precursors, enabling the use of lower synthesis temperatures and non-agglomerated nanosized powders. Detailed characterisation of the final materials and those found during the phase-evolution were performed using TG-DTA, dilatometry, XPS, XRD, IR and Raman spectroscopy, ToF-ERDA, PIXE, SEM-EDX/EBSD/TKD, (S)TEM-EDX/HAADF/EELS and Vickers micro-hardness.Starting materials with various carbon-to-zirconia ratios, composed of 2-4 nm sized ZrO2 particles embedded in amorphous carbon, were prepared by hydrolysis of Zr(OPrn)4 and sucrose. Following heating in argon and nitrogen gas yielded nano-phase ZrC and ZrN powders, respectively. Heating in argon gas to 1495 oC, yielded ca. 75 nm sized, loosely agglomerated ZrC particles after 30 minutes annealing when using a sucrose-C:Zr ratio of 5:1. Weakly agglomerated, ca. 30-40 nm sized ZrC particles with a dense 5 nm thick shell of micro-crystalline or amorphous Zr(C,O) and surplus-carbon was obtained after 3 minutes annealing, with a sucrose-C:Zr ratios of 12-20.The same starting components and heat treatment, but with a nitrogen atmosphere, yielded well-crystallised Zr(N,C) particles centred around 40-90 nm in size, with and without a 4-6 nm Zr(N,C,O) shell; the former with 5-6 sucrose-C:Zr and the latter with 7-8 sucrose-C:Zr, together with surplus carbon. The phase-evolution was studied for the sample with sucrose-C:Zr ratio of 8:1. This showed that the nitridation started as early as at 1200 °C and that shell-less c-Zr(N,C) was obtained at 1400 °C.The other part of this thesis describes solution-based nickel metal coating of a fine NbC powder and its sintering into dense NbC-Ni composites. The coating process produced well-distributed dot-like nickel metal coatings, after heating to 500 °C. The nickel dots sizes varied from 6 to 26 nm depending on the nickel loading. Pellets of the 14 wt. % Ni coated NbC powder were sintered at temperatures up to 1500 °C with 30 minutes annealing. The main sintering step started at 1110 °C and ended at 1375 °C, whereupon a final contraction took place ending at 1500 °C. Nearly full-dense composites were obtained after heating to 1375 and 1405 °C whereas a fully dense composite was obtained after heating to 1500 °C, with 30 minutes annealing. The composites were composed of networked, interconnected NbC grains with sizes of ca. 0.5-5 µm for the 1375 and 1405 °C samples. For the corresponding sample heated to 1500 °C, with a 30 minutes annealing, the size-range was 5-30 µm. The binder phases were well-distributed and highly textured.
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| 4. |
- Katea, Sarmad Naim, et al.
(författare)
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Fast, Low-Cost Synthesis of ZnO:Eu Nanosponges and the Nature of Ln Doping in ZnO
- 2020
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Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 59:11, s. 7584-7602
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Tidskriftsartikel (refereegranskat)abstract
- A low-cost template-free solution chemical route to highly porous nanocrystalline sponges of ZnO-EuO1.5 with 0-5 mol % Eu is presented. The process uses Zn- and Eu-acetate- nitrate and triethanolamine as precursors in methanol. After evaporation of the solvent and heating at 200 degrees C for 3 min, crystalline ZnO:Eu sponges with minor amounts of organic residues were obtained. Heating to 400 degrees C replaced the organics with carbonate, which in its turn was decomposed at temperatures below 600 degrees C, forming ZnO:Eu sponges. Samples heated to 200-1000 degrees C for 3 min were studied with XRD, SEM, TEM, TG, XPS and IR spectroscopy. The ZnO:Eu crystallite sizes could be tuned from below 10 nm for sponges prepared at 200-500 degrees C, to over 100 nm range at 900 degrees C, without sintering of the overall microstructure. XRD showed the presence of hexagonal ZnO:Eu (or at 700-1000 degrees C, ZnO:Eu and cubic Eu2O3) as the only phases present. The ZnO:Eu had slightly larger unit cell dimensions than the literature value of ZnO for samples obtained at 200-600 degrees C, while the unit cells of samples obtained at higher temperatures were quite close to the value of undoped ZnO. XPS showed that Eu was mainly in its 3+ state and well-distributed within the sponges but segregated at the ZnO sponge surface upon heating at 700-1000 degrees C, in accordance with XRD studies showing Eu2O3 formation.
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| 5. |
- Katea, Sarmad Naim, et al.
(författare)
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Low Cost, Fast Solution Synthesis of 3D Framework ZnONanosponges
- 2017
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 56:24, s. 15150-15158
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Tidskriftsartikel (refereegranskat)abstract
- An efficient, template-free solution-chemical route to nanostructured ZnO sponges is presented: A mixture of Zn(NO3)(2)center dot 6H(2)O, Zn(OAc)(2).2H(2)O, and triethanolamine in methanol was evaporated to a highly viscous liquid and rapidly heated to >200 degrees C for 1-3 min to achieve highly porous, nanocrystalline sponges of ZnO. The viscous precursor concentrate obtained on evaporation in air was characterized by TG, DSC, and IR spectroscopy, and the product ZnO sponges by XRD, SEM, TEM, and IR spectroscopy. The fast reaction forming ZnO started at 140 degrees C and finished within a few seconds. Scherrer analysis of the XRD peak broadening showed average crystallite sizes of 8 to 11 nm for ZnO prepared by annealing at 200-450 degrees C (3 min), while grain growth to 134 nm was observed from 500 to 900 degrees C (3 min). The ZnO powders obtained at 200-900 degrees C had cell dimensions of a = 3.25 angstrom and b = 5.21 angstrom, matching the ZnO literature data well. SEM and TEM analyses showed highly porous, bread-like 3D nanostructures built by ca. 30-70 nm thick walls of ZnO crystallites of the approximate average sizes given by the XRD Scherrer analysis. It seems that the crystal growth above 450 degrees C takes place within the ZnO 3D structure obtained at lower temperatures without much sintering of the larger porous structure.
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| 6. |
- Kumar, P. Anil, et al.
(författare)
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All-alkoxide based deposition and properties of a multilayer La0.67Sr0.33MnO3/CoFe2O4/La0.67Sr0.33MnO3 film
- 2021
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Ingår i: European Journal of Inorganic Chemistry. - : John Wiley & Sons. - 1434-1948 .- 1099-1948 .- 1099-0682. ; 2021:18, s. 1736-1744
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Tidskriftsartikel (refereegranskat)abstract
- Single and multilayer films of La0.67Sr0.33MnO3 and CoFe2O4 were deposited by spin-coating. The all-alkoxide precursors allowed inorganic gel films of extreme homogeneity to be formed and converted to phase pure complex oxides at low temperatures. La0.67Sr0.33MnO3 films were made with La- and Ca-methoxy-ethoxides and Mn19O12(moe)(14)(moeH)(10) as precursors at 800 degrees C. The CoFe2O4 films were obtained at extremely low 275 degrees C, using a new CoFe2-methoxyethoxide precursor. The decomposition and microstructural development on heating was described by TG, TEM, XRD and IR spectroscopy. XRD showed no spurious phases and the unit-cell dimensions coincided quite well with literature values of the targeted phases. The structural, magnetic and electronic properties of these films established their phase purity and high quality with physical properties comparable to films deposited by physical deposition methods. The magnetic and magneto transport results are presented for single, bi- and tri- layer structures. The magnetically soft La0.67Sr0.33MnO3 layer was exchange coupled to the magnetically hard CoFe2O4 layer, giving rise to interesting switching behaviour in magnetism and magneto-transport properties.
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| 7. |
- Marin, Riccardo, et al.
(författare)
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Europium-doped ZnO nanosponges : controlling optical properties and photocatalytic activity
- 2019
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Ingår i: Journal of Materials Chemistry C. - : Royal Society of Chemistry (RSC). - 2050-7526 .- 2050-7534. ; 7:13, s. 3909-3919
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Tidskriftsartikel (refereegranskat)abstract
- The optical features and photocatalytic activity of porous undoped and Eu3+-doped zinc oxide (ZnO) sponge-like structures, annealed at various temperatures, were assessed to establish the relationship between structure, morphology and photophysical properties. Upon monitoring the degradation of organic dye rhodamine B, undoped ZnO nanosponges annealed at 800 °C showed the highest photocatalytic activity, being among the best performing ZnO-based photocatalysts reported so far. The observed red- and blue-shift of the characteristic broad-band ZnO emission spectra as a function of the annealing temperature was ascribed to the nature of defects induced into the ZnO structure related to deficiency or excess of oxygen at lower (200 to 400 °C) and higher (500 to 1000 °C) annealing temperatures. These temperature-induced defects - along with the morphological sample characteristics - governed the photocatalytic performance. Doping-induced enhancement of the photocatalytic activity was noticed in specific samples and was found to markedly depend on the intrinsic properties of the undoped material. Overall, annealing temperature and europium doping concentration synergistically contributed to the defect structure, morphology and crystallinity, ultimately determining optical properties and therewith correlated photocatalytic activity.
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| 8. |
- Mukherjee, Soham, et al.
(författare)
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Entrapped Molecule-Like Europium-Oxide Clusters in Zinc Oxide with Nearly Unaffected Host Structure
- 2023
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Ingår i: Small. - : Wiley-VCH Verlagsgesellschaft. - 1613-6810 .- 1613-6829. ; 19:1
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Tidskriftsartikel (refereegranskat)abstract
- Nanocrystalline ZnO sponges doped with 5 mol% EuO1.5 are obtained by heating metal–salt complex based precursor pastes at 200–900 °C for 3 min. X-ray diffraction, transmission electron microscopy, and extended X-ray absorption fine structure (EXAFS) show that phase separation into ZnO:Eu and c-Eu2O3 takes place upon heating at 700 °C or higher. The unit cell of the clean oxide made at 600 °C shows only ≈0.4% volume increase versus undoped ZnO, and EXAFS shows a ZnO local structure that is little affected by the Eu-doping and an average Eu3+ ion coordination number of ≈5.2. Comparisons of 23 density functional theory-generated structures having differently sized Eu-oxide clusters embedded in ZnO identify three structures with four or eight Eu atoms as the most energetically favorable. These clusters exhibit the smallest volume increase compared to undoped ZnO and Eu coordination numbers of 5.2–5.5, all in excellent agreement with experimental data. ZnO defect states are crucial for efficient Eu3+ excitation, while c-Eu2O3 phase separation results in loss of the characteristic Eu3+ photoluminescence. The formation of molecule-like Eu-oxide clusters, entrapped in ZnO, proposed here, may help in understanding the nature of the unexpected high doping levels of lanthanide ions in ZnO that occur virtually without significant change in ZnO unit cell dimensions.
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| 9. |
- Naim Katea, Sarmad, et al.
(författare)
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Carbothermal nitridation of solution synthesised ZrO2-carbon nanocomposites; phase-development from precursor to nitride
- 2021
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Ingår i: Ceramics International. - : Elsevier. - 0272-8842 .- 1873-3956. ; 47:8, s. 10828-10847
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Tidskriftsartikel (refereegranskat)abstract
- A carbothermal nitridation route to nanophase ZrN1-xCx powder using nitrogen gas and a highly homogeneous solution derived carbon-ZrO2 nanocomposite is described. Composites with 2-4 nm sized ZrO2 particles were prepared by hydrolysis of Zr(OPrn)(4) and sucrose with 2-8 sucrose-C:Zr and heating to 600 degrees C. The phase-evolution on heating was elucidated in detail using: TG-DTA, XRD, XPS, IR- and Raman spectroscopy, SEM-EDS, and TEM-EDS. With 5-8 C:Zr samples heated to 1495 degrees C, somewhat agglomerated single-crystalline Zr (N,C) particles in sizes centred around 40-90 nm were obtained, in some cases with minor amounts of r-Zr7O8N4 and m-ZrO2. The c-Zr(N,C) had slightly larger XRD cell-dimensions (a = 4.589-4.606 angstrom) than the literature ZrN (a = 4.5775 A), due to some carbon substitution in the nitrogen positions. The single-crystalline Zr(N,C) particle cores of the 8C:Zr, and to some extent 7C:Zr, samples, had a 4-6 nm thick distinctive amorphous or microcrystalline Zr(N,C,O) shell.
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| 10. |
- Naim Katea, Sarmad, 1987-, et al.
(författare)
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Carbothermal nitridation of solution synthesised ZrO2–carbon nanocomposites; phase-development from precursor to nitride
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Tidskriftsartikel (refereegranskat)abstract
- A carbothermal nitridation route to nanophase ZrN1-xCx powder using nitrogen gas and a highly homogeneous solution derived carbon-ZrO2 nanocomposite is described. Composites with 2-4 nm sized ZrO2 particles were prepared by hydrolysis of Zr(OPrn)4 and sucrose with 2 to 8 sucrose-C:Zr and heating to 600 oC. The phase-evolution on heating was elucidated in detail using: TG-DTA, XRD, XPS, IR- and Raman spectroscopy, SEM-EDS, and TEM-EDS. With 5-8 C:Zr samples heated to 1495 oC, somewhat agglomerated single-crystalline Zr(N,C) particles in sizes centred around 40-90 nm) were obtained, in some cases with minor amounts of r-Zr7O8N4 and m-ZrO2. The c-Zr(N,C) had slightly larger XRD cell-dimensions (a = 4.589-4.606 Å) than the literature ZrN (a = 4.5775 Å), due to some carbon substitution in the nitrogen positions. The single-crystalline Zr(N,C) particle cores of the 8C:Zr, and to some extent 7C:Zr, samples, had a 4-6 nm thick distinctive amorphous or micro-crystalline Zr(N,C,O) shell.
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