Logo: to the web site of Uppsala University

Zinc oxide (ZnO) is a semiconductor with a wide range of applications, and often the properties are modified by metal-ion doping. The distribution of dopant atoms within the ZnO crystal strongly affects the optical and magnetic properties, making it crucial to comprehend the structure down to the atomic level. Our study reveals the dopant structure and its contents in Eu-doped ZnO nanosponges with up to 20% Eu-O clusters. Eu was distributed over the ZnO:Eu crystals, with an additional amorphous intercrystalline phase observed, especially in the 20% Eu sample. The electron pair distribution function revealed the presence of nonperiodic Eu3+-oxide clusters and proved highly effective for analyzing the coordination environment of Eu-O, ranging from 2.0 to 2.8 & Aring;. It uncovered three-, four-, and five-coordinate Eu-O configurations in the 20% Eu sample, and there were significant changes in Eu coordination between the samples, which is ascribed due to the intercrystalline phase. The proposed method offers a potential characterization routine for a detailed investigation of complex doped materials.

Proton irradiation with a primary ion energy of 2 MeV was used to simulate radiation damage in UN and (U,Zr)N fuel pellets. The pellets, nominally at room temperature, were irradiated to peak levels of 0.1, 1, 10 dpa and 100.0 dpa resulting in a peak hydrogen concentration of at most 90 at. %. Microstructure and mechanical properties of the samples were investigated and compared before and after irradiation. The irradiation induced an increase in hardness, whereas a decrease in Young’s modulus was observed for both samples. Microstructural characterization revealed irradiation-induced cracking, initiated in the bulk of the material, where the peak damage was deposited, propagating towards the surface. Additionally, transmission electron microscopy was used to study irradiation defects. Dislocation loops and fringes were identified and observed to increase in density with increasing dose levels. The high density of irradiation defects and hydrogen implanted are proposed as the main cause of swelling and consequent sample cracking, leading simultaneously to increased hardening and a decrease in Young's modulus.

Nanocrystalline ZnO sponges doped with 5 mol% EuO_1.5 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.

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 degrees 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 mu m 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 degrees C. The mechanical performance of the resulting cemented carbides was evaluated by measuring the hardness at temperatures between 20 degrees C and 700 degrees 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.m(0.5). 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.

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.

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.

The sintering of a 14 ​wt% nickel nano-dot coated NbC powder was investigated using dilatometry in combination with XRD, ToF-ERDA, PIXE, SEM-EDS/EBSD/TKD, and TEM-HAADF/EDS/EELS for analysis of samples heated to 1375, 1405 and 1500 ​°C, with a 30 ​min annealing. The main sintering step at 1110–1375 ​°C, was succeeded by a slower step, centred at 1405 ​°C, before the final smaller densification step to 1500 ​°C. NbC grain-growth took place on heating at 1500 ​°C for 30 ​min; from ca. 0.5–5 ​μm (average 1 ​μm) at 1375 and 1405 ​°C to ca. 5–30 ​μm (average 10 ​μm) 1500 ​°C for 30 ​min. The NbC_0.91-0.94 phase showed a micro-hardness of 1700–2300 HV_0.05. The binder phase consisted of the unprecedented textured c-Nb_0.15Ni_0.85 and h-Nb_0.2Ni_0.8 phases. The Nb–Ni binder phase yielded micro-hardness values of 1150–1250 (1375–1405 ​°C) to 810 HV_0.05 (1500 ​°C, 30 ​min), which greatly exceeds nickel and is higher than Fe₃Al.

Carbothermal reduction of Zr-sucrose gels powders into nano-phase ZrC, or ZrC-Zr(C,O) core-shell powders, via a composite of 2–4 nm sized ZrO₂ and amorphous carbon, is described. Samples with 1.7–20 sucrose-carbon:Zr ratio gels heated to 1495 °C at 10 °Cmin⁻¹, with 3 and 30 min hold time were studied in detail using; TG, XRD, SEM, TEM, STEM-EDX, and XPS with Ar⁺-ion etching. After 1495 °C, 3 min, the samples with 12-20C:Zr ratios yielded weakly agglomerated 30 to 40 nm sized ZrC particles, surrounded by a dense 5 nm thick shell of Zr(C,O). With 5C:Zr significant amounts of ZrO₂ was present after heating at 1495 °C for 3 min, while after 30 min annealing, ZrC particles without residual amorphous carbon was obtained. Minor amounts of zirconia was found in most samples, which in similarity with the 5 nm Zr(C,O) shell, is believed to stem from post synthesis oxidation.

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.

A fast, low cost solution route to nickel dot coated NbC powder using Ni(NO₃)₂6H₂O, Ni(OAc)₂4H₂O and triethanolamine (TEA) as precursors in methanol solvent was developed. After mixing the precursor solution with 50-300 nm sized NbC powder and evaporation of the solvent, heating to 150 ^oC, or higher, yielded NbC powder evenly covered with well-dispersed nickel metal dots, 6 to 25 nm in size, depending on the nickel loading and temperature. Coatings yielding 4 to 14 wt% nickel loading were studied at temperatures from 200 to 700 ^oC, using TG-DTA, XRD, SEM, XPS with Ar⁺-ion etching, TEM/DF, and STEM/HAADF/EDS.