Browsing by Author "Hannula, Simo Pekka"
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Item Cold gas spraying of a high-entropy CrFeNiMn equiatomic alloy(MDPI AG, 2020-01-01) Lehtonen, Joonas; Koivuluoto, Heli; Ge, Yanling; Juselius, Aapo; Hannula, Simo Pekka; Department of Chemistry and Materials Science; Advanced and functional Materials; Tampere University; Department of Chemistry and Materials ScienceCold gas spraying was used to make a coating from an equiatomic CrFeNiMn high-entropy alloy. This four-component alloy was chosen because it is Co-free, thus allowing application in nuclear industries as a possible replacement of currently used stainless steel coatings. The feedstock material was gas atomized powder with a particle size distribution from 20 to 45 μm. A number of parameters were tested, such as the powder feed rate and gas feed pressure, in order to obtain as dense a coating as possible with nitrogen as the process gas. Spraying was performed using a gas preheating temperature of 1000 °C, gas feed pressure ranging from 50 to 60 bar, and two powder feeding rates. The coating thicknesses ranging from 230 to 490 m and porosities ranging from 3% to 10% were obtained depending on the powder feed rate and gas feed pressure. The hardness of the cross-section of the coating was usually lower than that of the surface. The highest coating hardness obtained was above 300 HV0.3 for both the surface and the cross-section. The as-atomized powder consisted of a face-centered cubic (FCC) phase with a minute amount of body-centered cubic (BCC) phase, which was no longer detectable in the coatings. The microstructure of the coating was highly stressed due to the high degree of deformation occurring in cold gas spraying. The deformation leads to strain hardening and induces a pronounced texture in the coating. The 111 planes tend to align along the coating surface, with deformation and texturing concentrating mainly on particle boundaries. A high-entropy alloy (HEA) coating was successfully sprayed for the first time using nitrogen as a process gas. The coating has the potential to replace stainless steel coatings in nuclear industry applications.Item Comparison of magnetic field controlled damping properties of single crystal Ni-Mn-Ga and Ni-Mn-Ga polymer hybrid composite structures(Elsevier BV, 2018-05-26) Nilsén, Frans; Aaltio, Ilkka; Hannula, Simo Pekka; Department of Chemistry and Materials ScienceMagnetically controlled hybrid Ni-Mn-Ga composites are potential candidates for actuation and damping applications. The combination of ductile polymer and gas atomized large grained Ni-Mn-Ga powder has many advantages compared to bulk single crystals. These advantages include ease of manufacturing and freedom of shape, while still being magnetically controllable. In this report, Ni-Mn-Ga-epoxy hybrid composite structures are manufactured at three different filling ratios 25, 30 and 35 vol-% and damping properties of the composites are compared to those of 5M Ni-Mn-Ga single crystal. The damping properties are characterized using a laboratory made high-frequency dynamic mechanical testing instrument and a dynamic mechanic analyzer (DMA) in single cantilever mode. The mechanical cycling experiments revealed that the damping ability of the Ni-Mn-Ga composites depends on the filling ratio. The magnetic field induced stiffening observed in the mechanical cycling experiments of the single crystal sample at 100 Hz correlated roughly with that of the composite sample having filling ratio of 35 vol-%.Item Control of the size of silver nanoparticles and release of silver in heat treated SiO2-Ag composite powders(2018-01-05) Granbohm, Henrika; Larismaa, Juha; Ali, Saima; Johansson, Leena Sisko; Hannula, Simo Pekka; Department of Chemistry and Materials Science; Department of Bioproducts and Biosystems; Aalto Nanofab; Advanced and functional Materials; Bio-based Colloids and MaterialsThe growth of silver nanoparticles, the activation energy for silver particle growth, and the release of silver species in heat treated SiO2-Ag composite powders are investigated. The silver particle growth is controlled by heat treatment for 75 min of the as-synthesized SiO2-Ag composite powder at 300-800 °C. During heat treatment the mean size of the Ag particles increases from 10 nm up to 61 nm with increasing temperature, however, the particle size distribution widens and the mean size increases with increasing heat treatment temperature. Based on X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) studies, silver particles are crystalline and in a metallic state after annealing in all SiO2-Ag composite powders. The growth of Ag particles is suggested to take place via diffusion and Ostwald ripening. The activation energy for particle growth was determined as 0.14 eV. The dissolution of silver in aqueous solutions from the SiO2-Ag composites heat treated, at 300 °C, 600 °C, and 700 °C, was investigated by varying pH and temperature. The dissolution was reduced in all conditions with increasing silver particle size, i.e., when the total surface area of Ag particles is reduced. It is suggested that the dissolution of silver from the composite powders can conveniently be adjusted by controlling the Ag particle size by the heat treatment of the composite powder.Item Effect of ethanol on Ag@Mesoporous silica formation by in situ modified stöber method(2018-06-01) Chen, Qian; Ge, Yanling; Granbohm, Henrika; Hannula, Simo Pekka; Department of Chemistry and Materials Science; Advanced and functional MaterialsTunable core-shell Ag@Mesoporous SiO2 spheres were synthesized via an in situ modified Stöber approach by varying the amount of ethanol (EtOH) expanding their potentials in many applications. Mesoporous silica was generated by adding tetraethyl orthosilicate (TEOS) to the mixture of colloidal Ag particles prepared by reducing silver nitrate (AgNO3) with L-ascorbic acid and using hexadecyltrimethylammonium bromide (CTAB) as a template at the presence of ethanol and sodium hydroxide (NaOH) at pH 10 as a catalyst. The average sizes of the Ag cores at the three increasing volumes of ethanol were ~47 ± 6, 36 ± 4, and 11 ± 5 nm, while the silica particle size and the thickness of the silica shells increased, resulting in a blueshift of localized surface plasmon resonances (LSPR) of the Ag NPs. The corresponding specific surface areas of silica particles were 356 ± 10, 419 ± 20 and 490 ± 25 m2 g−1, and average pore diameters varied from 5.7, 5.0 to 3.3 nm according to BET and BJH analyses. TEM studies confirmed the core-shell structure, pore sizes and shapes of mesoporous shells. The dissolution tests demonstrated that the release of Ag from the powder samples is pH-sensitive and time-dependent.Item Effect of Morphology and Crystal Structure on the Thermal Conductivity of Titania Nanotubes(2018-01-01) Ali, Saima; Orell, Olli; Kanerva, Mikko; Hannula, Simo Pekka; Department of Chemistry and Materials Science; Tampere University of TechnologyTitania nanotubes (TNTs) with different morphology and crystal structure are prepared by chemical processing and rapid breakdown anodization (RBA) methods. The nanotubes are studied in terms of thermal conductivity. The TNTs with variable wall thickness below 30 nm have significantly reduced thermal conductivity than bulk titania, due to the phonon confinement, smaller phonon mean free path, and enhanced phonon boundary scattering. The amorphous nanotubes (TNTAmor) have comparatively thicker walls than both crystalline nanotubes. The TNTAmor has a thermal conductivity of 0.98 W m−1 K−1, which is slightly less than the thermal conductivity of crystalline anatase nanotubes (TNTA; 1.07 W m−1 K−1). However, the titania nanotubes with mixed structure (TNTA,T) and the smallest dimensions have the lowest thermal conductivity of 0.75 W m−1 K−1, probably due to the phonon confinement. The experimental results are compared with the theoretical study considering the size confinement effect with different wall dimensionsof TNTs and surface scattering. The results agree well with the surface roughness factor (p) of 0.26 for TNTA,T, 0.18 for TNTA, and 0.65 for TNTAmor, indicating diffusive phonon scattering and rougher surfaces for TNTA. Interestingly, the present results together with those presented in literature suggest that thermal conductivity reduction with respect to the wall thickness occurs also for the amorphous nanotubes. This is ascribed to the role of propagons in the thermal transport of disordered structures.Item Effect of sulfonating agent and ligand chemistry on structural and optical properties of CuSbS2 particles prepared by heat-up method(ROYAL SOC CHEMISTRY, 2018) Moosakhani, Shima; Sabbagh Alvani, Ali Asghar; Mohammadpour, Raheleh; Sainio, Jani; Ge, Yanling; Hannula, Simo Pekka; Department of Chemistry and Materials Science; Amirkabir University of Technology; Sharif University of Technology; Department of Applied PhysicsChalcostibite copper antimony sulfide (CuSbS2) is a promising candidate for application in solar cells. The functionality of CuSbS2 particles depends on particle size and morphology and controlling these two parameters during synthesis is of utmost importance. In this study, CuSbS2 particles were prepared by a facile heat-up synthesis method utilizing sulfur powder (Su) and thiourea (Tu) to investigate the effect of the sulfur source on the structural and physical properties of CuSbS2 particles. Different morphologies were observed when Su and Tu were employed. The results demonstrated that the shape uniformity can be improved by applying a coordinating sulfur precursor (Tu). Moreover, nanoplatelet- and nanobrick-shaped particles were obtained by changing the ligand chemistry, i.e., by using a different combination of oleylamine (OLA), 1-octadecene (ODE), and oleic acid (OL). Band gap calculations showed that CuSbS2 had direct and indirect bandgaps with a small difference of 0.2 eV. Composition analysis of samples obtained from the Tu precursor revealed that antimony contents varied resulting in differences of the lattice parameter c. Moreover, valence band (VB) and conduction band (CB) positions determined by cyclic voltammetry (CV) suggested that this material based on its composition can have dual applications: first, as an absorber in nanocrystalline solar cells and second, as a hole transport material in perovskite solar cells.Item Hierarchical microstructure of laser powder bed fusion produced face-centered-cubic-structured equiatomic CrFeNiMn multicomponent alloy(MDPI AG, 2020-10-02) Yang, Xuan; Ge, Yanling; Lehtonen, Joonas; Hannula, Simo Pekka; Department of Chemistry and Materials Science; Physical Characteristics of Surfaces and Interfaces; Nanochemistry and Nanoengineering; Advanced and functional MaterialsA cobalt-free equiatomic CrFeNiMn multicomponent alloy was fabricated from gas-atomized powder using laser powder bed fusion (L-PBF), also known as selective laser melting (SLM). The as-built specimens had a single face-centered cubic (FCC) structure, relative density of 98%, and hardness up to 248 HV0.5 for both the scanning speeds applied. In this work, we report the hierarchical microstructural features observed in the as-built specimens. These are comprised of melt pools, grains, cell structures including dendritic cells, elongated cells, equiaxed cells (~500 nm), and sub-cells (150–300 nm). The cell and sub-cell walls are composed of a notably high density of dislocations. In addition, segregation of Mn and Ni was detected at the cell walls, but only occasionally at the sub-cell walls. SLM exhibits the capability to produce FCC-structured equiatomic CrFeNiMn multicomponent alloy with the refined and hierarchical microstructure.Item Nanosilver–silica composite: Prolonged antibacterial effects and bacterial interaction mechanisms for wound dressings(2017-09-06) Mosselhy, Dina A.; Granbohm, Henrika; Hynönen, Ulla; Ge, Yanling; Palva, Airi; Nordström, Katrina; Hannula, Simo Pekka; Department of Materials Science and Engineering; Department of Bioproducts and Biosystems; Department of Chemistry and Materials Science; Biomolecular Materials; Advanced and functional Materials; University of HelsinkiInfected superficial wounds were traditionally controlled by topical antibiotics until the emergence of antibiotic-resistant bacteria. Silver (Ag) is a kernel for alternative antibacterial agents to fight this resistance quandary. The present study demonstrates a method for immobilizing small-sized (~5 nm) silver nanoparticles on silica matrix to form a nanosilver–silica (Ag–SiO2) composite and shows the prolonged antibacterial effects of the composite in vitro. The composite exhibited a rapid initial Ag release after 24 h and a slower leaching after 48 and 72 h and was effective against both methicillin-resistant Staphylococcus aureus (MRSA) and Escherichia coli (E. coli). Ultraviolet (UV)-irradiation was superior to filter-sterilization in retaining the antibacterial effects of the composite, through the higher remaining Ag concentration. A gauze, impregnated with the Ag–SiO2 composite, showed higher antibacterial effects against MRSA and E. coli than a commercial Ag-containing dressing, indicating a potential for the management and infection control of superficial wounds. Transmission and scanning transmission electron microscope analyses of the composite-treated MRSA revealed an interaction of the released silver ions with the bacterial cytoplasmic constituents, causing ultimately the loss of bacterial membranes. The present results indicate that the Ag–SiO2 composite, with prolonged antibacterial effects, is a promising candidate for wound dressing applications.Item Robustness of Adamussium colbecki shell to ocean acidification in a short-term exposure(Elsevier BV, 2019-08-01) Dell'Acqua, Ombretta; Trębala, Michal; Chiantore, Mariachiara; Hannula, Simo Pekka; University of Genoa; Department of Chemistry and Materials ScienceAtmospheric pCO2 has increased since the industrial revolution leading to a lowering of the ocean surface water pH, a phenomenon called ocean acidification (OA). OA is claimed to be a major threat for marine organisms and ecosystems and, particularly, for Polar regions. We explored the impact of OA on the shell mechanical properties of the Antarctic scallop Adamussium colbecki exposed for one month to acidified (pH 7.6) and natural conditions (unmanipulated littoral water), by performing Scanning Electron Microscopy, nanoindentation and Vickers indentation on the scallop shell. No effect of pH could be detected either in crystal deposition or in the mechanical properties. A. colbecki shell was found to be resistant to OA, which suggests this species to be able to face a climate change scenario that may threat the persistence of the endemic Antarctic species. Further investigation should be carried out in order to elucidate the destiny of this key species in light of global change.Item Round robin into best practices for the determination of indentation size effects(Multidisciplinary Digital Publishing Institute (MDPI), 2020-01-01) Ruiz-moreno, Ana; Hähner, Peter; Kurpaska, Lukasz; Jagielski, Jacek; Spätig, Philippe; Trebala, Michal; Hannula, Simo Pekka; Merino, Susana; de Diego, Gonzalo; Namburi, Hygreeva; Libera, Ondrej; Terentyev, Dimitry; Khvan, Tymofii; Heintze, Cornelia; Jennett, Nigel; Department of Chemistry and Materials Science; Advanced and functional Materials; European Commission Joint Research Centre; National Centre for Nuclear Research; Paul Scherrer Institute; Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas - CIEMAT; Centrum výzkumu Řež s.r.o.; Belgian Nuclear Research Centre; Helmholtz-Zentrum Dresden-Rossendorf EV; Coventry UniversityThe paper presents a statistical study of nanoindentation results obtained in seven European laboratories that have joined a round robin exercise to assess methods for the evaluation of indentation size effects. The study focuses on the characterization of ferritic/martensitic steels T91 and Eurofer97, envisaged as structural materials for nuclear fission and fusion applications, respectively. Depth-controlled single cycle measurements at various final indentation depths, force-controlled single cycle and force-controlled progressive multi-cycle measurements using Berkovich indenters at room temperature have been combined to calculate the indentation hardness and the elastic modulus as a function of depth applying the Oliver and Pharr method. Intra- and inter-laboratory variabilities have been evaluated. Elastic modulus corrections have been applied to the hardness data to compensate for materials related systematic errors, like pile-up behaviour, which is not accounted for by the Oliver and Pharr theory, and other sources of instrumental or methodological bias. The correction modifies the statistical hardness profiles and allows determining more reliable indentation size effects.Item Silica-silicon composites for near-infrared reflection(Elsevier Limited, 2021-06-15) Conley, Kevin; Moosakhani, Shima; Thakore, Vaibhav; Ge, Yanling; Lehtonen, Joonas; Karttunen, Mikko; Hannula, Simo Pekka; Ala-Nissila, Tapio; Centre of Excellence in Quantum Technology, QTF; Physical Characteristics of Surfaces and Interfaces; Department of Applied Physics; Department of Chemistry and Materials Science; Western UniversityCompact layers containing embedded semiconductor particles consolidated using pulsed electric current sintering exhibit intense, broadband near-infrared reflectance. The composites consolidated from nano- or micro-silica powder have a different porous microstructure which causes scattering at the air-matrix interface and larger reflectance primarily in the visible region. The 3 mm thick composite compacts reflect up to 72% of the incident radiation in the near-infrared region with a semiconductor microinclusion volume fraction of 1% which closely matches predictions from multiscale Monte Carlo modeling and Kubelka-Munk theory. Further, the calculated spectra predict a reddish tan compact with improved reflectance can be obtained by decreasing the average particle size or broadening the standard deviation. The high reflectance is achieved with minimal dissipative losses and facile manufacturing, and the composites described herein are well-suited to control the radiative transfer of heat in devices at high temperature and under harsh conditions.Item Solution synthesis of CuSbS2 nanocrystals(Elsevier Science, 2018-03-05) Moosakhani, Shima; Sabbagh Alvani, Ali Asghar; Mohammadpour, Raheleh; Ge, Yanling; Hannula, Simo Pekka; Amirkabir University of Technology; Sharif University of Technology; Department of Chemistry and Materials ScienceChalcostibite copper antimony sulfide (CuSbS2) micro- and nanoparticles with a different shape and size have been prepared by a new approach to hot injection route. In this method, sulfur in oleylamine (OLA) is employed as a sulfonating agent providing a simple route to control the shape and size of the particles, which enables the optimization of CuSbS2 for a variety of applications. The sulfur to metallic precursor ratio appears to be one of the most effective parameters along with the temperature and time for controlling the size and morphology of the particles. The growth mechanism study shows in addition to the CuSbS2 phase the presence of not previously observed intermediate phases (stibnite (Sb2S3) and famatinite (Cu3SbS4)) at the initial stage of the reaction. By increasing the ratio of sulfur to copper and antimony, wider and thinner CuSbS2 particles are obtained. The particles have nanoplate and nanosheet morphology with a good shape and size uniformity. Coalescence of very thin nanosheets occurs with increasing reaction time eventually leading to formation of thicker particles which can be called nanobricks. Band gap determinations demonstrate that the obtained CuSbS2 particles have both direct (1.51–1.57 eV) and indirect (1.44–1.51 eV) bandgaps. Transmission Electron Microscopy (TEM) studies revealed that the preferred growth directions are along the basis axes of the unit cell ([100] and [010]). Optical and structural properties of the obtained CuSbS2 particles are indicative for their great potential in different generations of solar cells and supercapacitor applications.Item Towards the additive manufacturing of Ni-Mn-Ga complex devices with magnetic field induced strain(Elsevier BV, 2022-01) Ituarte, Iñigo Flores; Nilsén, Frans; Nadimpalli, Venkata Karthik; Salmi, Mika; Lehtonen, Joonas; Hannula, Simo Pekka; Department of Mechanical Engineering; Department of Chemistry and Materials Science; Advanced Manufacturing and Materials; Technical University of Denmark; Tampere University; Mechanical Engineering and Metals Industry Standardization in FinlandLaser powder bed fusion (L-PBF) is used to produce foam-like Ni-Mn-Ga with tailored microscale and mesoscale features. Ni50-Mn28.2-Ga21.8 (at%) powder was gas atomised and processed in an L-PBF system with a range of energy density from 26.24 and 44.90 J/mm3. We characterised microscale and mesoscale properties, such as the chemical composition, crystal structure, magnetisation measurements, density, and porosity measurements as a function of process parameters, in a systematic design of experiment. Preliminary research on macroscale properties included tensile testing and magnetic field induced strain (MFIS) measurements. Results show how controlling process parameters allows tailoring the Ni-Mn-Ga polycrystalline microstructure. Hence, obtaining twinned martensitic structures with a predominant orientation going across the visible grain boundaries. All the processed samples showed a 56 Am2/kg magnetisation level, close to Ni-Mn-Ga 10 M single crystals. Mesoscale results show a distinctive porosity pattern that is tailored by the process parameters and the laser scanning strategy. In contrast, macroscale mechanical tensile test results show a brittle fracture of Ni-Mn-Ga due to the high porosity with yield stress 2–3 times higher than shown in single crystals. In sum, we built geometrically complex demonstrators with (i) microscale twinned martensitic structures with a predominant orientation going across the visible grain boundaries and (ii) mesoscale tailored periodic porosity patterns created by modifying power, scanning speed, and scanning strategy systematically. L-PBF demonstrates great potential to produce foam-like polycrystalline Ni-Mn-Ga, reducing grain boundary constraints and thus the magnetic force needed for MFIS.Item Tribological properties of thin films made by atomic layer deposition sliding against silicon(2018-01-01) Kilpi, Lauri; Ylivaara, Oili M.E.; Vaajoki, Antti; Liu, Xuwen; Rontu, Ville; Sintonen, Sakari; Haimi, Eero; Malm, Jari; Bosund, Markus; Tuominen, Marko; Sajavaara, Timo; Lipsanen, Harri; Hannula, Simo Pekka; Puurunen, Riikka L.; Ronkainen, Helena; VTT Technical Research Centre of Finland; Department of Chemistry and Materials Science; Department of Electronics and Nanoengineering; University of Jyväskylä; Beneq Oy; ASM Microchemistry Oy; Catalysis; Department of Chemical and Metallurgical EngineeringInterfacial phenomena, such as adhesion, friction, and wear, can dominate the performance and reliability of microelectromechanical (MEMS) devices. Here, thin films made by atomic layer deposition (ALD) were tested for their tribological properties. Tribological tests were carried out with silicon counterpart sliding against ALD thin films in order to simulate the contacts occurring in the MEMS devices. The counterpart was sliding in a linear reciprocating motion against the ALD films with the total sliding distances of 5 and 20 m. Al2O3 and TiO2 coatings with different deposition temperatures were investigated in addition to Al2O3-TiO2-nanolaminate, TiN, NbN, TiAlCN, a-C:H [diamondlike carbon (DLC)] coatings, and uncoated Si. The formation of the tribolayer in the contact area was the dominating phenomenon for friction and wear performance. Hardness, elastic modulus, and crystallinity of the materials were also investigated. The nitride coatings had the most favorable friction and wear performance of the ALDcoatings, yet lower friction coefficient was measured with DLC a-C:H coating. These results help us to take steps toward improved coating solutions in, e.g., MEMS applications.
