Wojciech Maziarz

Characterization and properties of a modified Si solar cell emitter by a porous Si layer

Abstract Porous Si (PS) has become an interesting material owing to its potential applications in many fields including microelectronics, optoelectronics and photovoltaics. PS layers on the front surface of n+/p monocrystalline, textured Si solar cells have been investigated with the aim of improving the performance of standard screen-printed cells, because an antireflection coating and a surface passivation can be obtained simultaneously in one chemical process. The results obtained could be useful in optimising the Si surface chemical treatment process. The surface morphology and microstructure of PS layers were investigated using SEM, TEM and non-contact AFM methods. The surface morphology of a PS layer depends strongly on the region where the pores are formed. The structure of PS layer is composed of macro-pores formed in p type Si (sizes vary over a large range up to 250 nm) and meso-pores formed in the n+ region of the p–n+ junction. The meso-pores of average size 20 nm on the pyramid slope elongate preferentially along the 〈111〉 direction. The interface between the PS layer and the substrate as well as the surface roughness are clearly defined. The results show that the PS layer on the pyramids is formed uniformly along the walls. Meso-pores created on the macro-pore surface are a characteristic feature of the surface between pyramids. Such a surface modification allows improving the Si solar cell characteristics.

SEM and TEM Studies of Magnetic Shape Memory NiCoMnIn Melt Spun Ribbons

Microstructure of Ni50-xCoxMn35.5In14.5 (x=0, 3, 5) melt-spun ribbons was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The typical layered microstructure consisting of oriented columnar grains and colonies of fine equi-axed grains was observed in the cross section of ribbons. The crystallographic structure of ribbons varied with the content of Co in alloys what affected of their Ms temperature. For the x=0 the single phase of monoclinic 14M modulated martensite was observed, but for x=3 and 5, a two phase structure of L21 austenite and monoclinic 14M or orthorhombic 10M modulated martensite were identified. Different temperature range of martensitic transformations were explained basing on valence electron concentration per atom e/a versus Ms relationship.

Nanostructured TiO2-based gas sensors with enhanced sensitivity to reducing gases

2D TiO2 thin films and 3D flower-like TiO2-based nanostructures, also decorated with SnO2, were prepared by chemical and thermal oxidation of Ti substrates, respectively. The crystal structure, morphology and gas sensing properties of the TiO2-based sensing materials were investigated. 2D TiO2 thin films crystallized mainly in the form of rutile, while the flower-like 3D nanostructures as anatase. The sensor based on the 2D TiO2 showed the best performance for H2 detection, while the flower-like 3D nanostructures exhibited enhanced selectivity to CO(CH3)2 after sensitization by SnO2 nanoparticles. The sensor response time was of the order of several seconds. Their fast response, high sensitivity to selected gas species, improved selectivity and stability suggest that the SnO2-decorated flower-like 3D nanostructures are a promising material for application as an acetone sensor.

Magnetostructural transition and magnetocaloric effect in highly textured Ni-Mn-Sn alloy

Ni49.4Mn38.5Sn12.1 near single crystal was obtained by the Bridgman method. At room temperature, it consisted of a mixture of the parent austenite phase with the cubic L21 Heusler structure (ac = 5.984 A) and modulated, tetragonal martensite phase 4M (at = 4.337 A, ct = 5.655 A). Under the application of a magnetic field, the specimen undergoes field induced reverse martensitic transformation, which combined with the Curie transition in austenite leads to the coexistence of direct and inverse magnetocaloric effects. The maximum entropy change at 280 K and under 5 T amounts to 3.4 J·kg−1·K−1 for the structural transition and at 316 K reaches −2.7 J·kg−1·K−1 for the magnetic transformation. The magnetic entropy change occurs over a wide temperature span leading to improved refrigerant capacity of 101 J·kg−1 (5 T). Hysteretic losses are considerably reduced, which is promising with respect to improved cyclic stability of such a material.

TEM and HRTEM studies of ball milled 6061 aluminium alloy powder with Zr addition

The effect of mechanical alloying on the microstructure of atomized 6061 aluminium alloy powder and 6061 powder with a zirconium addition was studied in the work. The atomized 6061 aluminium alloy powder and 6061 powder with addition of 2 wt.% Zr were milled in a planetary ball mill and investigated using X‐ray diffraction measurements, conventional and high‐resolution electron microscopy (TEM/HRTEM) and high‐angle annular dark field scanning transmission electron microscopy combined with energy dispersive X‐ray microanalysis. An increase of stresses was observed in milled powders after the refinement of crystallites beyond 100 nm. In the powder with zirconium addition, some part of the Zr atoms diffused in aluminium forming a solid solution containing up to 0.5 wt.% Zr. The remaining was found to form Zr‐rich particles containing up to 88 wt.% Zr and were identified as face centred cubic (fcc) phase with lattice constant a= 0.48 nm. That fcc phase partially transformed into the L12 ordered phase. Eighty‐hour milling brought an increase of microhardness (measured with Vickers method) from about 50 HV (168 MPa) for the initial 6061 powder to about 170 HV (552 MPa). The addition of zirconium had no influence on the microhardness.

Magnetic and Martensitic Transformations in Ni46Mn41.5-xFexSn12.5 Melt Spun Ribbons

Four alloys with nominal compositions Ni46Mn41.5-xFexSn12.5 (x=0, 2, 4, 6 at.%) were cast in an induction vacuum furnace and homogenized. Then they were melted in quartz tubes and ejected onto a rotating copper wheel to produce ribbons. The X-Ray phase analyses of as melt spun ribbons have shown that in both, the ternary as well as in the quaternary alloys a single phase of the Heusler L21 type ordered structure was found. The characteristic temperatures of magnetic (TC) and martensitic (Ms) transformations were determined by a vibrating sample magnetometer (VSM). Both the Ms and TC increase with the increase of Fe content in all alloys, which is in accordance with the theory of valence electron concentration (e/a) influence on Ms. The phase structures, chemical compositions, grains sizes and type of microsegregation were characterized by transmission electron microscope (TEM). The equi-axed grains of size from 0.95 to 1.7 μm were observed in all ribbons. The grains posses the L21 structure at room temperature, however in the alloys with higher Fe content the different type of martensite was observed at the grain boundaries of L21 phase. Appearance of this martensite was explained in relation to microsegregation of particular elements during melt spinning process and simultaneous change in the e/a ratio.

Structure investigations of ferromagnetic Co-Ni-Al alloys obtained by powder metallurgy

Elemental powders of Co, Ni and Al in the proper amounts to obtain Co35Ni40Al25 and Co40Ni35Al25 nominal compositions were ball milled in a high‐energy mill for 80 h. After 40 h of milling, the formation of a Co (Ni, Al) solid solution with f.c.c. structure was verified by a change of the original lattice parameter and crystallite size. Analytical transmission electron microscopy observations and X‐ray diffraction measurements of the final Co (Ni, Al) solid solution showed that the crystallite size scattered from 4 to 8 nm and lattice parameter a = 0.36086 nm. The chemical EDS point analysis of the milled powder particles allowed the calculation of the e/a ratio and revealed a high degree of chemical homogeneity of the powders. Hot pressing in vacuum of the milled powders resulted in obtaining compacts with a density of about 70% of the theoretical one. An additional heat treatment increased the density and induced the martensitic transformation in a parent phase. Selected area diffraction patterns and dark field images obtained from the heat‐treated sample revealed small grains around 300 nm in diameter consisting mainly of the ordered γ phase (γ’), often appearing as twins, and a small amount of the L10 ordered martensite.

HRTEM studies of amorphous ZrNiTiCu nanocrystalline composites

Ball milling of easy glass forming Ti25Zr17Ni29Cu29 alloys lead to the formation of an amorphous structure accompanied by a substantial increase of powder microhardness. The powders show clear glass transition effect and a few stage crystallization starting above 500°C. High‐resolution transmission electron microscope technique allowed identifying nanocrystalline inclusions as Cu12NiTi7 within the amorphous powder. The amorphous powders mixed with nanocrystalline iron or silver powders were hot pressed to form composites. A narrow 200 nm broad intermediate single‐phase layer at the amorphous‐phase/iron interface containing all elements present in the composite was identified using transmission electron microscope and high‐angle annular dark field detector techniques. scanning transmission electron microscopy energy dispersive spectroscopy line profile showed gradual change of composition within the intermediate zone. Amorphous phase contains small nanocrystals of size close to 10 nm identified using High‐resolution transmission electron microscope as Cu12NiTi7. Compression tests have shown better plasticity of composites than in the case of pure hot‐pressed amorphous powder; furthermore, high elastic limit of composites and the ultimate compression stress of about 1800 MPa for composites containing 20% Fe and near 700 MPa for those with 20% Ag.

SEM EBSD and TEM Structure Studies of α-Brass after Severe Plastic Deformation Using Equal Channel Rolling Followed by Groove Pressing

Commercial brass Ms36, 2mm thick was annealed and deformed in 6 passes in dual rolls equipment with attached equal channel equipment (DRECE). Then, material was deformed again using constrained groove pressing (CGP) by pressing of grooves 4.2 mm thick, and the groove angle of 45 deg. The experiment was performed 8 times (pressing out grooves and straightening at room temperature). Both methods allowed deformation without changing of the thickness of the sample, which was almost constant near 2 mm. The tensile experiment have shown the Yield Strength YS after 8x groove pressing of 210 MPa and Ultimate Tensile Strength UTS increased 27% up to 430 MPa. At the same time total elongation decreased from 34 to 15 %. The structure of the material after DRECE 6 passes was investigated using conventional TEM and have shown only rather uniform distribution of dislocations. After additional 8 groove pressing experiment, frequent, narrow deformation twins were observed accompanied by the formation of subgrains. Orientation imaging microscopy performed have shown average grain size after DRECE process near 5 μm, which decreased after 8 processes of groove pressing down to 2.9 μm. The fraction of low angle boundaries (below 5 deg) decreased after groove pressing down to 73% from 85% after DRECE process and annealing, while the fraction of high angle grain boundaries (>15 deg) increased after groove pressing up to 24% from 14%, however the total length of high angle boundaries increased more than 2 times since grain size decreased. The structure studies have shown rather mild effect on the grain refinement of both methods and they have to be modified to obtain material approaching nanosize range.

Structure and Martensitic Transformation in NiMnSn Alloy Ribbons with Partial Sn Substitution by Al

The influence of Al substitution for Sn in Ni44Mn43.5AlxSn12.5-x (x= 0, 1, 2, 3) ferromagnetic shape memory alloy ribbons on phase transformation and microstructure evolution is outlined in this paper. Ribbons produced by melt spinning technique showed fully crystalline structure, however non uniform. Energy dispersive spectroscopy microanalysis (EDS) confirmed the average composition of ribbons in accord with the initial alloys. The higher symmetry parent phase was identified with the aid of X-ray diffraction (XRD) as bcc L21 Heusler type structure. The unit cell parameters were determined applying the XRD profile fitting method. It was observed that with increase of Al content unit cell parameters and in turn unit cell volume decrease. This may be attributed to the fact that Al has a smaller radius compared to Sn, which it was substituted for. Differential scanning calorimetry (DSC) measurements did not allow to detect the martensitic transformation above -150°C.