Microstructure and electrical performance of eco-friendly thick film resistor compositions fired at different firing conditions

Perovskite ruthenates, viz. CaRuO₃, SrRuO₃, BaRuO₃, etc. although exhibit many interesting physical properties, have been seldom used in thick film resistors. In the preliminary attempts, we have formulated lead free thick film resistor paste compositions using CaRuO₃. Five different paste compositions by varying the ruthenate concentration were prepared. Thick film resistors were screen printed onto an alumina substrate. Different thick film firing conditions were adopted to fire the resistors. Although, for TFRs, the standard conventional firing cycle is 850 °C, our TFR compositions can be fired even at a peak temperature of 800 °C. The evolution of microstructure and electrical performance of these resistors were studied. The resistors fired in conventional thick film firing furnace (BTU make) were found to be morphologically more uniform than the resistors fired in normal tube furnace. However, the sheet resistance value ‘R_s’ is lower (54–1.36 kΩ/□) in case of resistors fired in tube furnace. On the other hand, resistors fired in thick film firing furnace exhibit higher range of sheet resistance (1.36–800 kΩ/□). The hot and cold TCR range is 180–560 ppm/°C in case of tube furnace fired resistors and 120–235 ppm/°C for BTU furnace, which is comparatively lower than those reported earlier (>600 ppm/°C) for other lead free resistors.     

Crystal growth of ferroelectric PbTiO3 nanocrystals on silicon substrate via intermediates

A unique aggregation-based crystal growth phenomenon of PbTiO₃ nanodots on Si (1 0 0) substrate was observed. Compared to conventional nucleation and diffusion mechanism, a process of aggregation, self-assembly and coarsen process was found to facilitate the growth of PbTiO₃ (PT) nanocrystals on Si (1 0 0) substrates. In addition, intermediates, which were constructed by self-assembly of Pb–Ti oxide nanocrystals, were observed during the crystal growth process on Si (1 0 0) substrates.     

The strain reduction and quality improvement in ZnO film by a 30° in-plane rotation with respect to the Al2O3 substrate

The in-plane orientation of epitaxial ZnO thin film on Al₂O₃(0 0 0 1) was determined by azimuthal scan of X-ray diffraction. Comprehensive structural characterizations, including the lattice strain in perpendicular direction, the defect density, were obtained from high resolution X-ray diffraction. It's found that a 30° rotation in ZnO against Al₂O₃, resulting in ZnO〈1 1 2 0〉//Al₂O₃〈1 0 1 0〉, can efficiently reduce the strain and defects in ZnO layer. Consequently, the optical property is significantly improved.     

Modulations in structural and ferroelectric properties due to tensile strain in BiFeO3 films on MgAl2O4 substrates induced by thermal-expansion

The effect of tensile strain on structural and ferroelectric properties of BiFeO₃ epitaxial films was investigated. The films grown by pulsed laser deposition on MgAl₂O₄ (0 0 1) substrates revealed monoclinic structure deviated from the bulk rhombohedral structure due to a tensile strain along the in-plane direction. The strain is induced by the difference in thermal expansion coefficients between the film and the substrate. A Poisson ratio is calculated from the in-plain and out-of-plain lattice constants at different temperatures measured by reciprocal space maps of X-ray diffraction. The small Poisson ratio compared to the bulk suggests a weaker elastic response at high temperature. The ferroelectric polarization of the tensile-strained film along the (0 0 1) is also decreased from the bulk value.     

Heteroepitaxial growth of δ-Bi2O3 thin films on CaF2(1 1 1) by chemical vapour deposition under atmospheric pressure

We have succeeded in achieving the heteroepitaxial growth of a δ-Bi₂O₃ thin film on a CaF₂(1 1 1) substrate by means of chemical vapour deposition under atmospheric pressure. The film grew with a strong (1 1 1) orientation. From X-ray pole figures, it was observed that the δ-Bi₂O₃ film grew on the CaF₂(1 1 1) substrate with 60° rotational in-plane domains. The growth mode was of a 3D island type, and the grain size decreased with increasing oxygen pressure during the δ-Bi₂O₃ film growth, improving the overall surface smoothness.     

The L12 CrPt3 underlayer effect on the ordering of L10 FePt films

This study discusses the effects of a CrPt₃ alloy underlayer on the magnetic properties and microstructure of FePt films. The L1₂ CrPt₃ phase was chemical long-range ordered at a temperature above 600 °C, and the 120-nm thick CrPt₃ film annealed at 800 °C, resulting in ferrimagnetic behavior. The long-range ordered CrPt₃ underlayer slightly increased the ordering of the L1₀ FePt film, as evidenced by the individual L1₂ CrPt₃ and L1₀ FePt (0 0 1) superlattice peaks. An X-ray slow scan of the [FePt/CrPt₃(T °C)]_400°C bilayer shows that the integrated intensity of the L1₀ FePt (0 0 1) peaks was higher on the long-range order L1₂-type CrPt₃ (T = 600, 800) underlayer and lower on the partial short-range order L1₂ and A₁-types CrPt₃ (T = 400) underlayer. However, optimal magnetic properties were obtained in the [FePt/CrPt₃(400 °C)]_400°C bilayer. When a FePt film is deposited on a ferrimagnetic CrPt₃(800 °C) underlayer, the resulting bilayer shows isotropic magnetic hysteresis loops and cannot be saturated at 1.8 T. Atomic force microscopy (AFM) and transition electron microscopy (TEM) show that the L1₂ CrPt₃(800 °C) underlayer exhibits coarse surface roughness and plate-like grains, respectively. From TEM images, all the FePt grains were isolated uniformly by CrPt₃ matrix.     

A molecular dynamics simulation of TiN film growth on TiN(0 0 1)

The growth of thin TiN films on the TiN(0 0 1) surface during reactive sputtering was simulated by molecular dynamics with the modified embedded-atom method potential. TiN₃ is found to be the smallest epitaxial island and the film grows via the layer mode. Vacancy concentration in the deposited films decreases with increasing the substrate temperature and kinetic energy of incident atoms, resulting from the enhancement of the thermal diffusion and kinetic energy assisted athermal diffusion. To get the stoichiometric TiN film, the N:Ti flux ratio should be larger than unity and be increased with higher incident energy due to the weak adsorption of atomic N on TiN(0 0 1).     

Nano-sized cube-shaped single crystalline oxides and their potentials; composition,assembly and functions

Nanocrystals, especially with an anisotropic shape such as cubic, are candidates as building blocks for new bottom-up approaches to materials assembly, yielding a functional architecture. Such materials also receive attention because of their intrinsic size-dependent properties and resulting applications. Here, we report synthesis and characteristics of CeO₂, BaTiO₃ and SrTiO₃ nanocubes and the ordered assemblies. The importance of shape as well as size distribution is clarified for the bottom-up development of 3D fine structures. CeO₂ nanocube assemblies with enhanced surface of specified crystal planes such as (1 0 0) and (1 1 1) would open new surface-dominant devices. BaTiO₃ and SrTiO₃ nanocubes with narrow size distributions and robust diversity in compositions were obtained. BaTiO₃ films made up of ordered nanocube assemblies were fabricated on various substrates by evaporation-induced self-assembly method. Regardless of the substrate, the nanocubes exhibited {1 0 0} orientations and a high degree of face-to-face ordering, which remained even after heat treatment at 850 °C. The supracrystal films exhibited distinct ferroelectric behaviors.     

Positive exchange bias in epitaxial permalloy/MgO integrated with Si (1 0 0)

In magnetic random access memory (MRAM) devices, soft magnetic thin film elements such as permalloy (Py) are used as unit cells of information. The epitaxial integration of these elements with the technologically important substrate Si (1 0 0) and a thorough understanding of their magnetic properties are critical for CMOS-based magnetic devices. We report on the epitaxial growth of Ni_82.5Fe_17.5 (permalloy, Py) on Si (1 0 0) using a TiN/MgO buffer layer. Initial stages of growth are characterized by the formation of discrete islands that gradually merge into a continuous film as deposition times are extended. Interestingly, we find that the magnetic features of Py films in early stages of island coalescence are distinctly different from the films formed initially (discrete islands) and after extended deposition times (narrow distribution of equiaxed granular films). Isothermal in-plane and out-of-plane magnetic measurements performed on these transitional films show highly anisotropic magnetic behavior with an easy magnetization axis lying in the plane of the film. Importantly, when this sample is zero-field cooled, a positive exchange bias and vertical loop shift are observed, unusual for a soft ferromagnet like Py. Repeated field cycling and hysteresis loops up to the fields of 7T produced reproducible hysteresis loops indicating the existence of strongly pinned spin configurations. Classical interface related exchange bias models cannot explain the observed magnetic features of the transitional Py films. We believe that the anomalous magnetic behavior of such Py films may be explained by considering the highly irregular morphology that develops at intermediate growth times that are possibly also undergoing a transition from Bloch to Neel domain wall structures as a function of Py island size. This study broadens the current understanding of magnetic properties of Py thin layers for technological applications in magneto-electronic devices, integrated with Si (1 0 0).     

Quickest ever single-step mechanosynthesis of Cd0.5Zn0.5S quantum dots: Nanostructure and optical characterizations

Multiphase Cd_0.5Zn_0.5S quantum dots (QDs) are prepared for the first time in a record minimum time of 30 min by mechanical alloying the stoichiometric mixture of elemental Cd, Zn and S powders at room temperature under Ar. Initially, the hexagonal phase is formed as major one and then gradually transforms to the cubic phase which is formed coherently on the hexagonal lattice with (0 0 2)_hexagonal||(1 1 1)_cubic mechanism. In the course of milling up to 15 h, the molar ratio of cubic to hexagonal phase becomes ～0.7:0.3. Both high resolution transmission electron microscopy (HRTEM) and X-ray microstructure analyses reveal that these QDs are isotropic in nature, their size reduces to ～5 nm after 15 h of milling with a very narrow size distribution and they contain a significant amount of stacking and twin faults. These QDs show very distinct quantum confinement effect and their optical band gaps are higher than that of the bulk CdZnS.     

Tailoring the domain structure of epitaxial BiFeO3 thin films

Control of the ferroelastic and ferroelectric domain structure of BiFeO₃ through the use of epitaxial growth on substrates with reduced symmetry is reviewed. The first approach presented utilizes orthoscandate substrates, specifically TbScO₃, to reduce the number of possible ferroelastic domains from 4 to 2. Experimental results and phase field simulations are presented which are in agreement with the theory of anisotropic strain relaxation, due to differing in-plane lattice parameters of the orthorhombic substrate, causing a reduction in the possible domains. The second approach that is presented involves the use of miscut cubic substrates, such as SrTiO₃, to tailor the domain structure from 4-domain to 2- or single-domain is presented, the former being achieved with a miscut in the [1 0 0] direction and the latter with a miscut in the [1 1 0] direction, assuming a film normal orientation of [0 0 1]. The use of these techniques in understanding the fundamental nature of the ferroelastic and ferroelectric properties in BiFeO₃, and the use of these methods in tailoring BiFeO₃ to meet the needs of future device applications is discussed.     

A general approach to monodisperse perovskite microspheres

BaTiO₃, PbTiO₃, SrTiO₃, and Pb(Zr,Ti)O₃ microspheres with uniform size and narrow size distribution have been successfully synthesized by a novel hydrothermal and annealing approach. In this approach, the chemical reaction and crystallization process of the ABO₃ perovskite oxides were separated in two steps. Spherical particles containing the B-site ions were obtained first via a controlled hydrolysis and aging process. Then, during hydrothermal treatment, the A-site ions were incorporated in situ into the microspheres to form amorphous perovskite microspheres. The particles were further crystallized with preserved spherical morphology under subsequent annealing treatment. The BET surface areas of the TiO₂ gel particles, the amorphous PbTiO₃ and the as-annealed PbTiO₃ microspheres were 245.7 m²/g, 41.67 m²/g and 4.53 m²/g, respectively, showing a significant change of the surface feature in the preparation process. This approach also allowed the microspheres diameters to be manipulated from 100 nm to 1500 nm in a controlled manner. Most of the microspheres were composed by closely packed nano-sized particles. Furthermore, the Pb(Zr,Ti)O₃ microspheres with an average diameter of 200 nm exhibited single crystal features, indicating highly oriented growth in the crystallization process. The microspheres were very stable, and still maintained spherical shape after higher temperature calcination.     

Synthesis of TiO2 thin films using single molecular precursors by MOCVD method for dye-sensitized solar cells application and study on film growth mechanism

For dye-sensitized solar cells application, in this study, we have synthesized TiO₂ thin films at deposition temperature in the range of 300–750 °C by metalorganic chemical vapor deposition (MOCVD) method. Titanium(IV) isopropoxide, {TIP, Ti(OⁱPr)₄} and Bis(dimethylamido)titanium diisopropoxide, {BTDIP, (Me₂N)₂Ti(OⁱPr)₂} were used as single source precursors that contain Ti and O atoms in the same molecule, respectively. Crack-free, highly oriented TiO₂ polycrystalline thin films with anatase phase were deposited on Si(1 0 0) with TIP at temperature as low as 450 °C. XRD and TED data showed that below 500 °C, the TiO₂ thin films were dominantly grown in the [2 1 1] direction on Si(1 0 0), whereas with increasing the deposition temperature to 700 °C, the main film growth direction was changed to [2 0 0]. Above 700 °C, however, rutile phase TiO₂ thin films have only been obtained. In the case of BTDIP, on the other hand, only amorphous film was grown on Si(1 0 0) below 450 °C while a highly oriented anatase TiO₂ film in the [2 0 0] direction was obtained at 500 °C. With further increasing deposition temperatures over 600 °C, the main film growth direction shows a sequential change from rutile [1 0 1] to rutile [4 0 0], indicating a possibility of getting single crystalline TiO₂ film with rutile phase. This means that the precursor together with deposition temperature can be one of important parameters to influence film growth direction, crystallinity as well as crystal structure. To investigate the CVD mechanism of both precursors in detail, temperature dependence of growth rate was also carried out, and we then obtained different activation energy of deposition to be 77.9 and 55.4 kJ/mol for TIP and BTDIP, respectively. Also, we are tested some TiO₂ film synthesized with BTDIP precursor to apply dye-sensitized solar cell.     

Synthesis of nanocrystalline GaN by the sol–gel method

Single-phase wurtzite GaN nanocrystals with an average diameter of 11 ± 3 nm were synthesized by the sol–gel technique from readily available Ga(NO₃)₃. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) confirmed they had a hexagonal structure and a narrow size distribution of the nanocrystals. X-ray powder diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurement showed that the GaN powder was of single-phase wurtzite structure with a considerable fraction of structural defects such as twin and stacking faults. The IR spectrum showed that only the Ga–N stretch is present at 600 cm⁻¹. The EDX pattern of as-prepared product showed their ratio approximate to 1:1. Room temperature photoluminescence (PL) measurement exhibited the band-edge emission of GaN at about 390 nm and defect emission peak at 610 nm.     

Effect of substrate on microstructure and mechanical properties of sol–gel prepared (K,Na)NbO3 thin films

Lead-free ferroelectric (K, Na)NbO₃ (KNN) thin films (～200 nm thickness) were prepared using a modified sol–gel method by mixing K and Na acetates with the Nb–tartarate complex, deposited by spin-coating method on Pt/Al₂O₃ and Pt/SiO₂/Si substrates and sintered at 650 °C. Pure perovskite phase of K_0.65Na_0.35NbO₃ in film on silicon were revealed, while film on alumina contained also small amount of secondary pyrochlore Na₂Nb₈O₂₁ phase. Homogenous microstructure of film on Si substrate was smoother with the lower roughness (～7.4 nm) and contained spherical (～50 nm) particles. The mechanical properties of films were characterized by nanoindentation. The modulus and hardness of KNN films were calculated from their composite values of film/substrate systems using discontinuous and modified Bhattacharya model, respectively. The KNN film modulus was higher on alumina substrate (91 GPa) in comparison with silicon substrate (71 GPa) and values of film hardness were the same (4.5 GPa) on both substrates.     

Bio-directed synthesis of gold nanoparticles using Ananas comosus aqueous leaf extract and their photocatalytic activity for LDPE degradation

A bio-directed synthesis of gold nanoparticles (Au NPs) was developed via the reduction of hydrogen tetrachloroaurate (III) (HAuCl₄·3H₂O) solution by the aqueous leaf extract of Ananas comosus. The polyphenol stabilized Au NPs were characterized by UV–visible, Fourier transform infrared (FTIR), powder X-ray diffraction (PXRD)/selected area electron diffraction (SAED), high resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDX) analyses. The HRTEM images revealed that Au NPs were well dispersed with spherical structures. The size ranges from 7.39 to 32.09 nm with average particle size of 18.85 ± 6.74 nm. The peaks of XRD analysis at (2θ) 37.96°, 44.06°, 64.54°, 77.50° and 81.73° were respectively assigned to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of the face-centered cubic (fcc) lattice of gold. The photocatalytic potential of Au NPs was studied through the solid-phase degradation of low-density polyethylene (LDPE) film. The photoinduced degradation of LDPE@Au nanocomposite film was higher than that of the pure LDPE film. The weight loss of LDPE@Au (1.0 wt%) nanocomposite film steadily increased and reached 51.4 ± 4.8% in 240 h under solar light irradiation, compared to the photo-induced LDPE with only 8.6 ± 0.7%. However, LDPE film with 1.0% Au NPs gave a weight loss value of 4.72 ± 0.71 under the dark condition at the end of 240 h. Thus, LDPE film with 1.0% Au NPs showed a degradation efficiency of 90.8% under solar irradiation after 240 h. The sustainability of the nanoparticles was confirmed through reusability in the photocatalytic degradation reaction up to five consecutive cycles without substantial loss in its catalytic performance.     

Application of carbonated apatite coating on a Ti substrate by aqueous spray method

The fabrication and characterization of a carbonate-containing apatite film deposited on a Ti plate via an aqueous spray method is described. The mist of the spray solution emitted from a perpendicularly oriented airbrush was made to strike a warmed Ti substrate. The thicknesses of the sprayed film and those heat-treated at 400 °C–700 °C under Ar gas flow were in the range 1.21–1.40 μm. The results of elemental analyses and Fourier transform infrared spectroscopy of the powders that were mechanically collected from the surface of the sprayed film suggest that the film was Ca₁₀(PO₄)6(CO₃) · 2CO₂ · 3H₂O. The presence of the carbonate ion and the lattice CO₂ molecule was confirmed via the aforementioned analyses; the finding was also consistent with the X-ray diffraction patterns of the films and the chemical identity of the sprayed and heat-treated films that were measured using X-ray photoelectron spectroscopy. The sprayed film comprises a characteristic network structure, which contains round particles within the networks, as was observed by field-emission scanning electron microscopy. A scratch test indicated that the shear stress of the sprayed film (21 MPa) significantly improved to 40 and > 133 MPa after heat-treatment at 600 °C and 700 °C, respectively, under Ar gas flow for 10 min.     

Effect of characteristics of (Sm,Ce)O2 powder on the fabrication and performance of anode-supported solid oxide fuel cells

Effect of characteristics of Sm_0.2Ce_0.8O_1.9 (SDC) powder as a function of calcination temperature on the fabrication of dense and flat anode-supported SDC thin electrolyte cells has been studied. The results show that the calcination temperature has a significant effect on the particle size, degree of agglomeration, and sintering profiles of the SDC powder. The characteristics of SDC powders have a significant effect on the structure integrity and flatness of the SDC electrolyte film/anode substrate bilayer cells. The SDC electrolyte layer delaminates from the anode substrate for the SDC powder calcined at 600 °C and the bilayer cell concaves towards the SDC electrolyte layer for the SDC powder calcined at 800 °C. When the calcinations temperature increased to 1000 °C, strongly bonded SDC electrolyte film/anode substrate bilayer structures were achieved. An open-circuit voltage (OCV) of 0.82–0.84 V and maximum power density of ～1 W cm^?2 were obtained at 600 °C using hydrogen as fuel and stationary air as the oxidant. The results indicate that the matching of the onset sintering temperature and maximum sintering rate temperature is most critical for the development of a dense and flat Ni/SDC supported SDC thin electrolyte cells for intermediate temperature solid oxide fuel cells.     

Crystallographic behavior of FeS2 films formed on different substrates

FeS₂ polycrystalline films were prepared on amorphous glass, monocrystalline Si (1 0 0), polycrystalline Al and microcrystalline TiO₂ film substrates by sulfuration annealing of magnetron sputtered iron films. The crystal microstructure and orientation distribution of the films were investigated. The FeS₂ films formed on Si (1 0 0) and glass substrates have relatively fine and uniform grains and small lattice distortion at the interface between the film and substrate but insignificant preferred orientation. The FeS₂ films formed on Al or TiO₂ substrates have relatively inhomogeneous microstructure, large lattice distortion at the interface and a (2 0 0) or (2 2 0) preferred orientation. High strain energy at the interface should be responsible for the preferred orientation and inhomogeneous microstructure in the films.     

Synthesis of SiO2 covered SiC nanowires with milled Si,C nanopowders

SiO₂ covered β-SiC nanowires were directly synthesized with a novel method, annealing the milled Si,C nanopowders at 900–1100 °C on Si wafer or Al₂O₃ substrate, and there is no any metal catalysts used. The diameters of the nanowires are range of 20 to 50 nm, and the lengths of the nanowires are up to several hundreds of microns. There is a uniform SiO₂ amorphous layer on the surface of SiC nanowires. The axial direction of SiC nanowires is < 111>, and there are stacking faults and twin lamellae in the SiC nanowires. The synthetic mechanism of SiC nanowires includes two solid–solid reactions and one gas–solid reaction between SiO, Si, C and CO.