B. Chenevier

CO and NO2 gas sensitivity of nanocrystalline tin dioxide thin films doped with Pd, Ru and Rh

Abstract The effect of Pd, Ru and Rh doping on microstructure, electrical, and gas sensor properties of nanocrystalline tin dioxide films is studied. SnO2 and SnO2(M) (M=Pd, Ru, Rh) films of 0.9–1-μm thickness and a doping metal content of 0.07–1.6 at.% are synthesized by aerosol pyrolysis. Studies at 50–400 °C of the evolution of conductivity of noble metal-doped tin dioxide films put in contact with gas mixtures containing small amounts of CO and NO2 give evidence of strong similarities of the interaction mechanisms. Correlations between the electrical response and oxygen affinity of noble metal clusters were found.

Electrical properties evolution under reducing gaseous mixtures (H2, H2S, CO) of SnO2 thin films doped with Pd/Pt aggregates and used as polluting gas sensors

Abstract An analysis of the electrical properties evolution of a series of SnO2 thin films doped with small amounts of Pd or Pt under pure air and air with 300 ppm CO is presented. As several types of chemical reactions are clearly involved in the solid–gas interactions at the film surface, it has been necessary to simplify the system by favouring interactions in absence of oxygen. Films were consequently also put in contact with N2 or Ar mixed with small amounts of H2, CO, H2S to favour behaviours in absence of oxygen gas. As under air+CO at 300°C, important features of Pd/Pt-doped film conductance closely resemble CO2 production rates of CO oxidation at the surface of Pd aggregates reported in the literature, an attempt of interpretation of the conductance evolution has been made along this line. Under CO and H2 mixed with neutral gas, dynamic (kinetic) electrical conductance measurements show that the dispersions of metallic elements induce a two-step time dependent behaviour. The first step is associated with a reduction of the oxygen molecules adsorbed at the SnO2 grain surface and an increase of electron density in the SnO2 depletion zone. The second step with a sharp conductance increase implies a reduction of the metallic aggregates and an electron transfer from the aggregates to the SnO2 grain conduction band. For H2S the conductance increases smoothly.

Electrical properties under polluting gas (CO) of Pt- and Pd-doped polycrystalline SnO2 thin films: analysis of the metal aggregate size effect

Abstract In Pt- or Pd-doped SnO2 thin films prepared by using a submicronic aerosol pyrolysis method where both metal elements are co-deposited, the best sensitivity to CO is obtained from low-concentration Pd/Pt precursor solutions. TEM observations from SnO2 thin films doped with small amounts of platinum have been combined with results obtained from in situ X-ray Absorption Spectroscopy (XAS) to give a schematic description of the metallic particle effect on the electrical properties of the films. It is demonstrated that the lower the concentration, the smaller are the metallic particles. The evolution of the particle size and density as a function of concentration can be understood in terms of nucleation-growth processes. The XAS analysis shows, in particular, that the Pt local environments are modified by metal concentration variations and by the gas in contact with the film surface. The modifications are discussed on the basis of competing bulk and surface contributions to CO reduction processes, the latter being dominant in smaller particles. It is inferred that smaller particles contribute as a whole to the electrical behavior whereas in bigger particles, a surface and a bulk contribution have to be considered.

Electron microscopy analysis of the microstructure of Ti1−xAlxN alloy thin films prepared using a chemical vapour deposition method

Abstract The microstructure of a series of Ti 1− x Al x N films prepared by Chemical Vapour Deposition (CVD) is investigated using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy imaging (EELS). A comparison is made with the microstructure of a film obtained using a magnetron sputtering method. CVD films were prepared with substrate temperatures ranging from 400 to 450°C. Plan-view and cross-sectional observations show that they are made of nanocrystallites, a majority of these being organised in columnar clusters. The sharpest electron diffraction patterns obtained from the films are indexed in terms of the fcc TiN structure-type with a lattice parameter a , decreasing slightly with increasing aluminum content. In the film deposited by magnetron sputtering the main structure is fcc TiN-type with a lattice parameter of 4.17 A and the observed grain size is 300 nm, a value much larger than the grain size obtained from CVD films, although the deposition was done at room temperature. The EELS imaging technique also shows that throughout the series, Ti, Al and N are homogeneously distributed.

Role of Pt Aggregates in Pt / SnO2 Thin Films Used as Gas Sensors Investigations of the Catalytic Effect

Fine dispersions of metal aggregates such as Pt, Pd, or Cu in metal oxides are well known to improve the electrical sensitivity and selectivity of solid‐state gas sensors put in contact with reducing gases (CO and , for instance) of solid‐state gas sensors. The improvement originates in the catalytic role played by the aggregates in regard to gas oxidation/reduction. The details of the solid‐gas interactions are nevertheless not fully understood. To further investigate the role of Pt aggregates in the sensing mechanisms results of in situ X‐ray absorption and Auger electron spectroscopy from films have been combined. Films with a Pt concentration in the range 0.6–6 atom % have been prepared using a modified chemical vapor codeposition method. Results show that the sensing mechanism in mainly driven by the CO oxidation catalyzed by the aggregates: first a spillover effect occurs between CO, chemisorbed oxygen, and aggregates, and in a second step, after a period of time depending on temperature, the aggregates are reduced/oxidized provided that they are small enough. The correlations between Pt particle behavior and film electric properties are discussed.

Influence of Si substrate orientation on stress development in Pd silicide films grown by solid-state reaction

In situ real-time measurements of stress are performed during solid-state reaction of a palladium thin film with Si(001) or Si(111) single crystals. The stress in Pd2Si is compressive in both cases at variance with the sign of epitaxial misfit. A large difference in stress relaxation kinetics between fiber textured [on Si(001)] and epitaxial [on Si(111)] Pd2Si films is evidenced. This difference is correlated with a considerable variation in stress buildup during silicide growth. The microstructure of the growing phase is thus a key parameter for stress development during solid-state reaction.

Anisotropy of thermal expansion in YAlO3 and NdGaO3

YAlO3 and NdGaO3 thermal expansion coefficients were measured using in situ powder x-ray diffraction in the temperature range of 28–650 °C. They exhibit a clear anisotropy: The expansion, quite similar along the [100] and [001] directions, is much lower along the [010] direction. The formation of cracks observed in YBa2Cu3O7−δ thin films deposited on YAlO3 and NdGaO3 substrates is likely related to the anisotropy. Stress value calculations have been performed in both systems. They indicate that the intrinsic components are specifically high in YAlO3.

Investigation of the interface stability in HfO 2 -metal electrodes

As the sharpness of the metal-oxide interface is of major importance to develop efficient structures, we have studied the thermal stability of interfaces in a series of thin film samples where the metallic component was either Pt, Al, Pd, Ni, Nb or Ti deposited on a thin HfO2 film. Thermodynamic considerations have been carried out to determine the possible products of metal-oxide reaction at a temperature of 500 °C. The evolution of the as-deposited thin film structures as a function of annealing temperature has been analysed by combining various techniques and specifically using X-ray reflectometry. This particular technique allows one to detect very thin embedded interfaces, distinguish small density variations and determine accurately the thickness, the normal roughness and the density of each layer. In addition, reflectometry results have been correlated with atomic force microscopy and transmission electron microscopy observations of selected samples. Our results indicate that platinum, palladium and nickel are stable up to 500 °C on hafnium oxide. Annealed aluminium, an interfacial layer forms, whereas niobium and titanium present a diffuse interface.

Pt Doping of SnO2 Thin Films A Transmission Electron Microscopy Analysis of the Porosity Evolution

In Pt-doped SnO 2 thin films prepared by using a submicroscopic aerosol pyrolysis method where the metal elements are codeposited, the best CO sensitivity is obtained from low concentration Pt precursor solutions. Transmission electron microscopy (TEM) observations of SnO 2 thin films doped by small amounts of platinum have been performed to analyze in detail the structural properties as a function of Pt doping concentration. Strong evidence of porous structure has been found in the TEM images and an assessment of the porous volume fraction could be done. Although this approach clearly underestimates the assessed porous fraction, results for undoped SnO 2 films nevertheless indicate that the film density is close to half of the bulk value. It increases with the film doping concentration, [Pt]/([Sn] + [Pt] + [O]), as films become thinner. For doping concentrations higher than 2%, no significant number of pores could be detected and the thickness reaches a plateau likely corresponding to the maximum density that can be obtained using our synthesis conditions. The usually observed maximum in the film sensitivity at solution concentrations close to 0.6% film doping, i.e., 3% in terms of solution concentration, coincides with a maximum in the product of the porous volume and the density of small Pt aggregates. Additionally high-resolution electron microscopy observations of selected film samples have been performed to examine grain arrangements and microstructural effects of Pt and SnO 2 nanocrystallites.

Critical current density distribution in superconducting oxide layer of Bi2Sr2CaCu2Ox/Ag composite tapes

Abstract The relationship between Jc and the thickness of the oxide layer of Bi2Sr2CaCu2Ox/Ag composite superconductor is studied to clarify the critical current distribution in the oxide layer. Jc of the oxide layer within 5 μm from the oxide/silver interface exceeds 3×105 A/cm2 at 4.2 K, 10 T, while Jc for the whole part of oxide (25 μm in thickness) stays at 1.2×105 A/cm2. The results indicate that the superconducting current is carried by thin and highly aligned oxide part along the oxide/silver interface. The results also suggest that the higher Jc can be expected with decreasing thickness of the oxide layer and with increasing the oxide/silver interface.