Flame spray pyrolysis (FSP) was used to make pure and Pt-doped tin dioxide nanoparticles in one-step. The aerosol generated by the dry FSP method was directly, in situ thermophoretically deposited onto interdigitated Pt-electrodes to form a porous, thick film of controlled thickness within the active sensor area. Tin oxide grain size (10 nm) and a high film porosity (98%) were preserved for all film thicknesses from 9 to 40 μm using different deposition times. The dependence of the film thickness on deposition time was theoretically estimated to enable precise control of the deposition process. Platinum doping did not affect the SnO2 grain size, crystallinity, or the porous film structure. These sensors exhibited high carbon monoxide (CO) sensor signals (8 for 50 ppm CO in dry air at 350 °C), good reproducibility, high analytical sensitivity and a remarkably low detection limit (1 ppm CO in dry air at 350 °C). The in situ platinum doping enhanced the overall sensor performance. Increasing the film thickness increased the sensor resistance and can be used to tune sensor performance.

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Direct formation of highly porous gas-sensing films by in situ thermophoretic deposition of flame-made Pt/SnO2 nanoparticles

In broad terms, hydrothermal synthesis is a technology for crystallising materials (chemical compounds) directly from aqueous solution by adept control of thermodynamic variables (temperature, pressure and composition). The objective of this review is to introduce the field of hydrothermal materials synthesis and show how understanding solution thermodynamics of the aqueous medium can be used for engineering hydrothermal crystallisation processes. In this review, powder synthesis, and their applications are introduced. In Section ‘Introduction’, we will focus on the hydrothermal synthesis as a materials synthesis technology by providing history, process definitions, technological merits and comments on its current implementation in the laboratory. In Section ‘Scope of hydrothermal synthesis in future’, we will describe the development of hydrothermal technology for materials synthesis, their results and comparison with other methods.

Review: hydrothermal technology for smart materials

The assembly of nanoparticle films by flame spray pyrolysis (FSP) synthesis and deposition on temperature-controlled substrates (323–723 K) was investigated for several application-relevant conditions. An exemplary SnO2 nanoparticle aerosol was generated by FSP and its properties (e.g., particle size distribution), and deposition dynamics were studied in details aiming to a simple correlation between process settings and film growth rate. At high precursor concentrations (0.05–0.5·mol/L), typically used for FSP synthesis, the nanoparticles agglomerated rapidly in the aerosol leading to large (>100 nm) fractal-like structures with low diffusivity. As a result, thermophoresis was confirmed as the dominant nanoparticle deposition mechanism down to small (≈40 K) temperature differences (ΔT) between the aerosol and the substrate surface. For moderate-high ΔT (>120 K), thermal equilibrium was rapidly obtained yielding a constant thermophoretic flux and film growth rate. A model was developed to predict the nanoparticle deposition rates by FSP synthesis at moderate-high ΔT that does not require detailed analysis of the aerosol composition. Comparison with previous studies having similar nozzle geometries showed that the deposition rates of FSP-made aerosols can be reasonably well predicted for various materials and flame conditions. © 2012 American Institute of Chemical Engineers AIChE J, 2012

Flame spray pyrolysis synthesis and aerosol deposition of nanoparticle films

In the current work, a novel combustion method is demonstrated for the direct synthesis of nanocomposite materials. Specifically doped tin dioxide (SnO2) powders were selected for the demonstration studies due to the key role SnO2 plays in semiconductor gas sensors and the strong sensitivity of doped SnO2 to nanocomposite properties. The synthesis approach combines solid and gas-phase precursors to stage the decomposition and particle nucleation processes. A range of synthesis conditions and four material systems were examined in the study: gold–tin dioxide, palladium–tin dioxide, copper–tin dioxide, and aluminum–tin dioxide. Several additive precursors were considered including four metal acetates and two pure metals. The nanocomposite materials produced were examined for morphology, phase, composition, and lattice spacing using transmission and scanning electron microscopy, x-ray diffractometry, and energy-dispersive spectroscopy. The results using the combustion synthesis approach indicate good control of the nanocomposite properties, such as the average SnO2 crystallite size, which ranged from 5.8 to 17 nm.

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A New Method for Direct Preparation of Tin Dioxide Nanocomposite Materials

Nanocrystalline SnOx particles (x= 0–2) were synthesized using tetramethyltin (Sn(CH3)4) vapor as the particle precursor reactant in hydrogen/oxygen/argon (H2/O2/Ar) flames. The particle composition and morphology were characterized using X-ray diffractometry, transmission electron microscopy, and nitrogen (N2) surface adsorption. By controlling the concentration of oxygen in the reactant gases and the flame temperatures, metallic tin (Sn), tin monoxide (romarchite SnO), and/or tin dioxide (cassiterite SnO2) were generated. The crystalline powders consisted of both discrete primary particles and agglomerates, with average primary particle sizes of 23–24 nm for SnO2 and 69 nm for Sn (based on specific surface area measurements of bulk powders collected in the exhaust region of the flame). The compositional results were interpreted using equilibrium and detailed chemical kinetics models.

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Combustion Synthesis and Characterization of Nanocrystalline Tin and Tin Oxide (SnOx, x= 0–2) Particles

The influence of electroless nickel plating on lead-based relaxor ferroelectric multilayer ceramic capacitors (MLCs) was studied by a comparison experiment. It was found that the influence is related to reactions in the electroless plating. It is proposed that adsorbed hydrogen atoms generated in electroless plating can diffuse into the ceramic bodies of MLCs and undergo some reduction reaction with them, resulting in the failure of electroless nickel plating. The implications for the negative influence of electroplating on MLCs and for the degradation in MLCs are also included, in which much attention is paid to the reduction reaction of hydrogen atoms generated by electrolysis of water.

Fredric Cosandey

Papers

Low‐Pressure Flame Deposition of Nanostructured Oxide Films