Peter Ryser

Tunneling-percolation origin of nonuniversality: Theory and experiments

A vast class of disordered conducting-insulating compounds close to the percolation threshold is characterized by nonuniversal values of transport critical exponent t, in disagreement with the standard theory of percolation which predicts t

Pyroelectric thin-film sensor array

Pyroelectric thin-film point detectors and 1 x 12 arrays have been fabricated and characterized. They consist of sol-gel-deposited PZT thin-film elements on micromachined Si3N4/SiO2 membranes. The measured current and voltage response as a function of modulation frequency of a 1 x 12 array element is compared with finite-element calculations. Voltage responsivities of almost 3000 V W-1 in vacuum and 800 V W-1 in air have been achieved for 0.4 mm x 0.9 mm elements. Some point detectors have been completely packaged and correct operation in a movement detection system has been demonstrated.

Solution of the tunneling-percolation problem in the nanocomposite regime

We noted that the tunneling-percolation framework is quite well understood at the extreme cases of percolation-like and hopping-like behaviors but that the intermediate regime has not been previously discussed, in spite of its relevance to the intensively studied electrical properties of nanocomposites. Following that we study here the conductivity of dispersions of particle fillers inside an insulating matrix by taking into account explicitly the filler particle shapes and the inter-particle electron tunneling process. We show that the main features of the filler dependencies of the nanocomposite conductivity can be reproduced without introducing any a priori imposed cut-off in the inter-particle conductances, as usually done in the percolation-like interpretation of these systems. Furthermore, we demonstrate that our numerical results are fully reproduced by the critical path method, which is generalized here in order to include the particle filler shapes. By exploiting this method, we provide simple analytical formulas for the composite conductivity valid for many regimes of interest. The validity of our formulation is assessed by reinterpreting existing experimental results on nanotube, nanofiber, nanosheet and nanosphere composites and by extracting the characteristic tunneling decay length, which is found to be within the expected range of its values. These results are concluded then to be not only useful for the understanding of the intermediate regime but also for tailoring the electrical properties of nanocomposites.

Size-Selective Diffusion in Nanoporous but Flexible Membranes for Glucose Sensors

A series of nanoporous membranes prepared from polyethylene-block-polystyrene were applied for size-selective diffusion of glucose and albumin molecules. Millimeter-sized test cells for characterization of such molecular diffusions were designed assuming an implantable glucose sensor. The prepared nanoporous membrane exhibits excellent flexibility and toughness compared to conventional nanoporous membranes of brittle alumina. Pore size of the membranes could be controlled from 5 to 30 nm by varying preparation conditions. All of these nanoporous membranes prepared in this study let glucose pass through, indicating a continuous pore connection through the entire thickness of the membrane in a few tens of micrometers. In contrast, membranes prepared under optimum conditions could perfectly block albumin permeation. This means that these vital molecules having different sizes can be selectively diffused through the nanoporous membranes. Such a successful combination of size selectivity of molecular diffusion in nanoscale and superior mechanical properties in macroscale is also beneficial for other devices requesting down-sized manufacture.

Application of graphite-based sacrificial layers for fabrication of LTCC (low temperature co-fired ceramic) membranes and micro-channels

Fabrication of sensors and micro-fluidic structures from low temperature co-fired ceramic (LTCC) sheets is a growing interest in the micro-packaging community. Such devices usually have inner cavities, whose production is quite complicated. The most elegant method to build such structures so far achieved is by a fugitive phase that is introduced into the multilayer and removed during firing. This paper, therefore, is aimed to introduce the graphite-based sacrificial paste developed for this purpose, and it is constructed in two sections: (i) selection of paste and determination of LTCC open-porosity elimination temperature, and (ii) fabrication and characterization of pressure sensitive LTCC membranes. In the former section, it is shown that increased heating rates (and decreasing tape thickness) shift the open porosity elimination temperature of LTCC by 20 °C, which is small compared to the shift of graphite oxidation temperature (about 100 °C). In the latter section, three parameters affecting the balance between the graphite oxidation and LTCC sintering are studied: heating rate, graphite phase thickness and width of the membrane inlet/outlet channels. As expected, larger heating rates and narrow inlet/outlet channels are found to hinder the oxidation of graphite and evacuation of the resulting products, which results in swollen membranes. Large graphite thickness, through the increased channel height, results in lower swelling in spite of the larger amount of graphite to be oxidized. Membranes with low swelling are found to exhibit excellent pressure sensing characteristics, whereas those with high swelling display hysteretic behavior.

Gauge factor enhancement driven by heterogeneity in thick-film resistors

We present a simple picture of the gauge factor (GF) enhancement in highly heterogeneous materials such as thick-film resistors. We show that when the conducting phase is stiffer than the insulating one, the local strains within the latter are enhanced with respect to the averaged macroscopic strain. Within a simple model of electron tunneling processes, we show that the enhanced local strain leads to values of GF higher than those expected for a homogeneous system. Moreover, we provide formulas relating the enhancement of GF to the elastic and microstructural characteristics of thick-film resistors.

Integrated LTCC Pressure/Flow/Temperature Multisensor for Compressed Air Diagnostics†

We present a multisensor designed for industrial compressed air diagnostics and combining the measurement of pressure, flow, and temperature, integrated with the corresponding signal conditioning electronics in a single low-temperature co-fired ceramic (LTCC) package. The developed sensor may be soldered onto an integrated electro-fluidic platform by using standard surface mount device (SMD) technology, e.g., as a standard electronic component would be on a printed circuit board, obviating the need for both wires and tubes and thus paving the road towards low-cost integrated electro-fluidic systems. Several performance aspects of this device are presented and discussed, together with electronics design issues.

Processing and properties of thin film pyroelectric devices

Pyroelectric PbTiO3 thin films devices with two temperature compensating elements on a SiO2/Si3N4 membranes have been fabricated and characterized. The measured voltage responsivity as a function of radiation modulation frequency has been compared to finite element model calculation. The relevant film properties have been compared for sputter and a sol-gel deposition techniques. The calculated responsivity amounts to 30 V/W. The measured pyroelectric signal is a few mu V.

Percolative properties of hard oblate ellipsoids of revolution with a soft shell

We present an in-depth analysis of the geometrical percolation behavior in the continuum of random assemblies of hard oblate ellipsoids of revolution. Simulations were carried out by considering a broad range of aspect ratios, from spheres up to aspect-ratio-100 platelike objects, and with various limiting two-particle interaction distances, from 0.05 times the major axis up to 4.0 times the major axis. We confirm the widely reported trend of a consistent lowering of the hard particle critical volume fraction with increase of the aspect ratio. Moreover, by assimilating the limiting interaction distance to a shell of constant thickness surrounding the ellipsoids, we propose a simple relation based on the total excluded volume of these objects which allows us to estimate the critical concentration from a quantity that is quasi-invariant over a large spectrum of limiting interaction distances. Excluded volume and volume quantities are derived explicitly.

Piezoresistivity and conductance anisotropy of tunneling-percolating systems

We propose a theory of the origin of transport nonuniversality in disordered insulating-conducting compounds based on the interplay between microstructure and tunneling processes between metallic grains dispersed in the insulating host. We show that if the metallic phase is arranged in quasi- one-dimensional chains of conducting grains, then the distribution function of the chain conductivities g has a power-law divergence for g-->0, leading to nonuniversal values of the transport critical exponent t. We evaluate the critical exponent t by Monte Carlo calculations on a cubic lattice, and show that our model can describe universal as well nonuniversal behaviors of transport depending on the value of few microstructural parameters. Such a segregated tunneling-percolation model can describe the microstructure of a quite vast class of materials known as thick-film resistors, which display universal or nonuniversal values of t depending on the composition.