Saturday, July 14, 2007

Organic Vehicles for Preparation of Thick-Film Paste

To prepare a paste for thick-film ink one way is to mix the following organic vehicles:

Terpineol Solvent
Ethyl Cellulose Resin

Terpineol solvent can be made by stirring α-Terpineol with Ethanol. I have no any idea for ratio of α-terpineol and the ethanol for now. But in some papers have been s
uggested by stirring α-terpineol 1wt% into ethanol 47wt%. After that we can dissolve the Ethyl Cellulose Resin into the Terpineol Solvent at room temperature in ratio of X:1 (X stand for terpineol solvent and can be varied from 5 to 15, e.g. the ratio 12:1 means we must dissolve the 92.3wt% Terpineol Solvent into 7.7wt% Ethyl Cellulose).

The metal oxide powder then can be added in desired ratio into this organic mixture and stirred to form homogenous paste by a magnetic stirrer. The ratio of metal oxide into the organic vehicle causes to change the rheology and viscosity of the paste. So we have to find the best ratio into the mixture. It’d better to prepare different sample and study their properties by try and error.

Wednesday, May 23, 2007

Thick Film Materials

The range of materials available for thick-film technology is determined by their capacity to be both printed and fired. Established thick-film technology is based on three classes of material supplied in the form of printing inks. Their classification and function are determined by the resistivity, and the three original classes of conductors, resistors and dielectrics. Emerging sensor technology prescribes a whole new range of materials with desirable properties of interaction with the external world. These new materials are naturally less developed and optimized than the established ones, and they therefore tend to lack some of the more desirable properties.

An important property in manufacture is adhesion, and in commercial inks this tends to be optimized so that structure bond strongly to the substrate. In specially developed sensor inks, we sometimes have to compromise in order to achieve the desired interaction properties. The other major property is the coefficient of thermal expansion, which needs to be matched to the substrate to prevent large thermally induced stresses during manufacture.

In general, thick film structure are created by printing patterns in inks composed of an organic carrier, a low softening temperature glass frit in the form of a finely divided powder and a characterizing ingredient. For conductors, the characterizing ingredient is a finely divided noble metallic power (gold, silver, platinum or Ag/Pd). After processing, the metallic particles fuse to form continuous electrical paths through the carrier glass. Sheet resistivities of the order of 10mΩ/cm^2 are typical. In sensor applications, conducting inks have an important function in the formation of electrode patterns, which range from simple rectangular structures to interdigitated pairs. Platinum is also used for resistance thermometry and for combined heaters and thermometers in areas such as gas sensing, where control at a variety of temperatures is an important technique.

Wednesday, May 2, 2007

Stability in Thick-Film Gas Sensors

Stability is a characteristic that takes into account the reproducibility of device measurements after long use. To avoid the effects of non-repeatability after repeated use, several gas sensor manufactureres submit the organic material (in active phase) and nobles to a thermal pre-treatment, which decrease subsequent material instabilities.
During treatments, samples are submitted to high calcination temperatures (from 400 °C to 1000 °C for 1 to 24 hours) to prevent instabilities in their working life (lifespan) in where they are heated 200-400 °C continuously.
Gas sensor should be stable in a variable ambient atmosphere, but it should also be reversibly unstable in the presece of the gases to be detected.

Friday, April 27, 2007

Thick-Film Gas Sensors Technology

Usually a thick-film gas sensor means a metal-oxide semiconductor sensor obtained by thick-film screen-printing technology. The chemically sensitive layer consists of a paste prepared from metal-oxide powder, inorganic additives and organic binders.
The paste is printed over an alumina substrate containing metal film electrodes and a back heating resistor; the paste is then sintered in a thermal or IR belt furnace.
Thick-film gas sensors are known for their high sensitivity especially for the large specific surface available for the chemisorption’s reactions. Moreover, with the proper choice of the semiconductor oxide and of the catalyst used, the sensing film can enhance the selectivity of the device towards a particular set of gases.
At room temperature the kinetics of the chemisorption’s reaction is too slow and not fully reversible, thus these sensing films are usually operated at temperatures ranging from 200 to 400°C. For this reason the overall power consumption of these devices is in general of the order 500 mW, mainly because of convection heat losses.
In next post I will describe all parts a thick-film-based gas sensor in detail.