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1. What are the major parts of thick film printing technology?
1. Printing, 2. Drying, and 3. Firing
2. What are two important characteristics of thick film pastes?
They are: a) Viscous fluids with a suitable rheology and b) Composed of two different multi-component phases.
3. Name two multi-component phases needed in fabrication of thick film pastes.
a) A functional phase to impart the electrical and mechanical properties and b) A vehicle phase to impart the proper rheology.
4. What are three basic categories of thick film pastes classified based-on thermal curing?
1. Polymer thick film (< 300°C) 2. Refractory thick film (>1300°C) 3. Cermet (Ceramic Metal Element) thick film (800 °C to 1000 °C)
5. What are the main components of ceramic metal paste?
1. Active element, 2. Adhesion element, 3. Organic binder, and 4. Solvent element
6. What are differences between “fritted” and “fritless” adhesion elements?
1. Thermal curing for fritted adhesion elements which contain glass elements is done at lower temperature (500 °C to 600 °C) but fritless composition is fired at 950 °C to 1100 °C. 2. Fritted adhesion is bound to the substrate based-on melting components. The fritless adhesions react with broken oxygen of the substrate to form a spinel structure (an AB2O4 oxide).
7. Name three major thermal treatments of powder preparation for thick film paste.
1. Drying after milling at about 100 °C. 2. Calcination at about 500 to 600 °C. 3. Sintering at above 800 °C.
8. What are the processes of thermal curing for printed thick film layer?
1. Leveling time at room temperature 2. Drying at 125 to 150°C 3. Firing at above 850 °C.
9. What are the three important parameters for characterization of thick film paste?
1. Fineness of grind, 2. Percent solids, 3. Viscosity.
10. What are the criteria of the conductive thick film pastes?
1. High purity of the fired layer
2. Have a porous structure (for gas and humidity sensors)
3. Have low resistance
4. High resolution
5. Good adhesion to the substrate
6. Be chemical stable
7. Be heat stable
8. Solderability
11. Name three material that are satisfied the requirements for conductive pastes?
Noble metals such as gold, platinum, palladium, copper, and silver. Then the paste can be formed in single-component (on the basis of silver or gold), or double component such as Ag-Pd, Au-Pt, Au-Pd, etc.
12. Why thick film components are called “Ceramic Films”?
Polycrystal films with thickness 10-100 µm are used for producing thick film components, because their structure is similar to the structure of the ceramics they are regarded as ceramic films.
13. Compare the earliest and the modern thick film resistors?
The earliest thick film resistors were made from material such as carbon, silver, and iron oxide. They were found to suffer from poor long-term stability, unpredictability of fired resistivity, and unacceptably high temperature coefficients. Modern thick film resistors are mainly based-on ruthenium dioxide (RuO2). This material has a high conductivity and is extremely stable at high temperatures.
14. Why we need gas sensors?
We need gas sensors to improve the environmental and safety control of toxic gases. There is also a great need for these kinds of sensors for optimizing combustion reactions in the emerging transport industry, and domestic and industrial applications.
15. What is the basic consideration to detect the gas by a planar gas sensor?
Atoms and molecules interact with semiconductor surfaces, and influence such surface properties as conductivity and surface potential.
16. What are the best materials for detection of gas by gas sensors?
The most suitable semiconductor materials for this type of sensor are metal oxides. Unlike other semiconductors which, under long-term or cycled heating in air, undergo irreversible chemical transformations by forming stable oxide layers, metal oxides bind oxygen on their surface in a reversible way. These materials can be classified as n-type or p-type according to whether their resistance increases or decreases when they are exposed to a reducing gas in an atmosphere of fixed oxygen partial pressure (air).
17. Name three materials that can be used as an active layer in a planar gas sensor.
Zinc Oxide (ZnO), Tin Oxide (SnO2), and Tungsten Oxide (WO3)
18. Beside the type of material used as active layer, what are the other parameters for a planar gas sensor to detect a target gas?
1. Particle size of active layer 2. Environment relative humidity 3. Reaction temperature provided by heater 4. Flow rate of the applied gas 5. Type of catalyst and filter above the active layer
19. Explain how the particle size of powder in active layer can affect the sensitivity of a planar gas sensor?
Planar gas sensors are worked based-on adsorption by trapping the oxygen molecule between the grain boundaries of active layer. Smaller grain size leads to more available surface of grain boundaries causes more sensitivity.
20. Briefly explain the terms of additive, doping, and catalyst in a planar gas sensor.
Additive: such as glass frit powders, are used to ensure good adhesion of film to the substrate. These materials usually increase the resistance of the film, because they fit active grains together to form a compact adhesive film on the substrate, but they have no catalytic properties.
Doping: Commercially available sensors normally contain a small quantity of noble metals such as Ag, Ti, Pd and Pt, which are dispersed on the oxide as activators or sensitizers to improve the gas selectivity and lower the operating temperature. They enhance the selectivity to some gases of active layer activity. They also make the basic material active for particular reactions, thus making the layer more sensitive.
Catalyst: The metal oxide-based sensors react to a wide range of gases with similar chemical properties. Catalytic filters can be applied to some of them so that selectivity is high. Recently some catalytic filters have been used in the construction of gas sensors. They prevent any reactions with some gases by removing these gases by combustion or some other reaction before they reach the active layer.
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