2 Technical University of Varna, Dept. of Mathematics, Studentska str. 1, 9010 Varna, Bulgaria, e-mail: meline_aprahamian@usa.net

Prototypes of surface heater elements for mean temperature heating are developed. The heaters are formed upon electroinsulated plates of vacuum thick alumina ceramics by magnetron sputtering or screenprinting of a resistive layer. Various compositions of resistive layer are investigated, including amorphous metal alloys. Electrical, thermal mechanical and climatic research of the heaters is made. Some prototypes with various compositions of amorphous metal layers are applied as sensors for hot – wire anemometer.

Foil heater elements have recently been widely applied in comercial and household heaters for low temperature heating. At present heater elements with insulating substrates of polyvinylchloride, polyester, polyamid, mecanite, silicon, enamelled metal , etc. are mainly used.

Having experience in developing and implementation of foil heater elements in production we faced the problem of developing surface heater elements for heating in the range of 300 – 600 ^oC. These were to be formed on dielectric substrate of a certain class of heat resistance. Having analized a lot of dielectrics avaible we found Al₂O₃ ceramics to be the most suitable one. It is widely used in microelectronics and in hybrid integrated circuits in particular. This material was prefered because of its advantageous properties, such as:

- It does not contain volatile components and thermal dissociation of oxides in the ceramics composition occurs at temperatures higher than those necessary for processing technology.

- Gas permeability is negligible. Practicaly no polluting substances are released in processing and high – grade conducting layers can be obtained.

- It has high volume resistance which practicaly remains constant in the process of wear and is slightly affected by temperature. It has high electric resistance and perfect insulating properties.

- It has high heat conductivity, much higher than that of standart insulating materials.

- It is inert to effects of various chemicals and highly resistant to humidity, fog, mist, etc.

- It has good contractability in all directions and appearance which practicaly does not change with ageing. Elements with a size of some fractions of milimeter up to some hundred milimeters can be made.

- Ceramics composition is mainly Al₂O₃, which is cheap and avaible.

In developing prototypes of surface heater elements on ceramic plates alumina ceramics produced by “Metaloceramics” ( Bulgaria ), “Rosenthal” and “Rubalit” ( Germany ) and 22XC type ceramics ( Russia ) were used.

All specimens were prepared on square ceramic substrates with a side of 2 inch ( 50.8 mm ) and 0.9 mm thick. Having analysed more than 20 possible procedures for forming a conducting layer upon a ceramic substrate, we prefered following two of them: magnetron sputtering [ 1 ] and screenprinting [ 2 ] of a resistive layer, both technologies being applied in integrated circuits production. Though principally different in implementation both of them enable high – grade layers to be obtained with:

• the coefficient of thermal expansion close to that of the substrate.

In forming heater elements by magnetron sputtering Russian stainless steel of X18H9T – type containing C – 1%, Cr – 18%, Ni – 9% and Ti – 0.5% is used as resistive layer material. Coating is performed in a vacuum – pumping plant of B – 90 type ( “Hochvakuum” – Germany ) with the folowing process parameters: cleaning voltage – 2 kV DC, cleaning time – 10 min, operating voltage – 390 V DC, operating gas – Argon, operating pressure – 2. 10^-3 Torr.

In forming heater elements by screenprinting special resistive pastes are used for resistive layer material, including pastes developed in Bulgaria by the Central Laboratory of Electrochemical Power Sources of Bulgarian Academy of Sciences [ 3 ], as well as those produced by “Heraeus” – Germany and “DuPont” – USA. DuPont conducting pastes are used for contact areas. Commercial DuPont Model 8010 printer is used, the parameters of the technological process being as follows: speed of the printing panel – 35 mm/s, thickness of the emulsion layer of the screen – 11 m m, mean baking temperature – 850 ^oC, time of maintaining maximal baking temperature – 10 min, paste viscosity – 105 Pa.s.

In experimenting the procedures mentioned parameters were changed many times and their effects upon the quality of the layer were studied by statistic analysis.

A protective layer was applied on specimens of heater elements by screenprinting, this layer enabling operation of the heater in direct contact with conducting objects without using additional insulation. Dielectric paste of “Heraeus” – Germany production was used for the purpose. Two types of protective layer were tested:

- entire layer covering entire plane of the element on the side of the resistive layer;

Configuration of surface heater elements is shown in Figs. 1, 2.

Prototypes are developed with the following parameters: area of the heater – 2, 58. 10^-3 sq. m, maximal power for this area – 60 W, maximal operating temperature – 380 ^oC. Power supply may be of direct or alternating current of up to 250 V. Main technical particulars of the heater elements are shown in Table 1.

Various types of heater elements were tested with as full as possible combination of types of ceramic substrates used, materials for resistive layer as well as types of protective layers. Some variants of formed and tested specimens are shown in Table 2. Through electrical, thermal, mechanical and climatic studies are made in accordance with standards used in Bulgaria. The results obtained prove the heaters to be practicable. Transient thermal process for various Watt density of the heater is studied – Fig. 3, as well as Watt density changes with temperature – Fig.4 for prototypes obtained by both procedures. The data obtained are very much alike and competible, i. e. both procedures give similar results.

Mechanical deformations of the elements due to thermal load are studied in order to choose optimal way of fitting elemnts in electro – thermal devices. Various types of fitting are tested and data for main mechanical stresses in the heater plane are found. Fitting with one fixed point ( by pressing ) and with 4 fixed points are prove to be most advantageous because dangerous tensile stresses are small and there are no great mechanical loads in the contact areas.

Test models of heater elements with amorphous resistive layer are formed by magnetron sputtering. Two amorphous compositions were tested: stainless X18H9T alloy with Titanium content of 13.9% and the same but with 15.5% Titanium. These elements are characterised with exceptionally high corrosion resistance and special uneven dependence between the amorphous metal layer resistivity and the temperature. This uneven change of resistivity may be due to recrystalyzation in the alloy structure when the temperature rises. These types of elements are especialy suitable for thermal relays and heat sensors in electric devices.

The procedure of measuring fluid flow velocity by hot – wire anemometers is based on the fact that an electrically heated thin metal wire being placed in a fluid flow, its temperature decreases and hence its resistance changes with the flow velocity increasing [ 4 ].

Having analized existing types of hot – wire anemometers, we came to a conclusion that it is possible to improve their sensitivity by replacing a silver wire sensor with a flat sensor consisting of a ceramic plate with an amorphous metal layer of certain configuration. Changes of resistivity of different sensors caused by the increase of the flowing fluid velocity is shown in Fig. 5. Let the rate of changing of a sensor resistivity relative to the fluid velocity be called its sensitivity. Then, as seen from Fig. 5, sensors Nos. 1 and 2 have comparable sensitivity while that the sensor No. 3 with amorphous structure is three times as high.

Heater elements on ceramic plates are also adopted in practice as thermostatic elements of computer systems. They are considered promising and we have been working on their further modifications.

1. Joly J., Ranger J.-B. The Use of Magnetron Sputtering for Hibrid Integrated Circuits Manufacturing, NTG Fachber, vol. 60, 1977
2. Mazda F. F. Integrated Circuits. Technology and Applications, Cambridge University Press, London, 1978
3. Draguieva I. and alt. Preparation and Properties of Magnetic Metal Powders by a Borohydride Reduction Process, Journal of Magnetism and Magnetic Materials, vol. 89, 1990
4. Madjirski V. Mechanics of Fluids, Tehnika, Sofia, 1989 ( in Bulgarian )