EP1443298B1 - Heating element for initiating pyrotechnical charges - Google Patents

Abstract

Heating element for igniting pyrotechnical charges comprises a structured resistance layer arranged on a base body, and contact fields arranged in an overlapping manner on the two ends of the resistance path. The heating element has a weight of 1.0 x 10-9-4.0 x 10-9 kg, a specific resistance of 1 x 10-6-2 x 10-6 omega m, and a specific heat capacity of 100-400 W/(kgK). An independent claim is also included for the production of the above heating element comprising pressing glass or a glass ceramic in a screen printing process on a base body, and drying and sintering the base body.

Description

The present invention relates to a heating element for igniting pyrotechnic charges comprising a base body, a patterned resistive layer disposed on the base body, and contact pads overlapped on both ends of the resistance layer. It further relates to a method for producing a heating element, in which optionally first glass or glass ceramic is printed by screen printing on a base body and then dried and sintered, these steps are repeated until the desired overall layer thickness is reached; after which the resistive paste is screen printed on the glass-ceramic substrate or coating and then dried and sintered; and after that the conductor paste for forming the contact pads is screen printed over the resistive layer and then dried and sintered.

The company Dynamit Nobel AG has been producing heating elements in thin-layer, sputtered-on heating technology for detonators in military use and in mining for many years. DE 2020016 A1 ). This type of heating element can be used in the automotive sector only with additional effort (external wiring).

LifeSparc Inc. and Auburn University also presented a laminar heating element (thin film, sputtered) on a semiconductor substrate ( US 4708060 A , which forms the basis for the preamble of claim 1, and US 4976200 A ). Here, too, an additional effort (external wiring with diodes in the semiconductor substrate) must protect the heating element against external influences if it is to be used in the automotive sector.

In the generic patent of the company. Schaffler & Co. ( AT 405591 B ) introduces a thick-film heating element. This can be used with appropriate application for igniting pyrotechnic sets, but also meets without additional effort (external wiring) also not the required specifications of the automotive industry in With regard to ESD (electrostatic discharge) and transient pulse while maintaining the required electrical resistance (eg 2 Ω) as well as the ignition delay time (eg maximum 2 ms).

The specifications to be met are e.g. the USCAR (Chrysler, General Motors and Ford) and the VW80150 (Volkswagen). In addition to the demands of environmental simulation (climate change tests and mechanical stress), the electrical requirements (sensitivity during ignition and resistance to interference pulses) are of utmost importance for the heating element. These tests are to be carried out on igniters (heating element with pyrotechnic kit installed according to the requirements of the automotive industry).

Ignition sensitivity is determined by so-called "all-fire" and "no-fire" tests (e.g., Bruceton, Logit, Run-Down). In the "All-Fire" test, the igniter must fire with a constant current pulse of 1.2 A for 2 ms (with a certain statistical probability). In the "no-fire" test, the ignitor must not ignite (with a certain statistical probability) with a constant current pulse of 0.5 A for 10 s.

If the igniter is subjected to the prescribed interference pulses, ignition must not occur. Interference pulses are predetermined amounts of energy which are introduced within a defined time and with a specific repetition frequency.

Example ESC interference pulse according to USCAR: Charge capacitor with 150 pF to 25 kV via a series resistor with 500 Ω via the igniter with installed heating element (2 Ω).

Example: Transient pulse according to USCAR: Current pulses with 5.3 A, a pulse duration of 4 μs (rise time 1 μs, decay time 3 μs) and a duty ratio 1: 1000 applied to the igniter with installed heating element (2 Ω) for 24 h.

The problem with detonators with all these known heating elements is that they only comply with these specifications fulfill additional electronics. So far there is no heating element in layer technology (thick film, thin film, semiconductor), which meets the requirements of the automotive industry according to specification without additional effort (external wiring).

It is an object of the present invention to provide a heating element in layer technology, so that a detonator equipped with it can be used without additional electronics in the automotive sector.

This object is achieved by a heating element of the type mentioned in the present invention, characterized in that the mass of the heating element of 1.0 · 10 ^-9 kg to 4.0 · 10 ^-9 kg, the resistivity of 1 · 10 ^-6 Ωm to 2 · 10 ^-6 Ωm and the specific heat capacity of the heating element is from 100 W / (kg · K) to 400 W / (kg · K).

The main difference to the heating element according to the AT 405591 B is that the mass is much larger (more than 10 times) and the resistivity is also much higher (more than 20 times). In this way, a similar total resistance (which is predetermined by the automotive industry) results, but due to the higher mass, the temperature of the heating element increases less if energy is released in the heating element by interference, so that the pyrotechnic charge can not ignite or the heating element can not be destroyed.

The cross section of the heating element is preferably 3.5 × 10 ^-10 m ² to 7.0 × 10 ^-10 m ² . This cross section is favorable in order to achieve usual resistance values, eg 2 Ω.

It is useful if the resistance layer of sintered Ag / Pd resistor paste or sintered Ag / Au / Pd resistor paste with 30-50 mass% Ag and 35-50 mass% Pd or sintered Pt / W resistor paste with 70- 90 mass% Pt and 5-20 mass% W exists. These materials are particularly suitable for providing suitable resistance values in the composition according to the invention. The rest contains oxidic additives and glass phase. The resistor paste normally also contains an organic agent before sintering.

It is also important for reliable ignition that too much heat can not be dissipated. It is therefore advantageous if the base body consists of a high temperature resistant glass or a glass ceramic or a ceramic with thermal thermal conductivity of at most 2 W / (m · K); or if the base body consists of a high-temperature-resistant glass or a glass ceramic or a ceramic with thermal heat conductivity of at most 3 W / (m · K) and on the body a heat barrier is applied, consisting of glass or glass ceramic coating with 20-80μm thickness and with a thermal thermal conductivity of at most 1.5 W / (m · K).

The preferred material for the contact pads is sintered AgPd or AgPt thick-film conductor paste with a Pd or Pt content of between 1 and 10% by mass. The rest contains oxidic additives and glass phase. The conductor paste normally also contains an organic agent before sintering.

You can produce the heating element according to the invention analogously as in the AT 405591 B described. However, it is preferred that the resistance layer is patterned only after the application of the contact fields by means of a programmable laser source. By appropriate shaping of the resistive layer by means of a programmable laser source, the heating rate (energy transfer) can be set individually by individual shaping. This structuring can relate both to the basic shape of the Glühbrücke by cutting out the corresponding geometry and to the height by areal removal. Compared to etching, shaping with a laser source is much more flexible. A change of the production is possible in a very short time only by a program change, whereas the etching a new etching mask must be created.

Preferably, by post-sintering at 800 ° C-900 ° C peak temperature over 10-20 minutes after sintering of the resistive layer or after sintering of the contact pads or after structuring the ignition element is improved in its stability to high electrical and thermal loads. Surprisingly, the speed of the ignition increases as a result of resintering. This makes it possible to use a larger volume (thereby decreasing the firing speed per se), so that the sensitivity to electrical interference can be reduced.

The present invention provides a heating element and a method with corresponding material combinations, which have not yet been realized in layer technology and meets the specifications of the automotive industry without additional electronics.

Calculations and simulation:

The proof of the resistance to ESD interference pulses and transient pulse according to USCAR can be carried out by means of thermodynamic calculation and subsequent numerical simulation.

Due to the analogy of the thermodynamic heat equation with the differential equations of an electrical conductor (telegraph equation), an exact one-dimensional simulation of the thermal conditions (temperature and heat quantities) can be performed over time after transformation of the thermodynamic quantities into electrical quantities.

Accompanying experiments and measurements with corresponding feedback of the test results in the computer simulation resulted in the measurement accuracy and the idealized boundary parameters (one-dimensional) agreement.

Q

eingebrachte Energiemenge in J (Störpuls)

m

Masse Heizelement in kg

c_p .

spezifische Wärmekapazität Heizelement in W/(kg·K)

ΔT

Temperaturänderung durch eingebrachte Energiemenge in °C

Comparison of a heating element according to AT 405591 B ("hitherto") with a heating element according to the invention ("new") using the example of the ESD interference pulse strength according to USCAR:
Thermal estimation of the heating element without heat dissipation via $$\mathrm{Q}\mathrm{=}\mathrm{m}\mathrm{\cdot }{\mathrm{c}}_{\mathrm{p}}\mathrm{\cdot }\mathrm{.DELTA.T}\mathrm{respectively}\mathrm{,}\mathrm{.DELTA.T}\mathrm{=}\mathrm{Q}\mathrm{/}\left(\mathrm{m}\mathrm{\cdot }{\mathrm{c}}_{\mathrm{p}}\right)$$

Q

introduced amount of energy in J (interference pulse)

m

Mass heating element in kg

c _p .

specific heat capacity heating element in W / (kg · K)

.DELTA.T

Temperature change due to introduced amount of energy in ° C

• "bisher": wirksames Volumen 5,74 10^-15 m³, mit spezifischem elektrischen Widerstand von 4,3 10^-8 Ω·m.
• "neu": wirksames Volumen 1,92 10^-13 m³, mit spezifischem elektrischen Widerstand von 1,4 10^-6 Ω·m.

Material Q [J] Masse [kg] c_p
[W/(kg·K)] ΔT [°C] "bisher": Au/Pd-Resinat 7,48 10^-5 1,09 10^-10 129 5319 "neu": Ag/Pd-Widerstand 1,40 10^-4 1,92 10^-9 337 217 The geometry, and therefore the mass, of the heaters has been chosen to meet conditions such as resistance, all-fire, and no-fire according to the automotive industry specification. From this, the amount of energy to be considered for the calculation is calculated, which assumes the following values on the basis of the materials used and with regard to the fulfillment of the necessary specifications:

• "hitherto": effective volume 5.74 10 ^-15 m ³ , with a specific electrical resistance of 4.3 10 ^-8 Ω · m.
• "new": effective volume 1.92 10 ^-13 m ³ , with specific electrical resistance of 1.4 10 ^-6 Ω · m.

material Q [J] Mass [kg] c _p
[W / (kg · K)] ΔT [° C] "so far": Au / Pd resinate 7.48 10 ^-5 1.09 10 ^-10 129 5319 "new": Ag / Pd resistance 1.40 10 ^-4 1.92 10 ^-9 337 217

The above-mentioned temperature change when the heating element is subjected to ESD interference pulses shows that due to the melting temperature of Au (1063 ° C.), the heating element is "destroyed" so far. This was confirmed not only theoretically, but also by a series of experiments.

In the single figure, the temperature of the heating element T (in ° C) is shown as a function of the time t (in s). The solid line refers to the heating element "hitherto", the dotted line to the heating element "new".

Taking into account the heat conduction of the individual materials, simulation yields approximately identical values, since this is an almost adiabatic process.

When applying the heating element with transient pulse according to USCAR shows a similar behavior both theoretically and in practical experiments, which also leads to the destruction of the heating element "so far".

Claims (8)

1. Heating element for igniting pyrotechnic charges, consisting of a base body, a structured resistance layer arranged on the base body, and contact fields arranged in an overlapping fashion on both ends of the resistance layer, the resistivity of the heating element ranging from 1·10^-6 Ω·m to 2·10^-6 Ω·m, characterized in that the mass of the heating element ranges from 1.0·10^-9 kg to 4.0·10^-9 kg and the specific heat capacity of the heating element ranges from 100 W/(kg·K) to 400 W/(kg·K).
2. Heating element according to claim 1, characterized in that the area of cross-section of the heating element ranges from 3.5·10^-10 m² to 7.0·10^-10 m².
3. Heating element according to claim 1 or claim 2, characterized in that the resistance layer consists of sintered silver/palladium resistance paste or sintered silver/gold/palladium resistance paste with a 30-50 mass% silver and 35-50 mass% palladium, or of sintered platinum/tungsten resistance paste with 70-90 mass% platinum and 5-20 mass% tungsten.
4. Heating element according to any one of claims 1 to 3, characterized in that the base body consists of a high temperature glass or a glass-ceramic or a ceramic with a thermal conductivity of not more than 2 W/(m·K).
5. Heating element according to any one of claims 1 to 4, characterized in that the base body consists of a high temperature glass or a glass-ceramic or a ceramic with a thermal conductivity of not more than 3 W/(m·K) and the base body is coated with a thermal barrier consisting of a glass or glass-ceramic coating with a thickness of 20-80 µm and with a thermal conductivity of not more than 1.5 W/(m·K).
6. Heating element according to any one of claims 1 to 5, characterized in that the contact fields consist of sintered silver-palladium or silver-platinum thick-layer conductor paste with a palladium or platinum content of between 1 and 10 mass%.
7. Method for the manufacture of a heating element according to any one of claims 1 to 6, wherein (if applicable) glass or glass-ceramic is initially printed on a base body by the screen printing process and then dried and sintered, these steps being repeated until the desired total film thickness is obtained; wherein the resistance paste is then printed on to the glass-ceramic substrate or coating by the screen printing process and then dried and sintered; and wherein the conductor paste to form the contact fields is then printed in an overlapping fashion over the resistance layer by the screen printing process and then dried and sintered; characterized in that the resistance layer is then structured by means of a programmable laser source.
8. Method according to claim 7, characterized in that the stability of the igniter element under high electrical and thermal loading is improved by post-sintering for 10 to 20 min with a peak temperature of 800°C-900°C after sintering the resistance layer or after sintering the contact fields or after structuring.

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