Carbon filaments were "flashed" by heating in a hydrocarbon vapor (usually gasoline), to improve their strength and uniformity. Metallized or "graphitized" filaments were first heated to high temperature to transform them into graphite, which further strengthened and smoothed the filament. These filaments have a positive temperature coefficient, like a metallic conductor, which stabilized the lamps operating properties against minor variations in supply voltage.
Sintering is based on diffusion and takes place below the melting temperature of the materials. Micro- or nanosized particles are needed, as the process is driven by the reduction of the total surface energy. The substrate and the sintered Ag are connected with a diffusion intermetallic phase. Usually, high pressure of up to 40 MPa is applied to the materials to assist the bonding during the process when microsized particles are used. Investment costs and manufacturing throughput can be optimized by using pressureless sintering with nanosized particles [24].
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TLP bonding shows a small but statistically significant deviation from the nominal value during the drift measurement at 10 bars. The mean long-term drift is 0.05% FSO/h, which is 0.04 bars in 8 h. Depending on the application, this can be ignored. The SEM image shows a good bond with few voids, and the EDX analysis indicates the formation of an intermetallic phase. Further work could be carried out to repeat the tests with reduced pressure. Another possibility for optimization is to vary the thickness of the Sn/Ag coating for the TLP bond. This could lead to different alloys and further improve the long-term stability of the pressure sensor. The measurement results from the characteristic curve are the best in this work; further optimization is not necessary.
The SEM images indicate good properties of the joints except for welding, where numerous cracks are visible and further optimizations of the process parameters are necessary. The EDX elemental analysis for TLP bonding is particularly interesting, as formations of intermetallic phases are indicated due to the elemental distribution. 2ff7e9595c
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