Palladium resistor

There are three major classes of palladium-based hydrogen sensors [4], The most popular class of palladium-based sensors is based on palladium resistors. A thin film of palladium deposited between two metal contacts shows a change in conductivity on exposure to hydrogen due to the phase transition in palladium. The palladium field-effect transistors (FETs) or capacitors constitute the second class, wherein the sensor architecture is in a transistor mode or capacitor configuration. The third class of palladium sensors includes optical sensors consisting of a layer of palladium coated on an optically active material that transforms the hydrogen concentration to an optical signal. 

Electronic Applications. The PGMs have a number of important and diverse appHcations in the electronics industry (30). The most widely used are palladium and mthenium. Palladium or palladium—silver thick-film pastes are used in multilayer ceramic capacitors and conductor inks for hybrid integrated circuits (qv). In multilayer ceramic capacitors, the termination electrodes are silver or a silver-rich Pd—Ag alloy. The internal electrodes use a palladium-rich Pd—Ag alloy. Palladium salts are increasingly used to plate edge connectors and lead frames of semiconductors (qv), as a cost-effective alternative to gold. In 1994, 45% of total mthenium demand was for use in mthenium oxide resistor pastes (see Electrical connectors). 

The largest uses of platinum group metals in electronics are ruthenium for resistors and palladium for multilayer capacitors, both applied by thick film techniques . Most anodes for brine electrolysis are coated with mixed ruthenium and titanium oxide by thermal decomposition . Chemical vapour deposition of ruthenium was patented for use on cutting tools . 

Several types of palladium-based hydrogen sensors have been reported in the literature. The most notable ones are based on Pd thin-film resistors, FETs, Pd nanowires, Pd nanoparticle networks, Pd nanoclusters, and Pd nanotubes as shown in Table 15.2. 

The sensor element constitutes a palladium-nickel alloy resistor with a temperature sensor and a proprietary coating. The sensor has a broad operating temperature range and a sophisticated temperature control loop that includes a heater and a temperature sensor, which controls the die temperature within 0.1°C. 

The cells shown in Figs. 28 and 29 all operate according to the same principles, which have been developed by Arup. The interior of the cell acts as the anode chamber, and a metal oxide cathode placed inside the cell in an alkaline electrolyte acts as the counter electrode. The hydrogen flux across the integrated membrane (coated with palladium on the internal surface) can be measured as the potential drop across a resistor placed between the membrane and the counter electrode. 

The thick-film design consists of four layers, to be separately screen printed and fired on a 1 in square alumina substrate (figure 14.9). Commercial formulations were used for electrodes, bridge trimming resistors, and passivation layers. The first attempted sensor layer was a commercial silver/palladium paste modified by the addition of palladium powder. Based on the performance of the first thick-film sensors, DuPont Electronics (Research Triangle Park, NC) specifically formulated a palladium-based thick-film paste for this application. 

The electrochemical cell itself consisted of essentially a flat cell, fixed inside in the optimal position within the EPR cavity so as to ensure maximum sensitivity. The working electrode employed was either a platinum mesh [102] or later, a partly laminated gold mesh [103, 104], An AglAgCl electrode was used as the reference, whilst two counter electrodes were employed to minimize oscillation of the cell voltage. The first counter electrode, a palladium sheet, is situated below the flat part of the cell, whilst the second, connected by a resistor to the potentiostat, is located above it. 

The first resistor ink system with a wide range of sheet resistivities was developed in 1958 by J. D Andrea. This palladium and silver system (PdO/ Ag) had a high firing temperature influence on the sheet resistivity, caused by the complicated chemical-dynamical process. The wide range of resistances that this composition could achieve was one main reason for the rapid growth of thick-film technology since then. 

Ceramic and ferrite components such as multilayer ceramic capacitors, chip resistors, and chip inductors are generally terminated with a fired-on silver or silver palladium paste. Because silver dissolves easily into molten Sn-Pb solder, a Ni/Sn or Ni/Au overplate is recommended. 

Although silver to palladium ratios of 3 1, 4 1, and 6 1 are commonly used in hybrid apphcations, thrifted versions utilizing Pd contents less than 5% have been shown to have good leach resistance in 62% Sn-36% Pb-2% Ag solders. Table 8.4 compares properties of various Ag Pd ratios. These materials meet the requirements of high-conductivity materials needed for circuits with higher speeds and densities. Similar performance can be met by small additions of platinum to silver (1 100). The leach resistance of a 1 % Pt addition is shown to be equivalent to a 10 percent Pd addition with minimum effect on conductivity. However, Ag migration such as dendritic growth and diffusion into resistors or capacitors when used as a termination material is stUl an issue with only minor Pd or E t additions. 

Silver-palladium compositions Ag/Pd ratio 79/21 TR-4846 (fired at 850°C) 18-25 pf2 cm Compatible with solder General wiring Resistor terminal Lead terminal... 

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