Metal glaze resistor definition
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The metal glaze resistor is a type of fixed resistor, which uses glass powder and metal particles (silver and palladium) mixture to restrict the flow of electric current to certain level.
What is metal glaze?
Metal glaze refers to the metals, which looks shiny because of the glass powder and the metal particles mixture. The resistors that are made from the metal glaze are called metal glaze resistors.
Metal glaze resistor construction
The film of the metal glaze resistor is constructed by using the glass powder and metal particles mixture. This mixture is then deposited on a ceramic substrate.
The combination of film and substrate is heated at 800°C. Because of this extreme heat, the glass powder and the metal particles are joined with the substrate. As a result, a thick layer is formed on the substrate. This layer is nothing but the thick film of metal glaze resistor.
The film made of glass powder and metal particles mixture acts as the resistive element to the electric current. Hence, the metal glaze film restricts the electric current to certain level. The resistance of the metal glaze resistors ranging from 100kΩ to 100MΩ.
In the metal glaze resistor, the desired resistance is achieved by cutting the metal glaze film in a helical manner along its length. Once the desired resistance is achieved, the cutting of metal glaze film is stopped.
The film of the metal glaze resistors is very thick. Hence, they operate at high surge current. A sudden rise of the electric current is called high surge current. The film of the metal glaze resistors is much thicker than the metal film and carbon film resistors.
The resistance of the metal glaze resistor is depends on the amount of metal and glass added
Metal glaze film with higher amount of metal particles and lower amount of glass provides low resistance to the electric current. Hence, a large amount of electric current flows through the metal glaze resistor.
Metal glaze film with higher amount of glass and lower amount of metal particles provides high resistance to the electric current. Hence, only a small amount of electric current flows through the metal glaze resistor.
Metal glaze resistors have low temperature co-efficient of resistance
The rate at which the resistance of the material changes with increase in temperature is called temperature co-efficient of resistance.
Metal glaze resistors have low temperature co-efficient of resistance. Hence, the resistance of the metal glaze resistors does not change easily with increase in temperature.
Advantages of metal glaze resistors
Low TCR (Temperature Co-efficient of Resistance).
High stability.
In case of this study, we focussed on thick-film pastes with graphite grains as a functional material and a glass as a bonding medium. The authors developed and fabricated novel material in a form or graphite'glass composite for application as screen-printed resistive layers. The material takes the advantages of both types of thick-film resistor pastes ' cermet and polymer. Low cost of the graphite, extended temperature of operation limited mainly by oxidation of graphite and possible application in standard thick-film circuits make the materials competitive to typical thick-film resistor material for standard applications.
Generally, typical thick-film pastes are based on particles of a conductive material, an organic vehicle and a glass as a bonding medium ( White, ). The organic vehicle is very important for the process of screen printing, because it gives proper viscosity. Weight content of the glass in pre-fabricated pastes should be checked experimentally. Percolation edge calculation could be not enough to fabricate such pastes. Besides, the organic vehicle ensures initial adhesion of grains to the substrate. Glass used in the paste mixture has significant influence on technology, cause after curing and reflow process of the glass, that material makes adhesion of conductive grains to the substrate. Adhesion also depends on the ratio of the functional material to the glass in the composite.
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Graphite and carbon-black are widely used as a conductive phase in a polymer thick-film resistive layers made with various matrix materials as PVC and PMMA ( Murthy et al., ; Zheng and Wong, ). The works cover the experiments on laser trimming of resistance, examination on resistance change in the function of humidity and temperature (temperature coefficient of resistance [TCR]) and impact of high voltage pulse on the resistance change. Also, new nanomaterials as graphene and carbon nanotubes were popular in printed electronics from their appearance, but generally in high conductivity layers, e.g. for using as detecting electrodes Hallam et al. () , Gómez-Mingot et al. () , Aragay et al. () . Polymer films do not require special ovens, because its curing temperature is below 300°C. Curing temperature has a big influence on their electrical parameters. The reduction of the process temperature down to 150°C can reduce costs but can also deteriorate the parameters. Besides, thanks to the lower temperature, the polymer layers based on carbon-black and graphite can be applied on flexible or FR4 substrates.
Graphite is a crystalline allotropic form of carbon with a hexagonal structure. This material is very common in the world, from the simplest applications such as pencils to more advanced ones such as control rods in nuclear reactors. Electrical parameters of that material allow to use it in a production of electronics, e.g. in a form of conductive inks or resistive layers in a form of a polymer thick-films. For the last purpose, typical materials used for resistive layers are IrO and RuO 2 in a glass matrix ( Pike and Seager, ). These materials present very good electrical stability and repeatability. However, comparing to polymer layers, cermet materials are much more expensive.
Test setup shown in Figure 4 was used during measurement of sheet resistance, TCR and impact of humidity for the series of test samples. The setup consisted of a programmable measurement unit with 20-channel multiplexer (34,972 A with 34,901 A card, Keysight). During TCR determination, the samples were placed on a hotplate with programmable temperature ramp. The data were logged on a PC.
The samples consisted of resistors with nine different lengths, as listed in Table 1 . Influence of length on the sheet resistance was important for compatibility testing of the contact material and resistive layer because it was possible to observe a contribution of contact resistance to total resistance of the component. The samples were arranged on the substrate as shown in Figure 3 . Such placement made possible to use edge connectors to multiplexed measurement unit and simultaneous testing of a whole series of the components.
Preliminary experiments showed that value b should be greater than 0.25 mm because the contact resistance is determined by not only contact area but also density of conductive grains, the most significant for of paste P1. Also, a porosity of printed layers determined a minimum number of layers. In the experiment, minimum two layers were printed. For examination of a sheet resistance of each paste, 2, 3 or 4 layers were screen-printed. Various firing temperatures were used, dependently on the atmosphere in the oven. The technology course of the samples is presented in Figure 2 .
During the experiment, three various pastes were fabricated with 3:1 mass ratio glass/graphite (P1), 1:1 mass ratio glass/graphite (P2) and 1:3 glass/graphite (P3) mass ratio. Synthetic graphite (Sigma Aldrich) with mean grains below 20 µm was used. Pastes were fabricated from glass (SG-683K, Heraeus) and acrylic vehicle. In the experiment, we used ceramic substrates Al 2 O 3 96% with screen-printed PdAg electrodes ( R, Du Pont). Electrodes were fired at a peak temperature of 875°C. Design of the resistors geometry is presented in Figure 1 , and their geometrical parameters are presented in Table 1 . For the graphite pastes, the deposition process was performed using 200-mesh stainless steel screen with 30-µm emulsion.
The resistance of test samples was determined at room temperature. Sheet resistance RSQ was determined from equation (1), where R is the resistance of the resistor, and n is the aspect ratio of the resistor (n = l/w):
(1)
RSQ= Rn
Mean values and standard deviation from the series of resistors were calculated. The results were presented in Table 2. In general, as expected, the sheet resistance was decreasing with increasing number of layers. The regularity was not visible for four layers of paste P2 fired in nitrogen. The effect could be caused by the glass flown from the paste on terminations' surface. In this case, lower resistance of the layer was dominated by higher resistance of the interface between resistor and termination layers. The flow of the glass in a sample fired in nitrogen atmosphere is visible in Figure 5(b), where area of the interface is shown on the image from the optical microscope (DM, Leica). Such effects were not observed for samples fired in the air [Figure 5(a)] at much lower temperature, as well for samples made of paste P3, where a low amount of the glass has not influenced resistance of the resistor-termination resistance.
Selected samples were covered with overglaze. The samples fired in the air at 550°C were covered by QQ550 (Du Pont) overglaze and fired at the same temperature as the resistor layer. The samples fired in nitrogen atmosphere, were covered using nitrogen fireable overglaze IP (Heraeus) and fired at 620°C. The results of sheet resistance measurement were shown in Table 3. Comparing the results with values for the layers without protection overglaze, the values of sheet resistance increased for samples fired in nitrogen and decreased for samples fired in the air atmosphere. The overglaze improved contact resistance between graphite grains for samples fired in the air. In contrast, the overglaze during penetration into the resistor layer deteriorated the contact between conductive grains in samples fired in nitrogen. The differences between PdAg contacts fired in air (550°C) and nitrogen (875°C) are presented in Figure 5.
Assuming the effects of the glass flow, visible in Figure 5(b), the contact resistance between resistor layer and terminations were estimated from the linear fitting or resistance as a function of resistors' aspect ratio. For direct comparison, only the four-layer resistors were considered. The results are shown in Figure 6(a) and (b) for layers fired in the air and in nitrogen atmosphere, respectively. For samples fired in the air, the resistance between two terminations and the resistor were about 760 Ω for paste P1 and about 72 Ω for paste P2. The values were high, but in relative relation to the sheet resistances, they can be acceptable. In this case, the number of conductive phase was important. For samples fired in nitrogen, the resistance of contact had higher contribution to total resistance than resistance of the resistive layer ' the values of about 4 and 8 Ω were estimated. Therefore, high deviation was noticed during sheet resistance calculation for these samples.
Temperature coefficient of resistance (TCR) is one of the most important parameters of resistors from application point of view. The TCR of examined resistors was calculated using the equation (2):
(2)
TCR= RT125-RT25RT25*ΔT* 10-6[ppm/°C]
RT125
is the resistance in 125°C,
is the resistance in 125°C,
RT25
is the resistance in 25°C and
is the resistance in 25°C and
ΔT
= 100°C. Change of the pastes TCR after various technology stages were studied. Results are presented in
= 100°C. Change of the pastes TCR after various technology stages were studied. Results are presented in Table 4 . Because of too high standard deviation of TCR for paste P3, the measurements were rejected. The results showed that the TCR is dependent on paste composition, firing temperature and atmosphere, as well as presence of top coating glass. For Paste P1 fired in the air, the value increased after firing overglaze layer, owing to changes of microstructure, also observed as a resistance change. For paste P1 fired in nitrogen, the values were not changed after overglazing. Considering the paste P2 fired in the air, the values of TCR were almost the same ' no changes of microstructure occurred, despite the resistance reduction was visible for that material and technology configuration. For the paste P2 fired in nitrogen, the values of TCR were similar for samples with two- and three-layer configuration. For the samples with four-layer resistors, the value of TCR was almost twice lower. That means the graphite grains were properly connected, and the TCR value was bringing closer to TCR of a bulk graphite, with the TCR of '280 ppm/°C ( Gajewska and Pietras, ).
Humidity can influence resistor layers, especially when the layers are porous and water vapour can penetrate the material, changing the resistance. Fabricated resistors were exposed to the environment with humidity in the range from 25% to 80% RH at 40°C in a climatic chamber (Binder MKF 115). The humidity in the chamber was increased up to specified limit at time of 2'h, and the values were measured using a hygrometer. Resistance change was measured in-situ for the series of resistors on a substrate. Mean resistance change and deviation were determined for specified material and process configuration. Results are shown in Figure 7 for different samples with and without the overglaze. In case of the layers fired in air, the impact of humidity was the highest, as visible in Figure 7(a). Nevertheless, after coating by the overglaze and firing, the impact of the humidity was reduced. It means that layers fired at 550°C were porous, and an additional coating sealed the pores between the graphite grains. In a case of the resistive layers fired in nitrogen, the results depended on paste composition and the number of printed layers. For the paste P1, an additional coating reduced the impact of humidity as shown in Figure 7(b). For the paste P2, no change was observed after top coating application, as visible in Figure 7(c). Therefore, the resistors made of P2 fired in nitrogen are not required for coating with protective overglaze.
Short-term overload (STOL) was determined by measurement of resistance change under consecutive load with a 5-s long electrical pulse with specified voltage. Automated setup consisting of programmable power supply and ohmmeter was used. The setup was equipped with the external switch for disconnecting the power supply from ohmmeter during the load and disconnecting the power supply from circuit for higher resistance measurement accuracy. The measurement was controlled by software working in LabView environment. STOL can be defined as an electric field intensity along the resistor, which result in resistance change of'±'1% from initial value. The parameter was determined for selected resistors with the same length for easier comparison between different material and firing atmosphere configuration. Resistance change during experiment for samples made of paste P1 and P2 are shown in Figure 8(a) and 8(b) for samples fired in air and nitrogen, respectively. In general, the overglaze on top makes the curves on the graph is changing more monotonic. Decreasing resistance in samples made of the P1 paste fired in air [Figure 8(a)] suggested a creation of new conductive paths in the microstructure, which means that the graphite grains were not properly sintered with the others. After applying and firing the overglaze, the effect was not so visible and in general, higher critical values were observed. In case of the layers fired in nitrogen, no difference was observed between coated and uncoated resistors, as visible in Figure 8(b).
The results of STOL determination for all examined configurations are shown in Table 5. It was noticed that the parameter is dependent on the resistance of resistors ' for higher resistance, the higher values of STOL parameter were determined. For direct comparison of resistors performance, a critical power density in a resistive layer at voltage corresponding to STOL values was determined. The critical power density was calculated from equation (3), where l is the length of the resistor, w is the width of the resistor and R is the resistance:
(3)
PDcrit =STOL2*lR*w
The values of critical power density are shown in Table 6. As visible, the overglaze improves the strength of resistors fired in air, especially for paste P1. The overglaze in that case stabilised and hermitised porous structure of a resistive layer. However, no significant impact was observed for pastes fired in nitrogen because the layers were properly sintered before overglaze, and its structure was not changed during the second firing with a top coating.
The microstructure of pastes P1, P2 and P3 after firing in 550°C (air) was studied using scanning electron microscopy (Auriga 60 Zeiss). In case of the paste P1 with the lowest contents of graphite, electric conduction was occurring probably mainly through agglomerates of the graphite surrounded by the glass material, as visible in Figure 9(b). The microstructure with non-uniformly distributed agglomerates can explain a relative high scatter of sheet resistance. The microstructure investigation revealed the best dispersion of graphite and glass phase for the paste P2 [Figure 9(b)], and thanks to that, the lowest scatter of sheet resistance and low impact of humidity were noticed. The paste P3 exhibited the highest amount of graphite, and hence the lowest sheet resistance was expected. However, as visible in Figure 9(c), the layer was porous hence the highest resistance was determined. Too low amount of the glass led to impossibility of its flow between grains, their wetting and impossible to proper densify the layer.
Ceramic glazes vary widely in their resistance to wear and leaching by acids and bases. The principle factors that determine durability are the glaze chemistry and firing temperature.
Ceramic glazes vary widely in their resistance to wear (cutlery marking, scratching) and leaching by acids and bases. The principle factors that determine durability are the glaze chemistry and firing temperature. In industry technicians are accustomed to evaluating glazes by looking at their oxide chemistry and rationalizing the relationship between it and fired durability. Glazes having plenty of Al2O3 and SiO2, for example, are more durable (when they are melted properly). By contrast, potters tend to focus on the recipe. This is a problem and it is the reason that potters can be found using very non-functional glazes (often flux saturated and therefore lacking in Al2O3 and SiO2) or firing them in a non-functional way (e.g. under firing).
Misconceptions are common in this area. Many feel that just because a glaze looks melted it is durable. Another common belief is that firing temperature is an indicator of durability, the higher it is fired the more durable it will be. These beliefs, when coupled with the active traffic in glaze recipes online, greatly increase the chance that non-durable ware will be made. Sadly, a large proportion of online glazes are not durable and possibly not safe, even though they are published in fancy formats from apparently reputable sources. It is thus important to have a critical eye when looking at new recipes.
Another factor in why many people are using glazes of poor durability is the lack of appreciation of the basic science. In excess of 99% of glaze recipes available are for special purpose colors and surfaces. But very few quality transparent recipes are available. Ones that are expansion adjustable, fire clear and transparent, work with stains, have the right melt fluidity, melt well yet do not blister or pinhole, are easy to use, etc. And people do not test these with their bodies and adapt them (e.g. thermal stress testing for shivering, crazing). But functional surfaces must be based on these. Further, most other glazes are simply transparents with added colors, opacifiers and variegators.
With some experience it is possible to quickly judge durability issues when looking at a new glaze recipe. For example, at low temperatures (cone 06-04) boron is essential, so we expect to see a significant frit presence (or a natural boron source like Gerstley Borate), up to 80% is not uncommon. At high temperature we expect to see no boron materials (feldspars, calcium carbonate, dolomite, wollastonite, strontium carbonate are active melters there). At middle temperature these do not melt well so they need help. That help is almost always boron, so you will see 10-40% frits or Gerstley Borate. Some middle temperature glazes use zinc and/or lithium in addition to or instead or boron (these are power melters). But it is important that melters not be in excess, this will make the glaze leachable (since the SiO2 and Al2O3 percentages are pushed down).
At all temperatures, you should see clay in the recipe. 15-30% is typical. At all temperatures you should see silica, from 5-40%. If there is no clay or silica that is a red light if it is supposed to be a durable functional glaze.
Testing durability is common sense. Expose it to an acid and to abrasion and scratching by hard materials.
Please see the Limit Formula glossary topic for more information on how to look at a formula and judge its balance.
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The glaze on the left is called Tenmoku Cone 6 (a popular, and old, CM recipe). It is 20% calcium carbonate, 35% Custer feldspar, 15% OM4 Ball Clay and 30% silica, 10% iron oxide. If you have any experience with glaze you will note two things that a fishy here: There is no boron, lithia or zinc sourcing material. How can this melt enough at cone 6? It looks melted, but the ease of scratching it shows it is not. So, it appears that if we saturate an incompletely melted glaze with a lot of refractory brown colorant on a dark body the effect can be black. A better idea is the glaze on the right. We start with a stable, reliable base transparent, GB. Then we add 5% Mason black stain (stains are smelted at high temperatures, quenched and ground, they are inert and relatively safe). A bonus is we end up with a slurry that is not nearly as messy to use and does not turn into a bucket of jelly.
Look at how fluid G is at cone 06 even though it has the Al2O3 and SiO2 of a cone 6 (or even cone 10 glaze)! It have found that glazes with lots of boron can tolerate amazingly high levels of Al2O3 and SiO2 and still melt very well. And they create many options to lower thermal expansion that would not otherwise be available. The GN recipe has the amazing ability to tolerate large additions of kaolin. Each addition sacrifices some melt fluidity but the glaze stays glossy and gets more durable (because of the increased Al2O3 and SiO2). And the thermal expansion drops even more. A highly melt fluid, super gloss with low thermal expansion is super difficult at cone 6, but here it is. The secret is high boron. From frits.
The outside glaze is a copper blue, but that is not the one we are interested in here. It is the clear glaze on the insides of these two identical cone 6 porcelain mugs. Why add such a small amount of zircon to it? It is not being added to opacify, it is being added to toughen the surface and reduce the thermal expansion.The presence of the 2% zircon has not affected the gloss or transparency of the glaze on the right. However the 3% on the left has opacified it just slightly and made the surface a little silky. So that is too much for this glaze (although it might be OK if the melt fluidity was higher). So, if you are interested in the most functional possible surface, consider a 2% zircon addition to your transparent.
This flow test compares the base and base-plus-iron version of a popular CM recipe called "Tenmoku Cone 6" (20% whiting, 35% Custer feldspar, 15% Ball Clay and 30% silica, 10% iron oxide). Although iron is not a flux in oxidation, it appears to be doing exactly that here (that flow is just bubbling its way down the runway, the white one also fires to a glassy surface on ware). It looks melted in the tray on the right but notice how easily it is scratching on the tile (lower left). This demonstrates that looks can be deceiving. Cone 6 functional glazes always have some percentage of a power flux (like boron, lithia, zinc), otherwise they just do not melt into a hard glass. Maybe a glaze looks melted, but it has poor durability.
This paper describes conduction processes in thick film resistors made from 1 MΩ/square and 10 MΩ/square pastes. Conductivity as a function of temperature indicates activation energies in the order of 1 meV. At high fields, conductivity is increased. These results are interpreted on the basis of a model where the main resistor characteristics are determined by hopping conduction in a narrow impurity band in the glass phase of the resistor.
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