A novel process for the fabrication of thermocouples using thick-film techniques has been developed. Thermoelectric reactions of 22 μV/°C per couple have been observed which are comparable to those of conventional thermocouples.
This work outlines the potential for a rapid, lowcost, low temperature manufacturing solution for the production of electrical temperature sensors

Duby, S

Harrison, DJ

Hay, GI

Ramsey, BJ

English

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Printed thermocouple devices

Brunel University Research Archive

Printed Thermocouple DevicesSam Duby, Blue Ramsey, David Harrison, Gareth HayBrunel University, Cooper’s Hill Lane, Englefield Green, Surrey, TW20 0JZ, UKSam.duby@brunel.ac.uk, www.brunel.ac.uk/research/cleanerAbstractA novel process for the fabrication of thermocouples usingthick-film techniques has been developed. Thermoelectricreactions of 22 µV/˚C per couple have been observedwhich are comparable to those of conventional thermocou-ples. This work outlines the potential for a rapid, low-cost, low temperature manufacturing solution for the pro-duction of electrical temperature sensors.KeywordsThermocouple, temperature sensors, printed electronicsINTRODUCTIONConsumer awareness and government legislation have putincreasing pressure on manufacturers to invest in and adoptmore environmentally-sensitive manufacturing processes.Directives such as WEEE (Waste Electrical and ElectronicEquipment) and RoHS (Restrictions of the use of certainHazardous  Substances) have extended producer responsibil-ity and have encouraged manufacturers to regard the impactof their processes on the environment as a primary concern.Research at Brunel University has been done into the appli-cation of additive printing processes to the manufacture ofelectrical components from simple capacitors and resistors[1,2], to more complex integrated filter structures, micro-wave circuit components and humidity sensors [3]. Thiswork has met with considerable academic and commercialinterest. The processes are high-speed, high-volume andoffer significant financial and environmental savings overtraditional manufacturing processes, both at the productionand end-of-life stages.There are many advantages that directly printed circuitryhas over the more conventional printed circuit board. Thefact that printing is a high-speed, high-volume processmeans that there are substantial reductions in productiontimes that lead to large cost reductions. Printing is an addi-tive process that results in substantial environmental bene-fits over standard techniques. Traditional circuit boardmanufacture incurs raw materials loses of over 80% throughthe waste stream, as an entirely copper coated board isetched away leaving only a small amount of conductivematerial to form the tracks. Many toxic materials are usedin the traditional process, from the photoresists to the etch-ing chemicals, cleaners and fixatives.Thermocouples have been printed using thick-film proc-esses, which demonstrate a repeatable and stable reaction totemperature and a potential difference per degree tempera-ture change comparable to conventional commercial cou-ples. Results also compare favourably to the thermoelectricperformance of other work on novel thermocouple manufac-ture methods, including that reported by Sihai Wen andD.D.L. Chung [4], and Kiyoshi Nogi et al. [5].This work is continuing to broaden the number of compo-nents which can be manufactured by printing processes,reducing material use and manufacturing complexity. Italso forms the first step in a more speculative research ob-jective, the development of a low-cost printed thermoelec-tric generator.Figure 1. Printed thermocouples 40mm longMETHODIn order to print a thermocouple, two thermoelectricallyactive materials with opposite bias need to be processedinto inks. In this case, the inks developed consist of metal-lic particulates suspended in a variety of polyester, cellu-lose and conductive polyaniline (Panipol M) resins. Theprincipal functions of the resin vehicle or matrix are to pro-vide suitable flow-characteristics for the ink, to ensure thatthe final printed track has structural integrity, and to ensurethe active particulate is adequately adhered to the polyestersubstrate. There are a variety of factors that become criticalat each stage of the process, and it is the optimisation andrefinement of each of these that constitutes the majority ofpractical work carried out.In order to maximise the thermoelectric reaction of aprinted sample, the proportion of thermoactive material to0-7803-8692-2/04/$20.00 ©2004 IEEE. 1098resin must be as large as possible. A small amount of ma-trix is necessary, as the particulate must remain adequatelyadhered to the substrate, however minimising the quantityof non-thermoactive material in the final dried printed sam-ple is desirable. In order for an ink to be printable, it needsto have appropriate rheological properties. These character-istics will only be derived from the resin vehicle, and notfrom the metal particulate itself. As a result, a balance mustbe sought between formulating an ink with enough resinvehicle to enable printing, and as little resinous material aspossible in the final dried printed sample.Figure 2. The principle of aprinted thermocoupleThis balance is addressed through the careful selection ofresins with an appropriate molecular weight. Very highmolecular weight polymers will lend more viscosity to astoichiometrically identical solution of lower molecularweight polymers. Only a small quantity of a high molecu-lar weight polymer will need to be added to a suitable sol-vent to give the resulting solution a relatively high viscos-ity and flow characteristics that enable printing. On drying,once all of the solvent has evaporated only a small amountof the polymer resin will remain adhering the particulate tothe substrate, maintaining the highest possible particulateto matrix proportion.The image below shows a 1000x magnified photograph ofa printed NiCr track. It can be seen that the particles havebeen flattened into flake-like form. This is a result of hav-ing been ground in a three-roller mill as part of the produc-tion process. Flattened particles lie against each other witha greater amount of overlapping surface-area resulting inhigher degree of inter-particle contact than nodular or granu-lar particles. This means that the conducting electron doesnot have as much non-conductive matrix to tunnel throughand can relatively easily hop from one particle to the next,facilitating conduction.Figure 3. Printed NiCr track-1000x magnificationEnabling better dispersion of the particulate in the resinresulted in a reduction to the quantity of resin that wasneeded to create a viable ink. This was done using ‘disper-sion’ or ‘wetting’ agents. The most effective wetting agentfor these purposes was found to be heptanoic acid. Thepolar nature of this carboxylic acid means that one end ofthe molecule is repelled by the solvent and attaches to thesolid particulate leaving behind a ‘tail’ that is attracted tothe solvent. This is an example of hydrophobic/hydrophilicaction and means that the solid particulates are effectivelyseparated from one another and dispersed in the solution.Various other additives were found to improve the inks.NaCl for example was outlined as a potentially useful addi-tive in the NiCr based n-type inks because of the fact thatchloride ions in solution are very effective at penetratingand destabilising a passive chromium oxide layer. Thechemistry of this mechanism has been closely scrutinisedand carefully documented because of the corrosive effectthat chloride ions have on the chromium in stainless steels.In this case it is inferred that the NaCl breaks down theprotective oxide layer of the chromium, leaving the metalexposed and open to an attractive reaction with a wettingagent, thereby enhancing the wetting mechanism.A test rig was built to gather thermoelectric data from theprinted thermocouple samples. The apparatus was designedsuch that one end of the thermocouple was held betweentwo air-cooled Peltier modules capable of maintaining aclosely controllable temperature of down to –20˚C, whilethe other was held between two flat-plate resistance heaterspowered by a 240V a.c. variable transformer capable ofmaintaining a stable temperature of between ambient and170˚C. The sample was only otherwise in contact with theair, ensuring that heat flow was predominately through theprinted track with a negligible amount through the polyes-ter substrate. Three mineral-insulated k-type thermocouplesprobes continually monitored the ambient, cold-junctionand hot-junction temperatures through a ThermocoupleData Logger connected to a PC. The data logger was capa-ble of measuring absolute potential difference with a resolu-tion of single microvolts and consequently was also usedto monitor the generated emf of the sample.1099RESULTS AND DISCUSSIONThermocouples have been printed using a variety of specifi-cally formulated N and P type inks. Internal resistanceshave been driven down to the order of tenths of ohms persquare. Experimental data from the test rig gives results forthe separate elements that when added together are in closeaccordance with theoretical data for a complete standardthermocouple (within 5% variance) [8].Figure 4. Discrepancy between published bulkmaterial values and those of printed samplesFigure 4 above shows the discrepancy between publishedbulk material values and those achieved with printed sam-ples. The differences are thought to result from the fact thatin contrast to a solid sample, the thermoactive material inan ink is held in a non-electrically conductive matrix. Thismeans that inter-particle conduction can only be achievedwith electrons tunneling through the resinous matrix fromone conductive particle to the next. Thus hindering thetransfer of electrons and hence conduction.Figure 5 below shows how the electrical reactions of twoprinted thermoelements compare with those of copper andNichrome wire, both of which are thermoelements fromstandard commercially available thermocouples. Data hasbeen gathered over the range of 0-150˚C. The reaction isstable and demonstrates a reasonable hysteresis, typical ofstandard thermocouple configurations. The best printablethermocouple configurations were found to be a combina-tion of a Nichrome based ink and a Nickel/Panipol blend.This is not a standard thermocouple combination. Data hasbeen gathered that demonstrates that different batches ofsamples printed with inks of the same formulation performconsistently with a standard deviation of <0.01.Figure 5. Generated Seebeck voltages of two standard and two printed thermoelements1100CONCLUSIONSA novel process for the manufacture of temperature sensingthermocouples has been developed that utilises thick-filmtechniques. The principal advantage that this process hasover traditional thermocouple manufacturing is that of cost.This technique demonstrates the potential application ofhigh-speed, high-volume manufacturing processes.The physical properties of the materials have defined thetemperature range in which these thermocouple sensors willbe able to operate. Results show that within the operationtemperature range defined by the materials, a reasonablethermoelectric sensitivity is demonstrated. Thermocoupleshave been manufactured with a thermoelectric sensitivity of22 V/˚C. This is comparable to commercially availablethermocouples.The experimental results indicate a potential for this ap-proach to offer a rapid, low-cost, low temperature manufac-turing process for effective electrical temperature monitor-ing devices or sensors, overcoming the need to place andsolder components and the requirement to ‘fire’ and lasertrim ceramic systems.One of the more exciting potentials that this work has al-lowed is the use of new materials as elements in a thermoe-lectric device. The inclusion of conductive polymers inspecific ink formulations had a positive effect on the ther-moelectric performance of the device and as such representsan interesting avenue of research.ACKNOWLEDGMENTSThis work has been funded by the EPSRC under grantnumber GR/S08244/01. The authors also wish to acknowl-edge the valued assistance and support received from Da-vidson Chemographics.REFERENCES[1] P.S.A Evans, B.J. Ramsey, P.M. Harrey and D.J.Harrison ‘Printed analogue filter structures’, Electron-ics Letters, 1999, 35, (4), pp. 306-308[2] P.M. Harrey, P.S.A. Evans, B.J. Ramsey and D.J.Harrison ‘Interdigitated Capacitors by Offset Lithogra-phy’, Journal of Electronics Manufacture, Vol. 10No.1, 2000. pp 69-77.[3] P.M. Harrey, B.J. Ramsey, P.S.A. Evans and D.J.Harrison ‘Capacitative-type humidity sensors fabricatedusing the offset lithographic printing process’, Sensorsand Actuators. B6361 (2002) 1-7.[4] Sihai Wen and D.D.L. Chung ‘Cement-based thermo-couples’, Cement and Concrete Research, 2001, 30,pp. 507-510[5] Kiyoshi Nogi, Takuji Kita and Xiang-Qun Yan ‘Pro-duction of iron-disilicide thermoelectric devices andthermoelectric module by the slip casting method’,Materials Science and Engineering, 2001, A307, pp.129-133[6] V.I. Kayadanov and T.R. Ohno ‘Process Developmentand Basic Studies of Electrochemically DepositedCdTe-Based Solar Cells’, Annual Technical Report–Colorado School of Mines, 1999, pp. 11-12[7] B. Yang, J.L. Liu, K.L. Wang and G. Chen ‘Simulta-neous measurements of Seebeck coefficient and thermalconductivity across superlattice’, Applied Physics Let-ters, 2002, Vol. 80, No. 10, pp 1758-1761[8] ‘Guide to Thermocouple and Resistance Ther-mometry’,TC Limited, Issue 6.01101

Capacitative-type humidity sensors fabricated using the offset lithographic printing process’, Sensors and Actuators.

Cement-based thermocouples’,

Guide to Thermocouple and Resistance Thermometry’,TC Limited,

Interdigitated Capacitors by Offset Lithography’,

Printed analogue filter structures’, Electronics Letters,

Process Development and Basic Studies of Electrochemically Deposited CdTe-Based Solar Cells’, Annual Technical Report –Colorado School of Mines,

Simultaneous measurements of Seebeck coefficient and thermal conductivity across superlattice’,

Takuji Kita and Xiang-Qun Yan ‘Production of iron-disilicide thermoelectric devices and thermoelectric module by the slip casting method’,

Printed thermocouple devices

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