LTCC

Thermoelectric energy harvester fabricated in thick-film/LTCC technology Piotr Markowski Faculty of Microsystem Electronics and Photonics, Wrocław University of Technology, Wrocław, Poland Abstract Purpose – The purpose of this work was fabrication of a small energy harvester. Design/methodology/approach – The multilayer thermoelectric power generator based on thick-film and low temperature co-fired ceramic (LTCC) technology was fabricated. Precise paths printing method was used to fabricate Ag/Ni and Ag/PdAg thermocouples on a number of unfired LTCC tapes. The tapes were put together to form a multilayer stack. The via holes were used to make the electrical connections between adjacent layers. Finally, the multilayer stack was fired in the appropriate thermal profile. Findings – It consists of 450 thermocouples and generates output voltage of about 0.45 V and output electrical power of about 0.13 mW when a temperature difference along the structure is 135°C. In the paper, individual stages of energy harvester fabrication process as well as its output parameters are presented. Originality/value – Miniaturized thermoelectric energy harvester based on thick-film and LTCC technology was fabricated. As materials, metal-based pastes were used. This is the first paper where multilayer thermoelectric harvester, fabricated with the aid of LTCC technology, was described. Keywords LTCC, Energy harvester, Generator, Thermocouples, Thermoelectricity, Thick-film Paper type Research paper

materials, connected by their ends. If the temperature difference appears between these ends, the thermoelectric force causes the flow of the electric current in the circuit (Seebeck effect, Figure 1) (Rowe, 2005). A single thermocouple is able to generate only a small voltage (ET) and a small electric power (POUT). ET is directly proportional to the Seebeck coefficient of the thermocouple (␣) and to the temperature difference ⌬T between “hot” and “cold” junction (Figure 2). Seebeck coefficient of the thermocouple is dependent on Seebeck coefficients ␣A and ␣B of the constituent materials, so:

1. Introduction Modern electronics need modern power supply. Requirements for such power supply have changed in past years. Today, electronic systems are smaller and need less power than they used to in the past. The modern trends focus on electrical energy generation using non-electrical energy present in the surrounding environment. It could be solar, wind, electromagnetic waves or thermal energy (von Büren and Tröster, 2007; Cetin et al., 2010; Sawetsakulanond and Kinnares, 2010; Zhang et al., 2013). Such electrical generators (or energy converters) are called energy harvesters (Cantatore and Ouwerkerk, 2006; Harb, 2011) and are objects of interest of many researchers and engineers. Some of the most important requirements of energy harvesters are their suitable size, reliability and long lifetime. Many modern electronic devices are designed as small, low-power consumption autonomous systems (Shi et al., 2011; Claes et al., 2002). They need small and inexhaustible power sources for their long working. The energy harvester connected to the accumulator (e.g. to a supercapacitor) to store surplus energy meets the requirements. The paper addresses the thermoelectric energy generators whose elementary components are thermocouples. A thermocouple consists of two arms, made of different

ET ⫽ ␣ · ⌬T ⫽ 共␣A ⫺ ␣B兲 · 共THOT ⫺ TCOLD兲

POUT ⫽

E2T R

(1)

(2)

where:

␣A, ␣B THOT, TCOLD R

– Seebeck coefficients of the A and B materials, – temperature of the “hot” thermoelectric junction, – temperature of the “cold” thermoelectric junction, – internal resistance of the thermocouple.

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In practice, a number of thermocouples are connected in series to obtain higher ET and POUT. Such a system is called a thermopile (Figure 3). Thermopile composed of n thermocouples generates a voltage n · ET and an electric power n · POUT.

Microelectronics International 31/3 (2014) 176 –185 © Emerald Group Publishing Limited [ISSN 1356-5362] [DOI 10.1108/MI-11-2013-0077]

This work was supported by the Polish National Science Centre, Grant No N N515 503240 and Wrocław University of Technology, Grants No S30066 and No B30104.

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Thermoelectric energy harvester

Microelectronics International

Piotr Markowski

Volume 31 · Number 3 · 2014 · 176 –185

Figure 1 Thermoelectricity: the illustration of the thermoelectric effect

screen-printing technique. Not all materials could be used in that method. Only selected ones are available in the market as thick-film pastes (DuPont, 2013a, 2013b; ElectroScience, 2013; Institute of Electronic Materials Technology, 2013). Previous works proved that conductor-based materials are the best choice for thick-film electrical energy generators (Markowski and Dziedzic, 2006; Markowski et al., 2006). That was the reason why only materials listed in Table I were taken into consideration in the presented research. In Table I, typical thick-film substrates are also listed. Their thermal conductivities are much smaller than metals. But it should be noted that typically substrates have much bigger cross-section than conductive paths. The cross-section of the example thermopile (single layer) is shown in Figure 4. Designed dimensions of the Ag and Ni paths are 150 ␮m in width and 20 ␮m in height. Low temperature co-fired ceramic (LTCC) substrate, upon the single thermocouple, has 600 ␮m in width (two arms plus two distances between arms – 4 · 150 ␮m) and 165 ␮m in height (thickness of single green tape). As can be easily calculated, the area of conductors is 2 · 0.003 mm2 ⫽ 0.006 mm2. The area of substrate is 0.1 mm2, and it is 16.5 times bigger. Thermal conductivity of silver is 130 times and nickel 27 times higher than ␭ of LTCC. It can be calculated that about 20 per cent of the heat is transported through the volume of the substrate. Parameters listed in the Table I are for bulk materials. Thick-film pastes consist of conductive powder but also glaze. It means that the impact of the substrate’s heat conductivity will be even higher, and it should be taken into account during designing the thermoelectric energy harvester. Also, other thermoelectric parameters of thick-film materials are different (usually lower) in practice. In Table II, parameters measured during previous research are presented.

THOT

TCOLD

ET R

Figure 2 Thermoelectricity: the thermocouple consists of two different materials (A and B)

THOT Material A (αA)

Material B (αB)

TCOLD ET Figure 3 Thermoelectricity: the thermopile consists of n thermocouples

n thermocouples ETn = n · ET

Table I Electric and thermoelectric parameters of selected materials [9,15-17]

Material Ag Au Ni Pt LTCC Al2O3

Seebeck coefficient, ␣a [␮V/K]

Electrical resistivity, ␳ [␮⍀·m]

Thermal conductivity, ␭ [W/Km]

⫹1.5 ⫹1.9 ⫺19.5 – – –

0.016 0.022 0.07 0.11 – –

430 320 91 72 3.3 21

Note: a the reference material was platinum

Single thermocouple, ET

Figure 4 The cross-section of the Ag/Ni thermopile screen-printed on the LTCC substrate

2. Thermoelectric materials for the energy harvester

20 µm

Different materials are characterized by different thermoelectric parameters. Their electrical resistivity and Seebeck coefficient are crucial for electrical energy harvesters. Thermal conductivity of the whole system should be calculated as the resultant of both, ␭ of thermoelectric materials and ␭ of the substrate, in case of thick-film technology (explanation is given below). Generators presented in the paper were fabricated using thick-film technology and

Ni

Ag

150 µm

LTCC

16 mm

177

165 µm

I = ET / R

ΔT



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Table II Electric and thermoelectric parameters of selected materials [18] The paste DP PdAg ESL PdAg DP Ag ESL Ag DP Au ESL Ni DP Pt

Seebeck coefficient, ␣a [␮V/K]

Sheet resistance, R [m⍀]

⫺7 ⫺8 0 0 1 ⫺21 ⫺1.5

65 50 3.5 3.5 5 40 300

The first stage of the work was fabrication of a series of planar thermopiles on the fired LTCC substrate. The mask designed for screen-printing contained 25 thermocouples, each 25 mm long. Arms width and spacing between them were equal to 200 ␮m. A mask for the type A (Figure 5) and type B (Figure 6) arms was designed. After assembly, they form the thermopile with measuring pads. The VS1520 AUREL Fine Line Stencil Screen-printer was used. After preparing the substrates (25 ⫻ 30 mm2), Ag-based arms were screen-printed (A-type mask). Then, after drying, PdAg or Ni arms were fabricated (B-type mask). It should be noted that the sequence of printing does not affect significantly the output parameters of the structures. The structures were fired in a box furnace at a peak temperature of 875°C. Fabricated thermopiles are shown in Figures 7 and 8. Output parameters of the thermopiles were characterized using the automatic measuring system (Markowski et al., 2006). “Hot” ends of the thermopiles were heated to a temperature of about 225°C while “cold” ones were held at

Note: a the reference material is ESL Ag paste

Pastes based on gold, platinum and silver are characterized by the same level of Seebeck coefficients. Pt has significant lower thermal conductivity than Ag and Au. However, the silver has the lowest electrical resistivity. Moreover, application of silver-based paste is much more favorable from the perspective of economic reasons. It was decided to use DP 6,145 ink. To the test, DP 6,146 (PdAg) paste was also selected, due to a good fit to the base material (green tape DP 951). The paste based on Ni (ESL 2,554-N1) was also used. The LTCC substrate was made of DP 951 green tape. It was decided to use the LTCC technique because it is dedicated to multilayer structures fabrication (Vasudev et al., 2013; Czok et al., 2013; Malecha and Golonka, 2009), which was the objective of the work. Before the firing process, LTCC is a flexible tape (green tape), on which the thermocouple’s arms can be easily performed. It can be easily shaped mechanically (cutting to the proper size, drilling via holes). Green tape can be put together to form a multilayer stack, in which the adjacent layers may be electrically connected. After firing at a suitable temperature profile (usually with peak firing temperature of about 875°C), the uniform ceramic structure with buried paths is obtained. The procedure of thermocouple’s arms fabrication onto the green tape was another issue that should be considered. In the presented investigation, the precise screen-printing technique was chosen. Although it does not allow to fabricate such precise patterns as photosensitive paste technique (Wyz˙kiewicz et al., 2006) or laser shaping technique (Nowak et al., 2009), it allows for quick and repeatable fabrication of many thermocouples, with a resolution of 100 –200 ␮m on the green tape substrate.

Figure 5 Masks for screen-printing: the A-type mask with pads

Figure 6 Masks for screen-printing: the B-type mask

3. DESIGN and fabrication of the energy harvester DP 951 green tape and pastes based on Ag- (DP 6,145), PdAg- (DP 6,146) and Ni-powders (ESL 2,554-N1) were selected to fabricate the generator. It was decided to fabricate two variants of thermopiles: Ag/PdAg and Ag/Ni. Different variants of planar (not multilayer) Ag/PdAg thermocouples on LTCC already have been fabricated by the author (Markowski et al., 2007). Ag/Ni thermocouples are characterized by better material parameters (higher Seebeck coefficient, smaller electrical resistivity and smaller thermal conductivity), but there are some problems with buried layers fabrication (Markowski et al., 2008). 178



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Figure 10 The output characteristics of Ag/PdAg and Ag/Ni thermopiles (consisting of 25 thermocouples): internal resistance R versus temperature of the “hot” junctions (THOT)

Figure 7 Fabricated Ag/Ni thermopile on the LTCC substrate: the whole structure

Figure 8 Fabricated Ag/Ni thermopile on the LTCC substrate: zoom of the junctions area

Figure 11 The output characteristics of Ag/PdAg and Ag/Ni thermopiles (consisting of 25 thermocouples): output electric power POUT versus ⌬T

room temperature. The measurements were made for several different ⌬T. Generated thermoelectric force and internal resistance were controlled. In Figures 9-11, the results of measurements are presented for Ag/PdAg and Ag/Ni thermopiles consisting of 25 thermocouples. Better parameters were achieved for Ag/Ni thermocouples, as expected. The measured average Seebeck coefficient for a single thermocouple was 20.8 ␮V/K (7.7 ␮V/K for Ag/PdAg Figure 9 The output characteristics of Ag/PdAg and Ag/Ni thermopiles (consisting of 25 thermocouples): output electric voltage ET versus temperature difference between “hot” and “cold” junctions (⌬T) ones). This is close to the expected value, and the small differences are due to measurement inaccuracies. However, Ag/PdAg material variant was chosen to fabricate multilayer thermoelectric energy harvester. It is characterized by worse output parameters than Ag/Ni variant, but co-firing of Ni-based paste and green tape substrate causes some technological difficulties, and their solution requires further work. Incompatibility of the ESL 2,554-N1 paste and DP 951 green tape shows up during co-firing process. In the areas where Ni paths are printed, delaminations of LTCC appear (Figure 12). The idea of multilayer thermoelectric harvester is shown in Figure 13. Harvester consists of 15 layers, 30 thermocouples on each. According to the author’s knowledge, this is the first paper where multilayer thermoelectric harvester, fabricated with the aid of LTCC technology, is presented. 179



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Figure 12 Delaminations of the LTCC ceramic after co-firing with buried Ni pattern

mask designed for screen-printing contained 30 thermocouples, each 17 mm long. Arms width was 150 ␮m, spacing between them was 75 ␮m. A mask for the A-type (Figure 14) and B-type (Figure 15) arms was designed. After assembly, they form the thermopile with measuring pads. The VS1520 AUREL Fine Line Stencil Screen-printer was used. To connect electrically, the adjacent layers via holes were located on the pads’ areas. They were cut out alternately – on the left or right pad. Otherwise, the adjacent thermopiles would not be connected in series. Via holes and positioning holes were fabricated using LPKF ProtoLaser U laser system. In Figures 16 and 17, patterns for laser system are presented. After cutting out via holes and connecting holes, the A-type arms were screen-printed. For the thermopiles with holes on the left pad, the Ag-based paste was used, for the others, PdAg-based paste was used. After printing the paste through a screen with an A-type mask (the via holes at the pad area were filled at the same time), the structures were dried. Subsequently, the second paste was screen-printed through a B-type mask, and the structures were dried again. As a result, a part of thermopiles was Ag/PdAg-type (Ag printed through the A-type mask); others were PdAg/Ag-type (PdAg printed

Figure 13 The idea of multilayer thermoelectric harvester

Figure 14 Mask for screen-printing and laser cutting: the A-type mask with pads

The first stage of the works connected with fabrication of multilayer harvester was related with reliable connection of LTCC sheets with printed thermopiles. Each thermopile fabricated on a single substrate has to be connected to the adjacent layers to provide electrical continuity of the multistack. In the system consisting of several hundred thermocouples, if there is electric discontinuity even in only one place, it would result in malfunctioning of the entire system. A series of test structures were fabricated and investigated. Optimal procedure of the interlayer connections manufacturing was elaborated. To eliminate a situation where a single defective interconnection prevents the action of the harvester, it was decided to increase number of interconnections. For each layer, there were five via holes, diameter equal to 200 ␮m, connecting the thermopile to the adjacent ones. Designed planar dimensions of the harvester were 17 ⫻ 15 mm2. Area of pre-prepared substrate was 30 ⫻ 25 mm2. On the additional area, the positioning marks and holes were arranged. Thickness of used DP 951 tape was 165 ␮m. The

Figure 15 Mask for screen-printing and laser cutting: the B-type mask

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Figure 16 Mask for screen-printing and laser cutting: the mask for laser cutting (Ag-based arms)

Figure 18 Fabricated energy harvester: view of the whole structure

Figure 19 Fabricated energy harvester: zoom of the junctions area

Figure 17 Mask for screen-printing and laser cutting: the mask for laser cutting (PdAg-based arms)

in length, 130 – 135 ␮m in width and 5 (Ag) or 9 (PdAg) ␮m in height. Figures 20-22 show a view of an exemplary layer (the profilogram taken using Talysurf optical profilometer, Taylor Hobson Precise) and the X-ray picture of the structure’s interior (the photo taken using NanoFocus X-ray inspection system and computed tomography, Nanomex 180). To determine the output parameters of the harvester (generated output voltage and power), the measurement system had to be modified. This was connected with measurement of the temperature of “hot” and “cold” thermocouples junctions. Previously, the temperatures of heating block and radiator were controlled. For single-layer structures, the measurement was accurate enough. However, for a structure with a thickness of 2.1 mm, significant inaccuracies arose. The temperature gradient appears not only along the thermocouple but also in a plane perpendicular to them (Figure 23). Therefore, the temperatures of “hot” and “cold” junctions were difficult to determine. This was the reason of using of a new mounting method of the structure on heater/radiator and using of a thermal imaging camera. The structure touched to the heater and heat sink as shown in Figure 24. An infrared camera, located above the structure, measured the temperature distribution along the harvester. The precision of the system increased, as the temperature gradient occurred only in one axis and the infrared camera allowed the accurate determination of ⌬T. Figure 25 shows the generated voltage as a function of the temperature difference ⌬T. Measurements were made in nine

through the A-type mask). The alternate printing is necessary. Therefore, in the presented system, the Ag/PdAg-type thermocouples were printed on the “odd” layers, PdAg/ Ag-type ones on the “even” layers. The structures were put together to form a multilayer (via holes filled with the paste ensured electrical contact between the layers) and laminated at a pressure of 200 atm at 70°C. In this way, 15 layers consisting 450 thermocouples were combined. The resulting structure was cut out to dimensions of 19 ⫻ 16 mm2 using a laser system – positioning marks and holes that were no longer needed were separated from thermocouples. In the last technological step, the energy harvester was fired in the box furnace with a peak temperature of 875°C. The characterization of the output parameters is presented in the next section.

4. Results Fabricated energy harvester is shown in Figures 18 and 19. Its dimensions are 16.8 ⫻ 14.1 ⫻ 2.1 mm3 – less than 0.5 cm3. It consists of 450 Ag/PdAg thermocouples integrated in the interior. Dimensions of single thermocouple’s arm are 14.1 mm 181



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Figure 20 Fabricated energy harvester: cross-section of the single layer – width and height of the Ag (L1–R1) and PdAg (L2–R2) arms

Figure 21 Fabricated energy harvester: profilogram of the single layer

Figure 22 Fabricated energy harvester: X-ray picture of the structure’s interior – side view (measuring pads and interlayer connections visible on the right side)

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Figure 23 The structure mounting methods on the heater and heat sink: version for thin structures (the problem with heat distribution in thick structures)

HEAT SINK

The structure

Figure 26 The output characteristics of Ag/PdAg energy harvester (consisting of 450 thermocouples) versus temperature difference between “hot” and “cold” junctions (⌬T): temperature difference along the fabricated structure – an exemplary infrared (IR) picture

HEATER

Figure 24 The structure mounting methods on the heater and heat sink: version for thick structures HEAT SINK

HEATER The structure

Figure 25 The output characteristics of Ag/PdAg energy harvester (consisting of 450 thermocouples) versus temperature difference between “hot” and “cold” junctions (⌬T): output electric voltage ET

Figure 27 The output characteristics of Ag/PdAg energy harvester (consisting of 450 thermocouples) versus temperature difference between “hot” and “cold” junctions (⌬T): internal resistance R

points. Every time ⌬T was read from the image generated by the infrared camera (an exemplary image is shown in Figure 26). At the same time, the internal resistance R and the output voltage ET were measured. Figures 27 and 28 show the internal resistance R and the output electrical power POUT versus ⌬T. For the temperature difference of 135°C (THOT ⫽ 214°C, TCOLD ⫽ 79°C) between the opposite ends of the structure, the harvester generates voltage 0.45 V at 1,545 ⍀ internal resistance. It provides electrical power of 0.13 mW. The power density is 0.26 mW/cm3.

knowledge, this is the first paper where multilayer thermoelectric harvester, fabricated with the aid of LTCC technology, was described. To fabricate the energy harvester, procedures of performing reliable interconnections and printing miniature conductive paths were developed. The elaborated methodology allows the fabrication of structures consisting of much higher number of integrated thermocouples than presented in the paper. This is possible by adding more layers (now 15, can be multiplied), or using thinner substrates for single thermopiles (currently 165 ␮m before firing, but 50-␮m-thick green tape can be used). Using thinner substrate needs prior analyses of the heat dissipation inside the whole structure. It allows building a harvester consisting of several thousands of thermocouples. Further research will focus on optimizing the fabrication process of the buried Ag/Ni thermocouples. Such a composition is

5. Summary The methodology of fabrication of the miniature integrated structures containing hundreds of thermocouples buried in the LTCC ceramic was elaborated. The prototype structure includes 450 Ag/PdAg thermocouples spread over a volume less than 0.5 cm3. A length of each one is about 14.1 mm. The generated output voltage was equal to 0.45 V and output electrical power 0.13 mW for temperature difference along the structure of 135°C. According to the author’s 183



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Figure 28 The output characteristics of Ag/PdAg energy harvester (consisting of 450 thermocouples) versus temperature difference between “hot” and “cold” junctions (⌬T): output electric power POUT

ElectroScience (2013), “Thick-film materials”, available at: www.electroscience.com/products.html (accessed 3 June 2013). Harb, A. (2011), “Energy harvesting: state-of-the-art”, Renewable Energy, Vol. 36 No. 10, pp. 2641-2654. Institute of Electronic Materials Technology (2013), “Thick-film compositions”, available at: www.itme.edu.pl/tl_ files/Oferta%20szczegolowa/Z16_E_Thick_Film_Technology_ and_Think_Film_Compositions.pdf (accessed 3 June 2013). Malecha, K. and Golonka, L. (2009), “Three-dimensional structuration of zero-shrinkage LTCC ceramics for microfluidic applications”, Microelectronics Reliability, Vol. 49 No. 6, pp. 585-591. Markowski, P. and Dziedzic, A. (2006), “Experimental characterization of thermoelectric properties of thick film composites”, IMAPS Journal of Microelectronics and Electronic Packaging, Vol. 3, pp. 95-99. Markowski, P., Dziedzic, A. and Prociów, E. (2006), “Thermoelectric microgenerators made in mixed thick/thin film technology”, Elektronika, Vol. 47 No. 12, pp. 7-8. Markowski, P., Pinczakowski, W., Straszewski, L. and Dziedzic, A. (2007), “Thick-film thermoelectric microgenerators based on nickel-, silver- and PdAg-based compositions”, Proceeding of the 30th International Conference ISSE, Cluj-Napoca, pp. 223-228. Markowski, P., Straszewski, L. and Dziedzic, A. (2008), “Sandwich-type three-dimensional thick-film thermoelectric microgenerators”, Proceeding of the 31st International Conference ISSE, Budapeszt, pp. 110-113. Nowak, D., Mis, E., Dziedzic, A. and Kita, J. (2009), “Fabrication and electrical properties of laser-shaped thick-film and LTCC microresistors”, Microelectronics Reliability, Vol. 49 No. 6, pp. 600-606. Rowe, D.M. (2005), Thermoelectrics Handbook: Macro to Nano, CRC Press. Sawetsakulanond, B. and Kinnares, V. (2010), “Design, analysis, and construction of a small scale self-excited induction generator for a wind energy application”, Energy, Vol. 35 No. 12, pp. 4975-4985. Shi, Y., Gwee, B.-H. and Chang, J. (2011), “Asynchronous DSP for low-power energy-efficient embedded systems”, Microprocessors and Microsystems, Vol. 35 No. 3, pp. 318-328. Vasudev, A., Kaushik, A., Tomizawa, Y., Norena, N. and Bhansali, S. (2013), “An LTCC-based microfluidic system for label-free, electrochemical detection of cortisol”, Sensors and Actuators B: Chemical, Vol. 182, pp. 139-146. von Büren, T. and Tröster, G. (2007), “Design and optimization of a linear vibration-driven electromagnetic micro-power generator”, Sensors and Actuators A: Physical, Vol. 135 No. 2, pp. 765-775. Wyz˙kiewicz, I., Lewandowska, N., Dziurdzia, B., Magon´ski, Z., Chudy, M., Brzózka, Z., Jakubowska, M. and Dybko, A. (2006), “Fabrication of ceramic-polymer microstructure for capillary electrophoresis with usage of photosensitive paste and thick film technology”, Elektronika: konstrukcje, technologie, zastosowania, Vol. 47, pp. 9-10.

characterized by two times higher Seebeck coefficient and lower electrical resistivity. The attempt to fabricate Ag/Ni thermocouples using photosensitive pastes technique will be also taken. It will allow for compaction of thermocouples and increasing their number on a single substrate. The author would like to thank PhD Przemysław Matkowski (Wrocław University of Technology) for making X-rays pictures. The author would like to thank Krzysztof Urban´ski, PhD, and Bartosz Płatek (Wrocław University of Technology) for making IR pictures.

References Cantatore, E. and Ouwerkerk, M. (2006), “Energy scavenging and power management in networks of autonomous microsensors”, Microelectronics Journal, Vol. 37 No. 12, pp. 1584-1590. Cetin, E., Yilanci, A., Ozturk, H.K., Colak, M., Kasikci, I. and Iplikci, S. (2010), “A micro-DC power distribution system for a residential application energized by photovoltaic–wind/fuel cell hybrid energy systems”, Energy and Buildings, Vol. 42 No. 8, pp. 1344-1352. Claes, W., Puers, R., Sansen, W., De Cooman, M., Duyck, J. and Naert, I. (2002), “A low power miniaturized autonomous data logger for dental implants”, Sensors and Actuators A: Physical, Vols 97/98, pp. 548-556. Czok, M., Bembnowicz, P. and Golonka, L. (2013), “Low-temperature co-fired ceramics system for light absorbance measurement”, International Journal of Applied Ceramic Technology, Vol. 10 No. 3, pp. 443-448. DuPont (2013a), “951 Green tape”, available at: www2. dupont.com/MCM/en_US/assets/downloads/prodinfo/ 951LTCCGreenTape.pdf (accessed 3 June). DuPont (2013b), “Microcircuit materials”, available at: www2.dupont.com/MCM/en_US/tech_info/products/ltcc. html#951 (accessed 3 June). 184



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Zhang, P., Li, W., Li, S., Wang, Y. and Xiao, W. (2013), “Reliability assessment of photovoltaic power systems: review of current status and future perspectives”, Applied Energy, Vol. 104, pp. 822-833.

WebElements (2013), “WebElements: the periodic table on the web”, available at: www.webelements.com (accessed 3 June (2013). Markowski, P. (2009), “Termoelektryczne właœciwoœci kompozytów grubowarstwowych (in Polish)”, PhD Thesis, Wrocław University of Technology, Wrocław, Poland.

Further reading Jantunen, H., Kangasvieri, T., Vaha kangas, J. and Leppa vuori, S. (2003), “Design aspects of microwave components with LTCC technique”, Journal of the European Ceramic Society, Vol. 23 No. 14, pp. 2541-2548.

Corresponding author Piotr Markowski can be contacted at: piotr.markowski@ pwr.wroc.pl

To purchase reprints of this article please e-mail: [email protected] Or visit our web site for further details: www.emeraldinsight.com/reprints

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