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mechanisms of video camera

auto-focus systems, or the ink ejecting mechanisms of inkjet printers. These components use piezoelectric elements that

expand and contract when a current is passed through them. PZT

(lead zirconate titanate) is the

name given to a special ceramic material that is a typical piezoelectric material. This expanding and contracting quality of PZT is put to use in motors, valves, speakers and other components. Unfortunately, lead is one of the key ingredients in PZT. Regulations like the RoHS Directive introduced earlier mean that lead-free piezoelectric materials are sought, but no piezoelectric material has been developed so far that has sufficient performance be a viable PZT replacement. Despite the RoHS Directive, the use of PZT as a piezoelectric material is permitted even though it contains lead. In the meantime, the development of lead-free piezoelectric materials 4) is advancing rapidly.

Paperless

Disc-less

Development of electronic paper 5) technology is progressing towards the ultimate goal of a paperless society through IT promotion. It is hoped that electronic paper technology will develop materials that can be bent and curved like paper, that have a surface like a TV screen whose contents can be easily changed, but can display images without using a lot of power. Many companies in Japan have already started to sell such products. In the same way that the memory stick has taken over from the floppy disc, we are about to see the day when electronic paper takes over from paper as the medium of information communication. The author does not have any information at hand about how the switch from floppy disc to memory stick and from paper to electronic paper has impacted on carbon dioxide emission assessment or industrial waste assessment. Nevertheless, the trend to switch over to these kinds of rewritable data media that can be reused and not thrown away is expected to help lower energy consumption and the environmental impact.

3. Summing up

The author has introduced some simple examples of the work that is being done in the electronics field to reduce the environmental impact and conserve energy. As discussed in the example of PZT materials, there are many materials containing toxic elements and compounds that we knowingly continue to use as there

are no viable substitutes. The earlier chapter on LEDs mentioned that fluorescent lamps use mercury –– a well-know toxic heavy metal. Unfortunately, we don’t have any other kind of lighting that has the energy efficiency, environmental performance or design aesthetics to replace existing fluorescent lighting –– which is why we continue to use it. Similarly, in the real world we have to find a trade-off between hazard, environmental impact and economic efficiency to decide the value of a product.

Like the example of the solder, the author hopes that electronic materials that place a lower impact on the environment will be developed, along with electronic devices that will use them. Maybe we need to reconsider whether our contemporary society, in which people can use game machines or personal computers that have the processing power of super computers of 20 years ago in their homes, really means we have achieved true affluence.

The author was assisted in the writing of this paper by Kenji Matsumoto, a NIMS junior researcher.

References

1)Intel Corporation homepage http://www.intel.com/

2)The Organization for Small & Medium Enterprises and Regional Innovation, JAPAN homepage http://www.rohs.eu/english/index.html

3)NIMS press release http://www.nims.go.jp/jpn/news/press/press215.html (dated 11 January 2008)

4)Ceramics, The Ceramic Society of Japan http://www.ceramic.or.jp/ihensyub/bulletin_ j/contents_ j/20 05_ j/2005_08.html

5)For example, M. Omodani, Kami e no Chosen Denshi Peipaa

(Electronic Paper Challenging Ordinary Paper) (Japanese language), Morikita Publishing Co., Ltd.

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tures, and if these cages are filled with large atoms, the thermal vibrations of those atoms drastically reduce the lattice component of thermal conductivity. Studies, chiefly in the United States, have focussed on skutterudite (shown in Fig 3.5.2 as p-CeFe4Sb12), whistler and clathrate materials.

In Japan and in the European Union, research is focussing on different materials, including Fe-Si, Mg-Si and Mn-Si based compounds. These have the advantages of being safe and being made from abundant raw materials. From the second half of the 1990s in Japan, research into Na-Co-O, Ca-C-O based and other strongly coupled oxides began. These materials have a Perovskite crystal structure consisting of alternating conducting and resisting layers, and are characterized by having a similar carrier concentration to metals and high Seebeck coefficients. Since the start of the new century, research, chiefly in Japan, has begun into polythiophenes and other organic polymers.

3.Applications in the environment and energy fields

3.1 Thermoelectric power generation

The efficient use of energy –– making sure that you do not waste any of it –– is important not only to energy policies but also to help reduce our impact on the environment. Thermoelectric power generation was originally developed to be a power source for use in outer space, the military and remote locations. Scientists are now looking into its applications in industrial plants, incinerators, boilers, automobiles and other heat sources to enable the generation of power from recovered waste heat. This power generation has the advantages of being compact, without any moving parts, silent and maintenance free.

3.2 Thermoelectric cooling (Peltier cooling)

Fig. 3.5.3 shows an illustration of a laser package 2) for optical telecommunications as an example of how thermoelectric cooling is assisting in energy conservation and technological innovation by enabling precise temperature control which in turn makes it possible to maximise system efficiency. Both the transmitter and exciter components of the semiconductor laser package are cooled by Peltier elements (size: several mm square), to enable wavelength division of the laser light so that a large among of data can be transmitted.

Fig. 3.5.3 Semiconductor laser package for highvolume optical data communication

4. Challenges in material science

At present we need materials that have a dimensionless figure of merit ZT equal to or greater than 2 for thermoelectric power generation and CFC-less cooling to become commonplace in society. In the last 10 years, research has attempted to increase this figure and has focussed on the relationship between material structure (including cage-like structures in crystals, substances with layered structures, and organic polymers) and thermoelectric performance.

From the mid-1990s, research, mostly in the U.S., has been conducted into creating quantum structures. This work has been based on theoretical predictions that if the dimensions of conductivity within a material are reduced by nano-dots, thin-film layers and other structures, then the Seebeck coefficient of that material can be greatly increased 3). Although scientists have yet to prove the validity of this theory, there have been reports that thermal conductivity has been reduced by at least a half, and that a ZT figure of greater than 2 has been achieved 4).

Material science researchers will have to grapple with the following issues.

1)A tie-up with computer science for study of atomic and molecular structure so that band gap control (already achieved through solid solution formation, distortion, doping) can be further improved.

2)There are many types of thermoelectric materials whose mechanisms of conduction need to be clearly understood.

3)Thermoelectric performance is studied in relation to average material structure; however, research that focuses on specific structure such as nano-particle dispersion (including quantum structure) is required.

4)Material research always uses raw materials of high purity; however, it is not clear how pure raw materials need to be.

5)The stability of the electrode on the hot side is the key to thermoelectric power generation. The development of electrodes that have both stable ohmic contact and contact strength is essential.

6)We need element-building technology that we can apply to a wide variety of device sizes and shapes.

References

1)X. Wu and I. W. Chen: J. Am. Ceram. Soc. 75 (1992) 2733.

2)B. Suzuki, D. Sato and C. Kato: Nature Mater. 4 (2005) 180.

3)T. Nishida, K. Shimazaki, Teion Kogaku (Low-Temperature Engineering) (Japanese language) 39 (2004) 115.

4)R. Klockenkamper: Total-Reflection X-ray Fluorescence Analysis, John Wiley & Sons.

For more information on thermoelectric materials,

1)M. Sakata, Thermoelectric conversion -Basics and applica- tion-, Shokabo Publishing Co. (2005)

2)The Ceramic Society of Japan and The Thermoelectric Society of Japan, Thermoelectric Energy Conversion Materials, Nikkan Kogyou Shinbun, Ltd. (2005)

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