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Journal of Cleaner Production 19 (2011) 58e63

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Journal of Cleaner Production

journal homepage: www.elsevier .com/locate/ jc lepro

Bench-scale synthesis of zeolite A from subbituminous coal ashes with highcrystalline silica content

Metta Chareonpanich a,b,*, Ornanong Jullaphan a, Clarence Tang c

aDepartment of Chemical Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, ThailandbNanotechnology Center, Kasetsart University, Bangkok 10900, Thailandc Siam Research and Innovation, Saraburi 18260, Thailand

a r t i c l e i n f o

Article history:Received 5 May 2010Received in revised form11 August 2010Accepted 12 August 2010Available online 21 August 2010

Keywords:Subbituminous fly ashBottom ashHigh crystalline silicaZeolite synthesis

* Corresponding author. Department of ChemUniversity, Bangkok 10900, Thailand.

E-mail address: fengmtc@ku.ac.th (M. Chareonpan

0959-6526/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.jclepro.2010.08.012

a b s t r a c t

In this present work, fly ash and bottom ash with high crystalline silica content were obtained from thecoal-fired boilers within the paper industries in Thailand. These coal ashes were used as the basic rawmaterials for synthetic zeolite production. The crystal type and crystallinity, specific surface area andpore size, and textural properties of zeolite products were characterized by using X-ray diffractionspectroscopy (XRD), N2 sorption analysis, and Scanning Electron Microscopy (SEM), respectively. It wasfound that sodalite octahydrate was selectively formed via the direct conventional (one-step) synthesis,whereas through a two-step, sodium silicate preparation and consecutive zeolite A synthesis process, 94and 72 wt.% zeolite A products could be produced from the fly ash and bottom ash, respectively. Thecation-exchange capacity (CEC) of fly ash and bottom ash-derived zeolite A products were closely similarto that of the commercial grade zeolite A.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Cleaner production concepts regarding activities includingresource use minimization through resource recovery and reuse,eco-efficiency, and source reduction which have positive effects onthe environment (Glavi�c and Lukman, 2007; Jegatheesan et al.,2009) have been widely used in various industries includingcement manufacturing, and pulp and paper industries. Concerningthese concepts, the utilization of coal ash which is a major by-product of coal-fired power plants through resource recovery hasbeen focused by many researchers.

Chindaprasirt and Rattanasak (2010) successfully synthesizedthe geopolymer with more reactive and higher strength from thepulverized coal combustion fly ash as compared to the use of thecoarser fluidized bed combustion ash. Recently, Geetha andRamamurthy (2010) also reported the utilization of bottom ash asa raw material for the aggregate preparation through the cold-bonded chemical activation resulting by the pozzolanic character-istics of bottom ash. From a different point of view, manyresearchers have focused on the use of coal ashes for zeolitesynthesis. Wide variety of zeolites including zeolite A (Rayalu et al.,

ical Engineering, Kasetsart

ich).

All rights reserved.

2001), phillipsite and faujasite (Rujiwatra et al., 2005) and GIS-typezeolites (Mouhtaris et al., 2003) were synthesized from low-ranklignite fly ash containing high amount of amorphous phases.Whereas a group of zeolites including zeolite X (Font et al., 2009),phillipsite and hydroxylcancrinite (Nascimento et al., 2009) weresynthesized from higher rank coal fly ashes.

It was clearly seen that the zeolite synthesis and the structuretypes of zeolites are strongly affected by factors including the Si/Almolar ratio, hydrothermal treatment temperature and pressure, thepresence of the structure-directing agent (Chareonpanich et al.,2004); type of alkaline solution, pH of mixture (Rayalu et al.,2001); and solid oxide contaminants (Ríos et al., 2009).

The synthesis processes of zeolite from coal fly ash can be simplyclassified into direct and indirect synthesis. In the direct synthesis,silica and alumina are initially extracted from coal fly ash by themean of alkaline extraction under high temperature, henceresulting in the mixture of silicate and aluminate extracts whichcan be considered as the precursors for the zeolite synthesis.However, due to a large variety in composition of coal fly ash, thedirect synthesis approach has primarily resulted in the mixedphases between zeolites and the remaining solid oxide contami-nants (Ríos et al., 2009; Jha et al., 2009). In order to selectivelysynthesize the pure phase of zeolite, it was essential that the bestavailable precursors of the desired compositions be used. However,due to the fact that during direct synthesis, the Si/Al molar ratio andalkaline concentration are well known to strongly affect the

M. Chareonpanich et al. / Journal of Cleaner Production 19 (2011) 58e63 59

selective formation of zeolites (Rayalu et al., 2001), the indirectsynthesis approach was more often applied in practice (Tanakaet al., 2004a, 2004b; Hui and Chao, 2006).

Practically, the use of coal fly ash containing high amount ofamorphous phases is more favorable for zeolite synthesis than thatof the highly crystalline phases due to the difficulty of silica andalumina dissolutions. In this present work, the challenge to find anappropriate method for resource recovery and reuse of subbitu-minous fly ash and bottom ash with high crystalline silica content,obtained from the coal-fired boilers within the paper industries inThailand, as the basic raw materials for zeolite A synthesis wasfocused. For the best of our knowledge, these high crystalline silicaashes have not been previously used as raw materials for zeolitesynthesis. The direct synthesis of zeolite A was firstly investigatedand after that, the indirect synthesis was subsequently appliedusing the preparation of silica and alumina precursors prior tozeolite A synthesis. The appropriate conditions for the bench-scalecleaner production of zeolite A were then identified. The coal ash-derived zeolite A products were tested for the cation-exchangecapacity (CEC) and compared to that of the commercial gradezeolite A.

It must be noted that the preparation of silica precursors fromcoal ashes in the form of sodium silicate solution is the outstandingenergy-efficient process since the conventional process involvesthe high sintering temperature in which silica sand (crystallinesilica or quartz) and sodium carbonate are fused at 1100e1200 �C.In our process, the relatively low temperature as 900 �C is appliedfor the production from coal ashes, therefore, a huge savings on fuelconsumption and environmental impact reduction are obtained.This process can practically prepare sodium silicate of similarquality, but with significantly lower fuel consumption.

2. Materials and methods

2.1. Materials

Subbituminous coal fly ash and bottom ash, obtained from theStockerBoiler andCirculatingfluidizedbed (CFB) reactorswithin thepaper industries in Thailand, were ground and sieved to the particlesize of smaller than 200 mesh (mean particle size <0.074 mm) andkept in the desiccator before use. Chemical compositions, crystalstructure and crystallite size, and specific surface area of the coalashes were determined by using X-ray Fluorescence Spectroscopy(XRF: ARL, 9400), X-ray Diffraction Spectroscopy (XRD: Philips,X’Pert, Cu-a radiation), and BET surface area analysis (Quantach-rome, Autosorb-1C), respectively.

2.2. Investigation of zeolite A synthesis from subbituminous coalfly ash

Subbituminous fly ash was used as the basic raw materials forzeolite A synthesis following both the conventional, directsynthesis method (Robson et al., 2001) and the modified indirectsynthesis methods. In the case of the conventional synthesis, fly ash

Table 1Chemical compositions and mineral contents of subbituminous coal fly ash and bottom

Sample SiO2 Al2O3 Fe2O3 CaO MgO

Fly Ash 51.55 23.64 8.10 5.62 2.01Bottom Ash 52.47 24.33 8.29 3.61 1.27

Sample Quartz Mullite Anhydrite Hema

Fly Ash 18.18 12.66 0.77 1.27Bottom Ash 19.11 e 8.89 0.81

of 34.93 g was mixed with 0.723 g of NaOH in 80 ml distilled waterto obtain the optimum synthesis pH value of approximately pH 11.The obtained mixture was then transferred to a Teflon-linedautoclave and hydrothermally heated at 120 �C for 4 h. The solidproduct was washed thoroughly with distilled water and dried inan oven at 100 �C for 12 h. The obtained product was analyzed byusing XRD to identify the crystal structure.

Since the major composition of the subbituminous coal fly ashwas crystalline structure consisting mainly a-quartz and mullite,the continuous leaching of silica and alumina in fly ash into sili-cate and aluminate formed zeolitic materials was considerablydifficult. As a result, severe leaching temperature and strongalkaline concentration were then essentially applied to theprocess. The conventional synthesis process was modified byvarying the operating conditions as follows: synthesis period,4e8 h; hydrothermal treatment temperature, 100e160 �C; NaOHamount, 0.723e10.845 g.

2.3. Preparation of zeolite A precursor from subbituminous coalfly ash

Due to the interference of zeolite A formation caused by themetal oxide contaminants, mainly Fe2O3, CaO, and MgO (Table 1);the iron oxide (hematite) was primarily removed from raw fly ashby magnetic separation. Subsequently, the remaining metal oxidecontaminants were removed from fly ash by the mean of acidtreatment. In this stage, fly ash was refluxed with 3 M hydrochloricacid solution (9 g of hematite-free fly ash/120 ml of HCl solution) at100 �C for 6 h. The solid product obtained from this stage, consistedmainly of silica (97.7% SiO2). The solid product was reacted withsodium carbonate to form sodium silicate at 900 �C for 1 h by usingthe molar ratio of silica: sodium carbonate of 1:1. The obtainedsodium silicate was subsequently dissolved in distilled water toproduce the silica precursor, sodium silicate solution (21wt.% SiO2).

2.4. Bench-scale synthesis of zeolite A from subbituminous coal flyash and bottom ash

In order to synthesize zeolite A from subbituminous coal flyash at the bench-scale order (0.5 kg of dry solid product), fly ash-derived sodium silicate solution was used as the silica precursor.The certain amount of sodium aluminate was slowly added to thesodium silicate solution, followed by slow stirring at 40 �C ina water bath while the pH of mixture was maintained at 11. Theobtained mixture was transferred to a Teflon-lined autoclave andhydrothermally heated at 120 �C for 4 h. The solid product waswashed thoroughly with distilled water, dried in air at 100 �C for12 h, and then calcined at 540 �C under atmospheric conditionsfor 6 h.

In the case of the bench-scale synthesis of zeolite A fromsubbituminous coal bottom ash, a similar process to that of the flyash was applied except that the bottom ash-derived sodium silicatesolution was used as the silica precursor.

ash (Data obtained from XRF and XRD analysis, respectively).

SO3 LOI Na2O K2O TiO2 P2O5

0.57 5.80 0.02 1.67 0.63 0.050.08 7.28 0.02 1.74 0.57 0.04

tite Magnetite Lime Calcite Amorphous

0.96 e e 66.470.86 4.70 4.50 58.87

M. Chareonpanich et al. / Journal of Cleaner Production 19 (2011) 58e6360

The crystal structure and purity of the obtained products wereanalyzed by using XRD, whereas textural structures were examinedby using Scanning Electron Microscope (SEM: JEOL, JEM-6301Fwith Au-coated, operated at 15 keV).

2.5. Cation-exchange capacity of zeolite A

The cation-exchange capacity (CEC) of fly ash and bottom ash-derived zeolite A products were characterized and compared tothat of the commercial grade zeolite A. The CEC data of zeolite Awere explained with respect to their specific surface area, Si/Almolar ratio (obtained from Inductively Coupled PlasmaeOpticalEmission Spectrometer (ICPeOES: PLASMA-1000)) and the chem-ical composition of the synthesized zeolite A products.

3. Results and discussion

3.1. Direct synthesis of zeolites from subbituminous coal fly ash

The chemical compositions and mineral contents of subbitu-minous coal fly ash and bottom ash are reported in Table 1. Themajor compositions of both fly ash and bottom ash are SiO2, Al2O3,Fe2O3, CaO, and MgO (90e91 wt.%) with approximately 40 wt.% ofsilica and alumina in the form of highly stable crystallineminerals. It should be noted that the amounts of reactive silicaand alumina precursors for zeolite synthesis strongly depend onsilica and alumina contents in the coal fly ash, especially in theamorphous form.

Fig. 1. XRD patterns of solid products synthesized at (a) various temper

Following the direct synthesis method (Robson et al., 2001) andthe method modified by varying the hydrothermal treatmenttemperature in the range of 100e160 �C, a-quartz was obtained asthe main crystalline products. This could be attributed to the lowhydrothermal treatment temperature and low alkaline (NaOH)concentration when the extraction of silica and alumina was per-formed. In the case of zeolite A synthesis process, the startingmaterials with relatively high concentrations of silicate andaluminate species were essential in order to initiate the alumino-silicate gel (zeolite precursor) formation. This could be consideredas the preparation stage for the synthesis of zeolite A.

In order to develop an optimum condition for zeolite Asynthesis, stronger alkaline concentrations were applied to firstlydissolve a-quartz, prior to the hydrothermal treatment in theautoclave. The pH value of mixture was then adjusted to the suit-able pH for zeolite A synthesis (approximately pH 11). Fig. 1(a, b)show the XRD patterns of the solid products synthesized at varioustemperatures for 4 h and the various temperatures and NaOHconcentrations for 8 h, respectively.

It was observed that a strong intensity of a-quartz was obtained,indicating that the reformation of a larger andmore stable crystal ofa-quartz was formed during the zeolite synthesis process. Thisresult implied the importance of the simultaneous dissolution of a-quartz (to form silicate species) and the formation of zeolitemicrostructure has occurred under the hydrothermal condition.With a higher concentration of alkaline solution (Fig. 1(a): 2.169 gor 0.69 M NaOH), it can be clearly seen that the crystallinity ofa-quartz gradually increased with increasing the hydrothermaltreatment temperature. This could be attributed to the ripening

atures for 4 h (b) various temperatures and NaOH amounts for 8 h.

Fig. 2. XRD patterns of (a) a-quartz (b) sodalite synthesized from pure chemicals.

M. Chareonpanich et al. / Journal of Cleaner Production 19 (2011) 58e63 61

phenomena caused by the dissolution of less stable silica to formsoluble silicate species, and the subsequent re-crystallization ofthese silicate species over existing crystalline silica (a-quartz)performed as the seeding materials (Break, 1979). However, atrelatively high temperature (160 �C), the crystalline silica wascompletely dissolved and the interaction between silicate andaluminate to form aluminosilicate product (in this case, sodaliteoctahydrate) was found. The reference XRD patterns of a-quartzand sodalite octahydrate synthesized from sodium silicate andsodium aluminate are shown in Fig. 2(a, b), respectively.

The similar trend of result to the investigation of temperatureeffect was found when both the alkaline concentration andhydrothermal treatment temperature were varied. However, atboth relatively high alkaline concentration and hydrothermaltreatment temperature, a-quartz was mainly observed, meanwhilethe formation of sodalite octahydrate was found. Under the abovecondition, a-quartz in the raw fly ash could be dissolved intosodium silicate and further reacted with aluminate in the aqueousmixture, resulting in the formation of aluminosilicate gel. Underthese conditions, only sodalite octahydrate was selectively formedwhereas zeolite A could not be formed.

3.2. Indirect synthesis: preparation of zeolite A precursor fromsubbituminous coal fly ash

The formation of zeolite A from fly ash can be interfered bynumerous factors, including the very stable silica content in theform of a-quartz in fly ash and the competitive formation ofsodalite octahydrate under the higher alkaline concentration. Inaddition, the presence of metal oxides contaminants could playsignificant role on the competitive formation of other zeolite typesduring the formative stages of the zeolite gel (Wu et al., 2006).Therefore, the synthesis of zeolite A from subbituminous fly ashwas modified by primarily purified fly ash followed by thesynthesis of zeolite A using the 2-step process via sodium silicatepreparation and zeolite A synthesis.

The physical separation of iron oxide (magnetite) contaminantsfrom fly ash was simply done by using a magnetic bar. Based on the

Table 2Chemical compositions of sodium silicate and zeolite A products prepared from various

Source SiO2 Al2O3 Fe2O3 CaO Na2O K2O

Sodium silicateCommercial 38.60 1.95 0.16 0.13 50.46 0.56Fly ash 78.57 2.56 3.32 2.11 e 4.15Bottom ash 54.69 7.29 13.45 4.24 12.62 4.55

Zeolite ACommercial 62.63 28.25 0.11 7.45 e 1.34Fly ash 60.71 28.60 1.09 5.14 e 1.12Bottom ash 44.22 22.09 5.30 23.99 e 2.64

dry weight of fly ash sample, the total weight loss after themagnetic separation process was approximately 30 wt.%. It shouldbe noted that parts of silica and alumina were also possibly lost inthe process since the iron oxide content in fly ash (includingmagnetite and hematite) was 8.1 wt.%. The magnetite-separated flyash was refluxed with 3 M hydrochloric acid solution in order toremove other metal oxide contaminants. During this stage, metaloxides mainly iron, calcium and potassium oxides were removed(36 wt.% based on the dry weight of fly ash sample). The obtainedsolid product was then reacted with sodium carbonate powder toform sodium silicate of which easily dissolved in distilled water.This process is very promising for commercial sodium silicateproduction as it is not only the outstanding energy-efficient butalso the environmentally friendly process. A huge savings on fuelconsumption due to the lower sintering temperature(w200e300 �C lower than the conventional process) and envi-ronmental impact reduction resulting from source reduction,resource recovery and reuse within this process are of greatadvantages.

The chemical compositions of sodium silicates obtained fromthis process were compared to that of the commercial one as shownin Table 2. In all cases, the presence of alumina content in sodiumsilicate was observed due to the fact that the silica sources of thecommercial and coal ashes-derived sodium silicates were the silicasand obtained from natural source, and the mineral matter in coalitself. Since the amounts of silica and alumina were varied in eachcase, in order to synthesize zeolite A in the next step of process,both the fly ash- and bottom ash-derived sodium silicate weresimilarly dissolved in distilled water to obtain 21 wt.% SiO2.

3.3. Bench-scale synthesis of zeolite A from subbituminous coal flyash and bottom ash

In this stage, fly ash and bottom ash-derived sodium silicatesolutionswere used as the rawmaterials for zeolite A synthesis. Thecrystal types and textural characteristics of the synthesized zeoliteA products were obtained by means of XRD spectroscopy, and SEMand N2 sorption analysis, respectively and compared to that of thecommercial grade zeolite A as shown in Fig. 3.

The XRD patterns of both bottom ash and fly ash-derivedzeolites A products were shown in Fig. 3(a, b). It evidentlyconfirmed that the selective formation of zeolite A from the bench-scale synthesis of the fly ash and bottom ash at 72 wt.% and 94 wt.%,respectively (calculated based on the 100 wt.% crystallinity ofzeolite A synthesized from pure chemicals). By using the Scherrerequation (Klug and Alexander, 1974), the calculated particle sizes ofzeolite A products were 160 and 200 nm for bottom ash and fly ash,respectively.

Moreover, the SEM photos clearly indicated the uniform;segregate spherical and cubic crystals of zeolite A productsobtained from both bottom ash and fly ash. The presence ofimpurities contained within the solid samples is unnoticeable. The

sources of silica and alumina.

TiO2 P2O5 ZrO2 SrO V2O5 Mn2O3 NiO ZnO

0.62 0.03 e e e e e e

8.53 e 0.56 0.09 0.10 e e e

2.67 e 0.22 0.14 e 0.13 e e

0.11 e 0.02 0.02 e 0.02 e 0.053.21 e 0.08 0.04 0.02 e e e

1.46 e 0.04 0.11 0.01 0.06 0.04 0.02

Fig. 3. XRD patterns, SEM photos and N2 sorption isotherms of zeolite A products synthesized from (a) subbituminous bottom ash (b) subbituminous fly ash (c) pure chemicals(commercial zeolite A).

M. Chareonpanich et al. / Journal of Cleaner Production 19 (2011) 58e6362

nominal size of the zeolite A was in the range of approximately1e10 mm.

The result from N2 sorption isotherms revealed type I isothermswith H1 hysteresis loop, indicating the existence of microporousmaterials. All the zeolite A products provided micropore structureswith one broad capillary condensation step at P/Po of 0.05e0.95,representing the broad pore size distribution of which normallyobserved with zeolite A.

3.4. Cation-exchange capacity of zeolite A

The cation-exchange capacities (CEC) of zeolite A products wereexamined by following the standard CEC analysis method asreported by García-Sosa and Solache-Ríos (2001). The CEC results ofdirect or indirect synthesized zeolites A were shown in Table 3.

It is clear that the CEC values of zeolite A products synthesizedfrom both subbituminous fly ash and bottom ash were extremelyhigh (4.00 and 3.77 meq/g, respectively) as compared to that of thefly ash (0.1 meq/g), bottom ash (0.03 meq/g). It is also observed thatthe CEC values of zeolite A synthesized from the subbituminous flyash and bottom ash are comparable to that of commercial gradezeolite A (4.79 meq/g).

Based on the theoretical formulae of the synthetic zeolite A asfollows: Na12(A112Si12O48).27H2O, the Si/Al molar ratio of zeolite Ais equal to one and the calculated CEC in terms of meq/g is 5.3.

Table 3Selected properties of subbituminous ashes and zeolite A products.

Materials Si/Ala Surfacearea (m2/g)

CEC(meq/g)

Fly ash 7.41 9.9 0.10Bottom ash 7.33 7.6 0.03Zeolite ACommercial 1.88 35.3 4.79Fly ash-derived 1.80 41.2 4.00Bottom ash-derived 1.70 36.1 3.77

a Si/Al molar ratios calculated from XRF results.

However, it should also be noted that the practical CEC value wasusually lower than that of the calculated CEC. This could beattributed to the mass transfer limit during ion exchange processdue to the critical aperture of zeolite A. Moreover, the higher Si/Almolar ratio observed in Tables 2 and 3 revealed the existence of theexcess amorphous silica and other metal oxides contaminants inthe bulk composition of zeolite A products.

Regarding the heterogeneous phases of zeolite products, thepractical CEC value could be strongly affected by not only the Si/Almolar ratio but also by the specific surface area and the crystallinityof zeolite A products. As the Si/Al molar ratio was theoreticallysimilar for which samples, it was concluded that the highest CECvalue was obtained from zeolite A product with a higher crystal-linity and specific surface area.

4. Conclusion

In this study, the cleaner production concept was successfullyapplied for zeolite A synthesis from subbituminous fly ash andbottom ash containing high crystalline silica content through the2-stage process. Sodium silicate was firstly prepared from coal ash-derived silica by thermal fusion with sodium carbonate at 900 �Cfor 1 h, and zeolite A was hydrothermally synthesized from theobtained sodium silicate and sodium aluminate at 120 �C for 4 h,consecutively. It was found that zeolite A of 94% and 72% crystal-linities with relatively high CEC values as 4.00 and 3.77 meq/g(compared to 4.79 meq/g of commercial zeolite A) were synthe-sized from subbituminous fly ash and bottom ashes, respectively.Furthermore, since the silica fusion temperature was 200e300 �Clower than conventional sodium silicate production, less fuelconsumption and significant environmental impact reduction aremainly achieved.

Acknowledgements

This work was financially supported by SCG Cement with SiamResearch and Innovation, the National Science and Technology

M. Chareonpanich et al. / Journal of Cleaner Production 19 (2011) 58e63 63

Development Agency (NSTDA Chair Professor and NANOTEC Centerof Excellence), and the Kasetsart University Research DevelopmentInstitute.

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Metta Chareonpanich, D.Eng. is currently an AssociateProfessor at the Department of Chemical Engineering,Kasetsart University, Thailand. She was awarded theMonbusho Scholarship from Japanese Government fordoctoral degree study in Tohoku University undersupervision of Professor Akira Tomita. Her researchfocuses are mainly on the synthesis of nanomaterialsincluding zeolites and mesoporous silica from low costand renewable resources using Cleaner Technologyconcept. For more information, please visit: http://www.che.eng.ku.ac.th/

Ornanong Jullaphan is currently a Ph.D. student at theDepartment of Chemical Engineering, KasetsartUniversity, Thailand. She has received the grant fromthe Commission on Higher Education, the Ministry ofEducation, under the Postgraduate Education andResearch Programs in Petroleum and Petrochemicals,and Advanced Materials for doctoral degree studyunder supervision of Associate Professor Metta Char-eonpanich. Her research focuses are mainly on thesynthesis of bimodal micro-mesoporous materials fromalternative resources.

Clarence Tang, PhD is currently a Senior Researcherwith Siam Research & Innovation, SCG Thailand. HisR&D activities focus primarily on developing renewablematerials derived from industrial waste products. Clar-ence Tang first graduated from the National Universityof Singapore and he was awarded a STINT Scholarship(Masters Degree) at the Chalmers University of Tech-nology, Sweden. Subsequently, he joined the SwissFederal Laboratories for Materials Testing and Research(EMPA)/Swiss Federal Institute of Technology (ETHZ)where he completed his PhD (Doctor of Sciences). Hehas published several papers in the international jour-nals, held several patents and presented at internationalconferences.