Review Report on

Feasibility Study of Environmental Barrier

Coatings for Ceramic Matrix Composite in High-

Temperature Energy Conversion Applications

Submitted by

E.O.B. Ogedengbe, Ph.D., P.Eng. President, Energhx Consulting

90 Woodridge Crescent, Suite 401

Nepean, ON K2B 7T1

to

K.E. Zanganeh, Ph.D., P.Eng. Group Leader

Zero-Emission Technologies Group

Clean Electric Power Generation

CANMET Energy Technology Centre-Ottawa

1 Haanel Drive, Nepean ON K1A 1M1

May, 2008

2

Table of Contents

Table of Figures.3

Abstract..4

1. INTRODUCTION.5

1.1 Background..5

1.1.1 Silicon-based Materials for High Temperature Applications

1.1.2 Material Recession in Water Vapour Environment

1.1.3 Corrosion Effects in Molten Salt Environment

1.2 Description of the Barrier Coatings.10

1.2.1 Thermal Barrier Coatings

1.2.2 Environmental Barrier Coatings

2. HIGH-TEMPERATURE APPLICATIONS .15

2.1 Solid-Electrolyte Fuel Cells.15

2.1.1 Interconnection Material Selection

2.1.2 Model Kinetics for the HTSEFC

2.2 Compact Heat Exchanger Design....18

2.2.1 Criteria for Material Selection

2.2.2 Maximum Thermal Conductivity

2.3 Ceramic Membranes....21

3. EBC-ENHANCED HEAT EXCHANGER OPTIMIZATION 23

3.1 Entropy Generation in Heat Exchanger.24

3.1.1 Basic Components of Entropy Generation

3.1.2 Minimization of Entropy Generation

3.2 Compatibility Criteria........................27

3.2.1 Material Compatibility

3.2.2 Performance Compatibility

3.3 Modelling Procedures in Heat Exchanger Design28

3.3.1 Thermo-fluid Design

3.3.2 Thermo-structural Design

4. CONCLUSION 31

References

3

Table of Figures

Figure 1 Chronological Evolution of Mullite in the Design of EBC................................ 13

Figure 2 Temperature Profile for Counterflow Heat Exchanger ...................................... 26

Figure 3 Description of the Offset Strip Fin Arrangement ............................................... 26

Figure 4 Three-layer Design of EBC................................................................................ 28

Figure 5 Section X-Y showing the 2-D computational domain for thermo-fluid modeling

........................................................................................................................................... 29

Figure 6 Section X-Z showing the 2-D computational domain for thermo-structural

modeling ........................................................................................................................... 30

4

Abstract

This report presents the feasibility study of the relevance of Environmental Barrier

Coatings (EBC) for improving the performance of the Ceramic Matrix Composite used in

the manufacturing of High Temperature Compact Heat Exchanger and Solid Electrolyte

Fuel Cell. Several industrial applications of advanced materials that endure hostile

chemical environments and more demanding thermal and mechanical conditions are

reviewed, in addition to high temperature heat exchanger applications, especially for gas

turbine, high temperature solid-oxide fuel cell, and aircraft engines. Major difference in

the selection criteria for EBC materials for High Temperature Compact Heat Exchanger

as differed from other existing applications is established. A micro-design approach for

enhanced performance of EBC based of the used of experimental validated numerical

analysis is proposed.

5

1. INTRODUCTION

1.1 Background

Integrated power generation resources exhibit great potential to provide more reliable

energy supply than the existing electric power supply systems. The performances of

high temperature system components like membranes, heat exchangers, electrostatic

precipitator, etc are subject to the material recession effects due to flue gases from the

combustor. Ceramic materials have superior properties that make them suitable for a

broad spectrum of high temperature engineering applications. These properties

include high temperature stability, resistance to oxidation and corrosion due to attacks

from demanding environments. Common environments that are peculiar to power

generation industries comprise the combustion and molten salt environments.

1.1.1 Silicon-based Materials for High Temperature Applications

Silicon-based ceramics are strong candidates for heat recovery energy components in

combustion environment because of the formation of silica surface that provide a

protective scale against potential attacks from the surrounding harsh environment.

Several applications including turbines, combustion liners and power generation have

long proposed the use of SiC and Si3N4. In dry oxygen, these ceramics form a self-

healing, passivating layer of silica, which imparts good oxidation resistance [1].

However, one of the obstacles to the widespread use of Silicon-based ceramics in future

long-term is that the carbon fibers oxidize at medium to high temperatures in an oxygen-

included environment, such as a combustion environment. Previous studies investigated

either the stressed oxidation behavior of SiC/C fibres reinforced SiC-based materials (i.e.,

SiC/SiC or C/SiC composite) in an oxidizing environment below 1500oC or the oxidation

kinetics without extra loads in combustion environments [2]. Opila et al. [3] studied the

effects of volatilization rate in combustion environments for various applications. The

complicating factors due to the actual combustion environment and commercial materials

are discussed. Probable vapour species were identified in both fuel-lean and fuel-rich

6

combustion environments. However, there is little knowledge about the oxidation of

C/SiC composites under extreme high stress at high temperatures up to 1800oC in a

combustion environment with oxygen, water vapour, and carbon dioxide. A fundamental

evaluation of the role of the environment in the damage mechanisms of material

subjected to mechanical loading is needed to assess the applicability of C/SiC

composites. These effects, including oxidation and volatilization, are described further in

the upcoming sections.

The main problem of monolithic ceramics, that are suitable for high temperature

applications, is their inherent brittleness and catastrophic failure mode that lead to low

damage tolerance of the component under service condition [4]. SiC/SiC or C/SiC

composites are more widely employed for high temperature applications in hot section

components, like combustors, shrouds, airfoils, aerojet engines, and thermal barrier

systems of aerospace vehicles. These materials exhibit high thermal conductivity,

excellent shock stability, oxidation resistance and improved toughness compared to the

monolithic material. Careful selection of these elements can constitute the matrix, the

fibres, the fibre/matrix interphase and the external coating that will be suitable for heat

exchanger applications. The process of manufacturing, including chemical vapour

infiltration (CVI), polymer impregnation, polymer impregnation pyrolysis (PIP), liquid

silicon infiltration (LSI), of these composites possess different microstructures and

likewise the properties of each process-borne composites differs.

A new promising ceramic matrix composite (CMC) for high temperature application,

especially as wear resistant component, incorporates carbon nanotubes (CNTs) into

different ceramic matrices (Al2O3, SiC and Si3N4) [4]. However, further investigations

are required in order to confirm the effect of CNTs to ceramic matrices, ad their potential

use as materials for high temperature demanding applications.

7

1.1.2 Material Recession in Water Vapour Environment

The combustion environment contains about 10% water vapour, independent of

hydrocarbon type and fuel-air ratio. In water-vapour-containing environments, which are

predominant in combustor liners, turbine vanes for propulsion and power generation

applications, SiC and Si3N4 undergo an oxidation and volatilization reaction. The kinetics

of the oxidation reaction are described by the parabolic rate constant for oxide formation,

kp, whereas the kinetics of the volatilization reaction are described by the linear rate

constant for oxide volatilization, kl [5]. These reactions occur simultaneously and are

described by paralinear kinetics. The paralinear kinetic model has been developed for

simultaneous oxidation and volatilization of Cr2O3 formers by Tedmon [6] and is directly

applicable to the oxidation of SiO2 formers in water vapour.

Using SiC as a case study, SiC is thermodynamically unstable in an oxidizing

environment and forms an outer scale of SiO2. Because the SiO2 forms a protective layer,

which grows at a slow rate, SiC has been proposed for use in high-temperature oxidizing

conditions, such as combustion environments. In combustion environments containing

O2, CO2, and H2O, SiC can oxidize by any or all of the following reactions [3].

SiC + 2

3 O2 (g) = SiO2 + CO (g) (1.1)

SiC + 3CO2 (g) = SiO2 + 4CO (g) (1.2)

SiC + 3H2O (g) = SiO2 + 3H2 (g) + CO (g) (1.3)

Water vapor is found to be the primary oxidant, based on a comparison of the oxidation

rates of SiC in each gas. Also, in mixed oxidizing/reducing gases, such as H2O/H2 or

CO2/CO mixtures, the SiO2 scale can, in turn, be reduced by one of the following

reactions to form volatile SiO(g).

SiO2 + H2 (g) = SiO (g) + H2O (g) (1.4)

SiO2 + CO (g) = SiO (g) + CO2 (g) (1.5)

8

Similarly, in water-vapor-containing environments, the SiO2 scale may react to form

volatile hydroxides or oxyhydroxides by one of the following reactions.

SiO2 + H2O (g) = SiO(OH)2 (g) (1.6)

SiO2 + 2H2O (g) = Si(OH)4 (g) (1.7)

2SiO2 + 3H2O (g) = Si2O(OH)6 (g) (1.8)

SiO2 + 21 H2O (g) = SiO(OH) (g) +

41 O2 (g) (1.9)

2SiO2 + 3H2O (g) = Si2(OH)6 (g) + 21 O2 (g) (1.10)

Under conditions such as combustion environments, where both SiC oxidation and SiO2

volatilization occur, paralinear kinetics are observed. The overall sample weight change

observed for paralinear kinetics is the sum of the weight gain caused by the growth of the

scale and the weight loss caused by volatilization of the SiO2. At long times, oxide

growth occurs at the same rate that oxide volatilization occurs, so that a constant oxide

thickness is formed. After a constant oxide thickness is established, linear weight loss and

linear SiC recession rates are observed. Under conditions at which the volatility rate is

much greater than the oxidation rate, nearly linear weight loss and recession rates are

observed, even at short times. Thus, the rate of SiC recession is controlled by the

volatility rate of SiO2 rather than the oxidation rate of SiC [5].

1.1.3 Corrosion Effects in Molten Salt Environment

Silicon-based ceramics are widely used for high temperature industrial applications

because of their high oxidation resistance at temperature up to 1500oC. This resistance is

enhanced by the formation of protective silica layer on the surface of the materials during

oxidation. However in molten salt environment, the protective oxide layer is destroyed

and this lead to corrosion etching [7]. Gogotsi et al [7] investigated the salt-assisted

oxidation effects on mechanical properties of silicon-based ceramic materials. Their

studies reveal that molten sea salt and NaCl produced a very mild corrosion while molten

Na2SO4 contributed to a very severe corrosion.

9

Molten salts are ionic liquids obtained by the fusion of solid salts. The general

characteristics of molten salts are as follows: (a) liquid state over a large range of

temperature, (b) ability to dissolve a relatively large amount of many inorganic and

organic compounds, (c) low vapour pressure and stability at normal pressures, (d) low

viscosity, as the ions are mutually independent, for most of the cases, (e) chemical

inertness (no reaction with air or water), (f) high heat capacity per unit volume. These

and other characteristics allow their utilization in many processes not possible with

normal solvents [8]. The molten salts listed in Table 1 were selected for high temperature

heat recovery operations because of their high fusion points and large heats of

solidification [9]. The processes occurring during the corrosive Na2SO4 attack start with

the oxidation of the silicon-based material near 800C, and the salt melting occurs only at

890C [7].

Si3N4 + 3O2 = 3SiO2 + 2N2 (1.11)

2SiC + 3O2 = 2SiO2 + 2CO (1.12)

Table 1 Thermal Storage Capabilities of Molten Salts

Molten Salt Fusion Point

(K)

Heat of Fusion

(kJ/kg)

Specific Heat

(kJ/kg K)

NaCl 1073 483 1.228

Na2CO3 1131 280 1.819

Na2SO4 579 N/A N/A

Then after the salt melting, its interaction with the silica layer starts

2SiO2 + 2Na2SO4 = 2Na2SiO3 + 2SO2 + O2 (1.13)

leading to the dissolution of the silica layer. After the dissolution of the SiO2 layer, the

reaction of the salt with Si3N4 and SiC starts

10

6Na2SO4 + 2Si3N4 + 3O2 = 6Na2SiO3 +4N2 + 6SO2 (1.14)

Na2SO4 + SiC + O2 = Na2SiO3 + CO + SO2 (1.15)

with the silicon carbide dissolution being much more intensive than that of silicon nitride.

Takeuchi et al [10] studied the corrosion resistance of ceramic materials in pyrochemical

reprocessing using molten salts. It was reported that ceramic materials, which show good

corrosion resistance (0.1 mm/y), are Silicon Nitride (Si3N4), Alumina (Al2O3), Beryllia

(BeO), Mullite (Al6Si2O13) and Cordierite (Mg2Al3(AlSi5O18)). While zirconia (ZrO2) is a

top coat material for high temperature application in water-vapour environment, it is not

so excellent in molten salt environment because of impurity as CaO.

1.2 Description of the Barrier Coatings

A dry oxidative environment presents a suitable atmosphere for excellence performance

of silicon-based ceramics for high-temperature applications. Here, the growth of a

protective silica layer present a consistent oxidation resistance, and increases the stability

of the material under high temperature. However, in water-containing and/or corrosive

environments, deteriorating dimensional changes of the ceramic component take place.

Also, silicon-based ceramic materials are faced with the pressure of improving the

efficiency and performance of energy systems, especially diesel and turbine engines, by

increasing the operating temperature above the melting point of the constituent materials.

In either of these scenarios, the design of a barrier coating system is inevitably required to

reduce the effects of the harsh oxidative, corrosive and thermal environments.

1.2.1 Thermal Barrier Coatings

A Thermal Barrier Coating (TBC) system is composed by four constituents: i) the nickel,

alumina or cobalt-based superalloy substrate, ii) the metallic bond-coat layer of 75150

m in thickness made of platinum or nickel aluminide or of MCrAlY alloy (M = Ni, Co,

11

Fe), iii) the thermally grown oxide (TGO), mainly a -Al2O3, with a thickness ranging

from 1 to 10 m, which is created by the oxidation of the bond-coat during ceramic top-

coat deposition and grows during service operation, and iv) the thermal insulator ceramic

top-coat with 100500 m in thickness (depending on the deposition technique used),

where 78 wt% of yttria-stabilized zirconia (YSZ) layer is the ceramic most common

used [4]. The excellent mechanical, chemical and thermal properties of YSZ, which are

the basic requirements a ceramic barrier coating must fulfil, include high thermal

stability, chemical inertness, no phase transformation between room temperature and the

operating temperature, low sintering rate of the porous ceramic microstructure, excellent

adherence to the rest of the TBC system and, in particular, low thermal conductivity.

Atmospheric plasma spraying (APS) and electron-beam physical vapour deposition (EB-

PVD) are the most common deposition techniques to obtain a ceramic thermal barrier

coating although, to a lesser extent, low-pressure plasma spraying (LPPS), high-velocity

oxygen fuel spraying (HVOF) and chemical vapour deposition (CVD) are also employed.

Comparing the two main deposition processes, i.e. APS and EB-PVD, great differences

in the properties of the YSZ top-coatings can be found due to the distinct coating

morphologies. While APS leads to the orientation of the pores between the splats parallel

to the substrate surface, reducing the YSZ bulk theoretical thermal conductivity values

from 2.2-2.6 Wm1K1 to 0.81.7 Wm1K1, the columnar grain microstructure

created by EB-PVD process contains channels between the columns and pores within the

grains which are oriented perpendicular to the substrate surface, provoking a lower

reduction in the thermal conductivity than APS (1.5-2.0 Wm1K1) [4]. Conversely,

the columnar distribution of the grains allows an increase on the strain tolerance of the

TBC and, hence, EB-PVD coatings present greater durability than APS ones. Taking into

account the different behaviour of the TBC deposited by both techniques, large

components operating at relatively low temperatures are mainly coated by APS, such as

blade and vanes in gas turbine engines, and fuel vaporizers, after-burner flame holders

and stator vanes in aircraft engines. On the contrary, relatively small components

working in harsher applications, such as blade and vanes in aircraft engines, are coated by

EB-PVD despite its higher manufacturing cost.

12

Increasing research activities are ongoing by using the EB-PVD techniques to improve

the thermal properties of the TBC, including compositional changes, multiple layer

design and depositional techniques. New design methodology of implementing these

techniques, using micro-scale numerical modelling, can be incorporated for effective

analysis and reduction of product design cost and time. Major aims to achieve the desired

thermal property improvement include: 1. to use many thin alternating layers in order to

achieve significant interfacial resistance; 2. to increase or control porosity; and 3. to

decrease the inherent thermal conductivities by increasing the atomic scale disorder. It

has been established that micro-structural analysis of TBC can improve the performance

of high-temperature ceramic materials [11].

1.2.2 Environmental Barrier Coatings

In water-vapour-containing environments, the excellent protective silica layers of silicon-

based ceramic materials are degraded by reacting with impurities, such as molten salts

[12] and/or water vapour [13]. Figure 1 presents the development of EBC, starting from

the use of mullite as coating for a solar turbine, in order to protect SiC heat exchanger

tubes from corrosion. By the mid 1990s, the volatilization of silica in water vapor and the

resulting rapid recession of silicon-based ceramics emerged as major challenges for the

use of silicon-based ceramics in combustion environments, shifting the focus of coatings

13

Copyright by Surface and Coating Technology*.

Figure 1 Chronological Evolution of Mullite in the Design of EBC

research to protection from water vapour [14]. Major durability concerns in the mullite-

based coating system include 1. Through-thickness cracking in the mullite; 2. Weak

boding of mullite onto silicon-based ceramics; and 3. Interface contamination [14].

Through-thickness-cracks open up access for water vapour to oxidize the substrate,

leading to the eventual failure of the system. The YSZ overlay coating failed to seal the

cracks in mullite since YSZ also cracked due to the large CTE mis-match between the

two layers. However, it is believed that the development of through-thickness-cracks in

mullite is due to stresses in the coating. The presence of second phases, such as residual

amorphous mullite and alumina, in the mullite coating and the resulting volumetric

* 133-134 (2000) 1-7

[7] J.R. Price, M. van Roode, C. Stala, Ceramic oxide-coated silicon carbide for high temperature corrosive environments, Key Eng. Maters. 72-74, 1992, 71-84.

[8] J.I. Federer, Alumina base coatings for protection of SiC ceramics, J. Mater. Eng. 12, 1990.141-149. [9]- [17] K.N. Lee, R.A. Miller, N.S. Jacobson, New generation of plasma-sprayed mullite coatings on silicon-carbide, J. Am.

Ceram. Soc. 78, 3. 1995. 705-710 [18] M.L. Auger, V.K. Sarin, The development of CVD mullite coatings for high temperature corrosive applications, Surf. Coat.

Technol. 94-95, 1997.46-52 [19] J.A. Haynes, K.M. Cooly, D.P. Stinton, R.A. Lowden, W.Y. Lee, Corrosion-resistant CVD mullite coatings for Si3N4, Cer-

amic Engineering and Science Proceedings, vol. 20, 4., The American Ceramic Society, Westerville, OH, 1999, pp. 355-362

14

shrinkage and CTE mis-match are suggested to be the major sources for the stresses in

the coating. Mullite does not form a strong chemical bond with SiC according to a

diffusion couple study. Interfacial contamination can degrade coating durability by

altering the physical and chemical properties of the silica scale, especially growth rate,

viscosity and porosity.

15

2. HIGH-TEMPERATURE APPLICATIONS

High-temperature applications and utilization for power generation offers immense

possibility of reduced emissions, enhanced energy efficiency, and smaller

equipments. Recoveries of both energy and ecology, in effect, have continued to

represent ongoing research investigation towards efficient processing, optimization

and utilization of high-temperature materials. Power generation procedure through the

combustion of fossil fuel produces both high energy-carrying and ecology-polluting

flue gases upon which all emerging recovery technologies (For example, heat

recovery and membrane separation technologies) are subjected. Energy conversion

optimization processes (including enumerate-accelerating computations) involving

thermal coupling application of the high temperature waste heat from the flue gases

can increase the efficiency of the power generation plant. Example of such energy

conversion systems include high-temperature Solid Electrolyte Fuel Cells [15], and

high-temperature Compact Heat Exchanger [16] (whose optimization strategy will

briefly described in the upcoming chapter) as presented below.

2.1 Solid-Electrolyte Fuel Cells

Conventional power plant with coal combustion can waste up to 60% of the useful

energy from the combustion process, unless an effective energy recovery system is

integrated within the process cycle. Since the irreversibility due to the combustion

process is proportional to the chemical affinity of the reaction, fuel cell is capable of

circumventing the combustion irreversibility by electrochemically lowering the

chemical potential of the fuel and/or oxygen while producing electricity. High-

Temperature Solid-Electrolyte Fuel Cell (HTSEFC) is a high-efficiency device which

can be thermally coupled with coal gasifiers [15]. The thermal coupling of the

endothermic coal gasification reactor and the exothermic HTSEFC device has

enabled the direct utilization of the high-temperature waste heat generated by the fuel

cell. However, one of the major problems with HTSEFCs commercialization is the

16

need for stable interconnection materials that are compatible with other fuel cell

components.

2.1.1 Interconnection Material Selection

High temperature fuel cells are promising source of alternative energy, promising low

environmental emission and high efficiency. However, the corrosion of the anodic side of

the fuel cell is the limiting factor against optimized performance. The operating

conditions on the anode side, in fact, are more severe and the material must resist to a

reducing and carburizing environment at high temperature and in presence of molten

carbonates that partially impregnate the anode [17]. Ceramic fuel cells are all-solid state

energy conversion systems that directly convert chemical energy of a fuel to electricity in

a clean and efficient way. Conventional ceramic fuel cells with yttria stabilized zirconia

(YSZ) as electrolyte are operated at high temperatures (8001000 oC) to achieve

sufficient ion conduction in the electrolyte, which brings about various disadvantages in

long-term stability of the cell, manufacturing cost, etc. upon commercialization [18].

Current developments in high-temperature ceramic fuel cells include the minimization of

the degradation due to strontium diffusion from the cathode material to the electrolyte,

thereby maintaining the stability performance of the fuel cell over a long period of time.

Uhlenbruck et al[19] observed an improvement in the performance of ceramic high

temperature fuel cells by applying a vapour-deposited (Ce, Gd)O2 diffusion barrier

coating. However, there is still an uncertainty behind the performance deviation when the

substrate is heated to a temperature up to 800 oC. For example, yttrium doped ceria-based

electrolytes become unstable in the reducing fuel environment because of the instability

of ceria. Two major methods of mitigating the instability problem include the use of a

nanostructured thin-film ionic-doped ceria to enhance the ionic conductivity and

chemical instability. The other method is to prepare and use two-phase composite

ceramic materials, like GDC-NaCl and GDC-Al2O3, to suppress electronic conduction,

thereby enhancing the stability of the material in the fuel cell environment [20]. Typical

kinetics of the transport systems within a solid electrolyte fuel cell is outlined in the next

section.

17

2.1.2 Model Kinetics for the HTSEFC

The ideal driving force for the H2-Air fuel cell is given by

( )F

GK

F

RTFvoltsinforceDriving

OH

P2

ln2

)(, 2

= (2.1)

where ( )PK , R and F are the equivalent, gas, and Faraday constants respectively. OHG 2

is the Gibbs free energy change of the electrochemical reaction, which is a function of

temperature at the interface of the electrolyte and the anode. The fluids in the fuel cell

channels have multiple components. In an air channel (cathode), the fluid consists of

oxygen and nitrogen. In a fuel channel (anode), it consists of hydrogen and water vapour.

The heat/mass transfer and fluid flow are coupled with each other through the

interconnection material.

The governing equations for flow, temperature and species concentration are:

( ) 0= u (2.2) ( ) ( )upu += (2.3) ( ) ( ) qTkuTcP += (2.4) ( ) ( )

mimii SYDuY += , (2.5)

The domain borders between cell stacks and along the ribs for heat/mass transfer are

assumed to be adiabatic. The conditions for the chemical species at the interfaces

between the active solid and gas streams must satisfy the mass flux balances by following

equations [21]:

2222

2

2

, HHOHH

H

HYD

A

M= (2.6)

OHOHHOH

OH

OHYD

A

M

2222

2

2

, = (2.7)

2222

2

2

, OONO

O

OYD

A

M= (2.8)

18

where 2H

A , OHA 2 and 2OA are the active interface area of solid and hydrogen, water

vapour, and oxygen respectively. The total heat produced in an operating fuel cell

consists of three parts including chemical reaction heat, irreversibility heat and Joule

heat. The chemical reaction heat is calculated based on the electrochemical reaction heat

through the thermodynamic relationship and considered to be applied evenly where

electrochemical reaction occurs. Ongoing studies of irreversibility in fuel cell have not

been able to estimate losses beyond activation and concentration changes [21]. Micro-

structural changes due to flow kinetics within the interconnection materials and barrier

coatings can affect the performance of the fuel cell.

2.2 Compact Heat Exchanger Design

The design of thermofluid systems, upon which all energy systems involved in combined

power generation cycle are based, depend on the modeling of the transport of constituent

mass and heat quantities within the system. The complexities of these models will depend

on the nature of constituency flows, the boundary and the operating conditions. While

adequately set-up experimental techniques represent consistent modeling tool, by

producing more reliable physical data; these techniques can be prohibitively expensive

and time consuming. Since the advent of computers, numerical modeling of complex

problems provides an alternative effective tool for product development. Consequently,

the design cycle approach involves the use of experimentally validated numerical model

for the development of all energy system components.

Recently, combined cycles have been proved to offer the most efficient way to generate

electricity. With the combustion of gas, the flue gas can directly drive the combined

turbine cycle. However, the combustion of coal results in flue gases with ash particles

and chemically aggressive slag can quickly damage the turbine vanes, if the turbine is

internally combined with the combustor. Since the Vertical Combustor (VC) is designed

for different modes of combustion, including the combustion of coal and bitumen,

externally run turbine cycle can be integrated with the existing system. Consequently, the

19

proposed design will require advanced high-temperature heat exchanger, for needed

energy recovery from flue gases.

Compact heat exchanger are been in use for many applications, including automobile

radiators, air-conditioning evaporators and condensers, electronic cooling devices,

recuperators and regenerators, and cryogenic exchangers. Compact surfaces offer high

heat recovery advantage due to their lightweight, space-saving features. The basic plate

fins for this heat exchangers includes plain rectangular, plain triangular, wavy, offset

strip, perforated, louvred, etc. [22].

Conventionally, the heat duty requirement in the design of heat exchangers demands the

determination of the Colburn j factor and the Fanning friction f factor as functions of

Reynolds number. Different surface configurations depicted in various plate fins, stated

above, present different size and shape relationships with varied performances. However,

of these many enhanced fin geometries, offset strip fins are widely used, especially for

high temperature applications. They offer a high degree of surface compactness, and

substantial heat transfer enhancement. This is due to the inherent periodic building and

collapsing laminar boundary layers over the uninterrupted channels formed by the fins

and their dissipation in the fin wakes [23]. Three effects are three-dimensional and cannot

be captured without stable and accurate model. Experimental validated numerical

approach proposed in this project is capable of modeling these surfaces with three-

dimensional flow-solver.

2.2.1 Criteria for Material Selection

Considering the complexity of satisfying the material demands of high-temperature heat

exchangers, for use in a vertical combustor which is subjected to varied mode of

combustion, a separation of function had to be applied by using an environmental barrier

coating (EBC) that ensures the corrosion stability. This selection focuses on the thermo-

mechanical stability of the material and the gas impermeability. Suitable materials under

consideration for this application include Nimonic PK33 [24], Reaction Bonded SiC [25]

20

and Calcined Alumina [26]. The criteria for selection will include, but not limited to, low

coefficient of thermal expansion CTE, low Youngs modulus E, high thermal

conductivity k, durability and cost. Available properties of these materials are shown in

Table 2.

Table 2 Comparison of the material properties at elevated temperature

900

1127

217

10.6

27.2

PK3390

16001700920Tmax [oC]

4003001175b,bend[MPa] (20oC)

410340204E [GPa] (20oC)

4.67.512.7CTE [10-6 K-1] (960oC)

40627.9K [W/m K] (1000oC)

SSiCAl2O3Nimonic

900

1127

217

10.6

27.2

PK3390

16001700920Tmax [oC]

4003001175b,bend[MPa] (20oC)

410340204E [GPa] (20oC)

4.67.512.7CTE [10-6 K-1] (960oC)

40627.9K [W/m K] (1000oC)

SSiCAl2O3Nimonic

2.2.2 Maximum Thermal Conductivity

Table 2 above shows a comparison of the material properties of some selected materials

proposed for the manufacturing of the plate-fin ceramic heat exchanger. In order to obtain

the complete comparison with the selection of a suitable EBC for the operating condition,

material testing experiments will be conducted in the laboratory. This effort will enable a

proper protection of the designed energy system from substance from oxidation.

Considering the fact that combination of the different materials is inevitable for the heat

exchanger design, additional challenge is posed in order to ascertain that an optimized

material selection with combined maximum thermal conductivity is presented. This

criteria is different for other high-temperature applications where higher gradient in the

temperature distribution is encouraged with the design of the required

thermal/environmental barrier coatings.

21

2.3 Ceramic Membranes

High temperature ceramic membranes are increasingly important for applications

requiring chemical reactions and separation at elevated temperature. This procedure is

extremely relevant for post-combustion separation of CO2 in the flue gases from fossil

fuel combustion for power generation. Porous nanostructured materials are ideal

candidates for this purpose because of their high surface area and large porosity. The

simple fact that the initial surface areas of these materials are very high, usually above

100 m2/g, will make them texturally unstable during high-temperature treatments [27].

The three concepts that have emerged on how to include CO2 capture in power generation

processes that employ fossil fuels 1. Post-combustion capture, where the CO2 in the

exhaust gas coming from a standard gas turbine combined cycle, or a coal-fired steam

power plant is captured through the use of chemical or physical solvents (e.g. amine

scrubbing); 2. Oxy-fuel combustion capture, where O2 is used as fuel oxidising agent

instead of air. The use of the term oxy-fuel combustion often refers to combustion of

natural gas or coal with CO2 recycled from the exhaust as the inert gas to keep

combustion temperatures at a permissible level; and 3. Pre-combustion fuel

decarbonisation, where the carbon of the fuel is removed prior to combustion, whereby

the fuel heating value normally is transferred to hydrogen [28].

Current advances in membrane separation for gases involve the use of an integrated high

temperature separation technology. This technology will enable more efficient power

generation system, by reducing pressure loss and wastage of process heat. This promising

technology involves the use of inorganic membrane, where the most relevant separations

are integrated oxygen separation/production from air, separation of hydrogen from

CO2/CO/CH4/H2O, or separation of carbon dioxide from H2/H2O/CO/CH4. However, the

separation of carbon dioxide, unlike others, with the high-temperature CO2-selective

membrane is still in an early technological stage. Most of other separations, involving

H2O, H2, CO, and CH4, with much smaller molecules compared to CO2 can be separation

using Knudsen diffusion mechanism. Integrating molten carbonate fuel cell technology,

22

with liquid carbonate membranes, may also have potentials in CO2 capture. Both

generation of electricity and simultaneous CO2 capture, and electrochemical pumping of

CO2 are modes of operation represent an exciting integration cycle with potential

increase in plant efficiency.

Ceramic based composite materials have been found useful candidates for coating

barriers for high-temperature applications, especially for power generation systems.

These applications, however, presented rigorous challenges depending on the specific

operating conditions of the energy conversion system. This feasibility study focuses on

the challenges involved with the application of ceramic materials for the design of

Compact Heat Exchanger.

23

3. EBC-ENHANCED HEAT EXCHANGER OPTIMIZATION

Environmental Barrier Coatings (EBC) for High Temperature Compact Heat Exchanger

prevent the material degradation of the silicon-based ceramics used in the design of the

component, without significant increase in the temperature gradient across the coatings.

Optimization of the performance of the system, therefore, must take due consideration of

the heat/mass transfer of the fluid systems and the micro-structural kinetics within the

EBC. The Second Law equation describes the state of irreversibility within the

boundaries of energy systems. Due consideration of this law in addition to the First Law

can provide better estimate of the quality of available heat energy recoverable via the

serving stream of the heat exchanger. These two laws of thermodynamics can be written

as [22]:

+++== out

t

in

t

n

li

iO hmhmWQQdt

dE&&&&& (3.1)

and

++== out

t

in

t

n

li i

i

O

O

gen hmhmWT

Q

T

Q

dt

dSS &&&

&&& (3.2)

By eliminating OQ& from Eqns (3.1 & 3.2), the work rate output can be maximized as:

[ ] ( ) ( ) genOOtout

Ot

in

i

n

li i

O

O STsThmsThmQT

TSTE

dt

dW &&&&& +

+=

=

1 (3.3)

Since genS& cannot be negative, the maximum possible work from the system is obtained

at the minimum value of genOST& , known as the lost available work or Gouy-Stodola

theorem. In order to understand the application of this theorem to heat exchanger design,

it will be useful to comprehend the process of entropy generation via the interaction of

the streams with the walls.

24

3.1 Entropy Generation in Heat Exchanger

System optimization demands exergy analysis for all energy systems where power or

refrigeration effect is operational. In this case and as it applies to heat exchanger design,

the First Law which deals with the conservation of energy will not be adequate, in order

to capture the heat and work interaction through the conjugate system.

3.1.1 Basic Components of Entropy Generation

From the First Law:

dxqdhm =& (3.4)

And assuming steady state condition with no work and heat loss or gain from the

environment, the Second law states that:

0+

= dsmTT

QdSd gen &

&& for each side, (3.5)

while the sign denotes either the hot or cold stream of the heat exchanger. Now, the

canonical entropy relationship states that

dx

dp

dx

dsT

dx

dh

1+= (3.6)

Therefore, entropy generation term (after linking Eqns 3.4 - 3.6) is

( )

+

+

==

dx

dp

T

m

T

Tq

dx

dSS

gen

gen

&&

12 (3.7)

25

where TT /= , the dimensionless temperature difference. This equation reveals that

the two basic components of entropy generation, including the temperature gradient term

and the pressure gradient term. Since the heat transfer gradient is directly proportional to

the temperature gradient, it implies that the entropy generation rate for the thermal

component is proportional to the square of the dimensionless temperature difference ,

and this term plays a vital role in the minimization of the generation of entropy within the

energy system.

3.1.2 Minimization of Entropy Generation

Figure 2 reveals the temperature profile for a typical counterflow heat exchanger. Writing

Eqn 3.7 in a differential form,

( ) ( )S

S

SS

P

P

PP

SP

genp

dpRm

p

dpRm

TTT

dxq

TTT

dxqSd &&&

+

+

+

= (3.8)

Fictional entropy generation due to pressure drop for liquids (and for limiting perfect gas

flow assumption) is negligible, due to the high density in the last two terms in Eqn 3.8.

Simplified analysis, with this assumption of zero pressure drops, has adopted two

approaches including the balanced counterflow [29] and the flow imbalance [30]

procedures. However, the design of compact heat exchanger with imbalanced streams

and with possible differential pressure ratios cannot be analysed based on this

assumption.

Substantial correlations for the heat transfer and pressure drop in offset strip fin heat

exchanger (see Figure 3) are available in the literature [31]. Although, many of these

efforts are dominated by experimental investigation [32], analytical models and

numerical solutions [33] have also been developed. Despite the preceding investigative

efforts, the prediction of the heat transfer and pressure drop along the channels of offset

strip fin heat exchanger remains difficult, and grossly oversimplified.

26

T1P

T2P

T2S

T1S

T

{ }cp

cm&

{ }hp

cm&

wall

dxT1P

T2P

T2S

T1S

T

{ }cp

cm&

{ }hp

cm&

wall

T1P

T2P

T2S

T1S

T

{ }cp

cm&

{ }hp

cm&

wall

dx

Figure 2 Temperature Profile for Counterflow Heat Exchanger

Considering the broad application of offset strip fin heat exchanger, reliable prediction of

heat transfer and friction factors is necessary.

Figure 3 Description of the Offset Strip Fin Arrangement

X Y

xyz

X

Z

X Y

xyz

X

Z

27

Apart from the geometry of the fins, the thermal properties of the flue gas can play a

significant role in the heat transfer and pressure drop characteristics. The effect of flue

gas radiation on the performance of a compact ceramic heat exchanger has been reported

by Chen et al [34]. It was reported in their numerical study that, the predicted Nusselt

number with surface and gaseous radiation heat transfer was averagely higher than the

Nusselt number without radiation heat transfer by 7%. Similar trend was observed for the

friction factor comparisons, while the increment in this case was 5%.

3.2 Compatibility Criteria

3.2.1 Material Compatibility

One of the major challenges with the design of EBC-enhanced heat exchanger is the

compatibility of the silicon-based substrate with the combination of materials for the

environmental barrier coatings. Figure 4 shows a three-layer design comprising a bond

coat, intermediate coat, and top coat. For example, Zhu and Miller [35] designed a

thermal barrier coating for an advanced propulsion engine system including ZrO2-Y2O3-

Nd2O3(Gd2O3, Sm2O3)-Yb2O3(Sc2O3), and their thermal conductivity, sintering behavior

and cyclic durability were investigated at high temperatures. The advanced TBC systems,

typically consisting of a 180-250 m ceramic top coat and a 75-120 m NiCrAlY or

PtAl intermediate bond coat, were either plasma-sprayed or electron-beam physical

vapour deposited on to the 25.4 mm diameter and 3.2 mm thick nickel base superalloy

(Ren N5) or mullite/mullite+barrium strontium aluminosilicate (BSAS)/Si coated

SiC/SiC CMC disk substrates. The plasma-sprayed coatings were processed using

prealloyed powders. The ceramic powders with designed compositions were first spray-

dried, then plasmareacted and spheroidized, and finally plasma-sprayed into the coating

form. The advanced EB-PVD coatings were deposited using pre-fabricated evaporation

ingots that were made of the carefully designed compositions.

28

ZrO2/YS/CAS/BSAS/MAS

Mullite

SiC/Si3N4

Bondcoat

Substrate

TopcoatIntermediate coat

ZrO2/YS/CAS/BSAS/MAS

Mullite

SiC/Si3N4

Bondcoat

Substrate

TopcoatIntermediate coat

Figure 4 Three-layer Design of EBC

3.2.2 Performance Compatibility

Because of the relatively porous nature of the ceramic coating, thermal conductivity may

increase considerably due to coating sintering and phase structure changes after a long-

term thermal exposure. Therefore, evaluation of the initial and post-exposure thermal

conductivities, and the rate-of-conductivity-increase is crucial in characterizing the

coatings performance. The coating thermal conductivity change kinetics for 8YSZ can

be expressed as [36]

=

t

RTkk

kkO

cc

O

cc exp168228

exp2.102inf

=

RT

41710exp5.572

where ck is the coating thermal conductivity at any given time t, O

ck and inf

ck are

ceramic coating thermal conductivity values at the initial time and at infinitely long time,

respectively, R is gas constant, and is the relaxation time.

3.3 Modelling Procedures in Heat Exchanger Design

3.3.1 Thermo-fluid Design

Figure 5 shows the computational domain from section X-Y of the heat exchanger.

Navier stokes equations, comprising the conversion of mass, momentum and energy will

29

be used for Nusselt number and pressure drop calculations. The discrete transfer radiation

model will be incorporated into the numerical model for radiation calculation.

Figure 5 Section X-Y showing the 2-D computational domain for thermo-fluid

modeling

The ceramic heat exchanger will be designed based on counter flow configuration and

analyzed using LMTD-method.

3.3.2 Thermo-structural Design

Thermo-structural analysis of the heat exchanger represents a significant aspect of the

design because of the brittleness of the ceramic materials. This analysis will investigate

the stress distribution of the base element and the core section at the operating condition.

The predicted temperature distribution for the flue gas and the process gas will be used as

the boundary condition.

Figure 6 shows the computational domain from section X-Z of the heat exchanger. Three

dimensional finite element modeling of the structure, including the environmental barrier

coatings, base plate and the offset strip fins, will be carried out. Micro-structural changes,

phase and molecular transformation within the environmental barrier coatings require

Computational domainPeriodic boundary

Inlet Outlet

Offset strip fins Base plate

Computational domainPeriodic boundary

Inlet Outlet

Offset strip fins Base plate

30

modeling that cannot be predicted with continuum assumption. Therefore, hybrid

computational technique, comprising both solution with Navier-Stoke equations for the

fluid system and molecular dynamics for the coating systems will be inevitable.

Figure 6 Section X-Z showing the 2-D computational domain for thermo-structural

modeling

Process gas

Flue gas

Computational domain

Offset strip fins Base plate

l

h

Process gas

Flue gas

Computational domain

Offset strip fins Base plate

l

h

31

4. CONCLUSION

A feasibility study of the use of ceramic materials as environmental barrier coatings for

high temperature energy conversion applications has been presented. High temperature

applications have the potential of boosting the efficiency of the power generation cycle,

when integrated with other system components (like fuel cell, membranes and heat

exchanger) in such as a way that process heat wastage and pressure drop are reduced. The

design procedure for compact heat exchanger with environmental barrier coatings was

illustrated. The computational techniques, using hybrid formulation of both continuum

Navier-Stoke equations and molecular techniques, are proposed. The design procedure

will enable comprehensive material selection process, thermo-fluid design and thermo-

structural modeling for efficient and optimized energy conversion.

32

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