Addressing Rare-Earth Element Criticality: An Example from the Aviation Industry
Addressing Rare-Earth Element Criticality: An Example from the Aviation Industry
0 1.-GE Global Research , 1 Research Circle, Niskayuna, NY 12309, USA. 2.-GE Aviation, 1 Neumann Way, Evandale, OH 45215, USA. 3.-Solvay, 350 George Patterson Drive, Bristol, PA 19007-3624, USA. 4.-
Rare-earth (RE) elements are enablers for a wide range of technologies, including high-strength permanent magnets, energy-efficient lighting, hightemperature thermal barrier coatings, and catalysts. While direct material substitution is difficult in many of these applications because of the specific electronic, optical, or electrochemical properties imparted by the individual rare-earth elements, we describe an example from the aviation industry where supply chain optimization may be an option. Ceramic matrix composite engine components require environmental barrier coatings (EBCs) to protect them from extreme temperatures and adverse reactions with water vapor in the hot gas path. EBC systems based on rare-earth silicates offer a unique combination of environmental resistance, thermal expansion matching, thermal conductivity, and thermal stability across the service temperature window. Several pure rare-earth silicates and solid solutions have been demonstrated in EBC applications. However, all rely on heavy rare-earth elements (HREEs) for phase stability. This article considers the possibility of using separation tailings containing a mixture of HREEs as a source material in lieu of using the high-purity HREE oxides. This option arises because the desired properties of RE-silicate EBCs derive from the average cation size rather than the electronic properties of the individual rare-earth cations. Because separation tailings have not incurred the costs associated with the final stages of separation, they offer an economical alternative to high-purity oxides for this emerging application.
Rare-earth (RE) elements enable a wide range of
technologies, including high-strength permanent
magnets, energy-efficient lighting,
high-temperature thermal barrier coatings, and catalysis. Their
importance to industry has led several
organizations, including the U.S. Department of Energy, the
European Union, and some private companies, to
classify them as ‘‘critical’’ materials.1–4 This
designation is applied to important raw materials where
concerns exist about the supply. In the case of
rareearth elements, this supply risk comes from the
recent concentration of the supply chain in China
and was most acutely felt during a pricing bubble in
In recent years, there has been increasing focus
on a subset of the rare-earth elements that face the
tightest balance of supply and demand: europium,
terbium, neodymium, praseodymium, and
dysprosium. Direct material substitution for these
elements is difficult because of their specific electronic,
optical, or electrochemical properties. For example,
the electronic band gaps of europium and terbium
make them specifically suited for use in red and
green phosphors, respectively.6 Similarly,
dysprosium and terbium are uniquely suited to improving
the high-temperature performance in NdFeB
permanent magnets. Although these elements deserve
heightened attention in the near term, it is
important to recognize that technology development can
radically alter the situation in the longer term. For
instance, the introduction of material or system
substitutes can mitigate some of the supply risk,
while the emergence of new applications can exert
upward pressure on demand.
This article considers the use of heavy rare-earth
elements for an emerging application in the aviation
industry, namely environmental barrier coatings
(EBCs) for ceramic matrix composites (CMCs).
Heavy rare-earth elements are the least abundant,
making their markets especially sensitive to the
introduction of new applications. However, the
technical needs for the EBC application afford some
flexibility in the specific rare-earth elements
required and thus presents an opportunity to reduce
the supply risk through creative use of the available
materials. After a brief introduction of the technical
requirements driving the use of heavy rare-earth
elements (HREEs) in EBCs, we describe the
possibility of using tailings comprising an unseparated
mixture of HREEs and present experimental data
showing the technical feasibility of this approach.
MATERIAL SELECTION FOR
ENVIRONMENTAL BARRIER COATINGS
EBC Requirements and Candidate Material
CMCs are a class of rapidly emerging materials
that enable increased efficiency in turbine engines for
both aviation and power generation applications.7 In
aircraft engines, SiC-SiC CMCs offer the added
benefit of weight savings because components made from
CMCs have approximately one-third the mass of a
comparable part made using a nickel superalloy.8
CMC components running in the hottest engine
locations require EBCs to reduce the CMC
operating temperature and to prevent recession due to
reaction with water vapor in the hot gas path.9 The
CMC/EBC system is shown in Fig. 1.
The key material requirements for an EBC
system include: 9
1. Environmental resistance under engine
operating conditions. The coating must be resistant to
reaction with water vapor while blocking water
ingress into the underlying CMC.
2. Thermal expansion matching with the
underlying CMC. Low coefficient of thermal expansion
(CTE) mismatches are necessary to prevent
cracking, delamination and spalling of the
coating. The CTE of SiC and SiC/SiC materials is
between 4.5 and 5.5 9 10 6/ C.
3. Low thermal conductivity. Candidate EBC
materials should have a thermal conductivity below
3 W/m-K so as to provide an effective thermal
barrier between the combustion gases and the
4. Phase stability over the service temperature
window. Candidate materials should be phase
stable and thermochemically compatible with the
underlying CMC up to at least 1350 C.
Crystallographic phase changes accompanied by large
shear or dilational strains are especially to be
avoided, as these can lead to cracking,
delamination, and spallation of the coating.
EBC development has progressed through several
generations of materials systems.10 Initial work
through the 1990s focused on mullite, which had a
good CTE match but suffered from silica
volatilization at high temperatures. This issue was partially
mitigated through the addition of a
yttrium-stabilized zirconia (YSZ) top coat, but the multilayer
coating was limited in cyclic durability because of
the relatively high CTE of the zirconia. EBCs based
on barium strontium aluminosilicates (BSAS) were
introduced in the 2000s. Field tests of these coatings
over thousands of hours raised concerns about the
long-term stability and temperature capability of
this material system. Since then, EBCs based on the
rare-earth silicates have been identified as top
candidates for high-temperature, high-cycle
Fig. 1. Schematic of a CMC/EBC system in a jet engine.
Rare-Earth Silicate EBCs
Rare-earth silicates exist as monosilicates (RE2
SiO5) and disilicates (RE2Si2O7). While the
monosilicates exhibit lower silica activity and hence
are more stable in high-temperature water vapor,
the disilicates have a better CTE match to SiC.11 Of
the disilicates, Y2Si2O7 possesses perhaps the best
balance of properties and affordability; however, it
exhibits up to five polymorphs up to its melting
point, including a Type C C2/m (b) to Type D P21/c
(c) monoclinic phase transition between 1300 C and
1350 C.10,12 This phase transformation has been
seen to be accompanied by exaggerated grain
growth and subsequent microcracking after thermal
cycling (because of the highly anisotropic thermal
In the rare-earth disilicates, the b fi c phase
transition temperature seems to increase with
decreasing size of the rare-earth cation, as shown in
Fig. 2.13–15 Thus, the smallest-cation disilicates,
Yb2Si2O7 and Lu2Si2O7, do not exhibit polymorphs
to temperatures over 1700 C. This trend also
applies to solid solutions, wherein the average ionic
radius of the rare-earth elements is the key
parameter governing the phase stability.16 For
example, substitution of Lu into (Y1 xLux)2Si2O7
was shown to produce solid solutions that progress
from four high-temperature phase transformations
between 1,225 C and 1,535 C to a single phase
exhibiting stability to temperatures above
1650 C.15,17 A similar stabilizing effect was
observed with increasing Yb substitution into
(Y1 xYbx)2Si2O7.12 In quantitative terms, rare-earth
disilicate stability seems to require an average
cation radius less than about 0.88 A.12
Based on these observations, we sought to
establish whether phase stability can be demonstrated
with rare-earth disilicates based on unseparated
mixtures of heavy rare-earth elements from
tailings, provided that the average cation radius
criterion is satisfied.
FEEDSTOCK SUPPLY CONSIDERATIONS
The rare-earth separation process is shown
schematically in Fig. 3.18 Rare-earth elements are
mined together. Raw ores are pulverized in a
beneficiation process, and the rare-earth elements are
dissolved in a solvent. Individual rare earths are
separated using a solvent extraction process.
Lighter rare-earth elements are more abundant and
typically are extracted earlier in the separation
process. Heavier rare-earth elements, such as
europium, gadolinium, terbium, and dysprosium
follow. Yttrium is a lighter rare earth that is often
separated alongside these heavier rare-earth
elements because of its chemical similarity. The
heaviest elements, such as Tm, Yb, and Lu, occur at
much lower concentrations and are separated at the
end of the process. Given the small quantities of
material and the costs needed for complete
separation, these elements are commonly retained as
unseparated tailings. Tailings samples that have
the proper mixture of Tm, Yb, and Lu represent a
potential feedstock for rare-earth element disilicate
The key technical hypothesis is that tailings
containing mixtures of Tm-Yb-Lu that meet the
average ionic radius criterion can form disilicates
without polymorphism to temperatures of at least
To test this hypothesis, a tailings sample
containing a mixture of Y, Tm, and Yb was obtained
from Solvay. The sample was chemically analyzed,
mixed with an appropriate amount of silica, and
pressed into pellets. These pellets were heat treated
to convert them into sintered RE disilicate test
specimens, and x-ray diffraction (XRD) was used to
determine whether the samples had undergone the
irreversible b fi c phase transition.
Tailings Sample Composition
The chemical composition of the tailing samples
were determined using inductively coupled
plasmaoptical emission (ICP-OE) spectroscopy. Table I lists
the compositions of the as-received material. The
tailings are enriched in Yb because of the enhanced
natural abundance of rare-earth elements having
even atomic numbers.
The average ionic size for the tailings sample can
be computed from the composition and ionic radii of
the constituent rare-earth elements. The cation
sizes for Ho, Er, Tm, Yb, Lu, and Y are 0.90 A˚ , 0.89 A˚ ,
0.88 A˚ , 0.868 A˚ , 0.861 A˚ , and 0.89 A˚ ,
respectively.16,17 The weighted average ionic size is
0.87 A˚ , comfortably below our stability criterion of
0.88 A˚ .
The powders were mixed with silica and pressed
into pellets. Fully stoichiometric Yb2Si2O7 contains
76.6 wt.% Yb2O3. The target masses were:
23.1 wt.% SiO2 and 76.9 wt.% RE2O3, so as to yield
a slightly rare-earth rich RE2Si2O7 (as a result, the
pellets were expected to contain about 2 mol% Yb2
SiO5). Four pellets were pressed from homogenized
powder and sintered for 4 h at 1550 C in air.
The phase of the pellets was determined using XRD.
The pellets were loaded with clay in a deep sample
holder and scanned using a Bruker D8 Advance
system (Bruker Corporation, Billerica, MA), equipped
with a SOL-XE Energy Dispersive x-ray Detector.
Patterns were collected over a 2h range from 5 to 90
using 0.03 steps with 4 s integration per step. The
sample was rotated during data collection.
The samples made from the tailings sample
powder showed majority b phase with a minority
monosilicate Yb2SiO5 phase detected, corresponding
to less than 10% by weight (Fig. 4). No c phase was
detected in either sample, confirming the hypothesis
that phase-stable RE2Si2O7 can be produced from
SUMMARY AND OUTLOOK
A preliminary technical evaluation suggests that a
phase-stable b-RE2Si2O7 rare-earth disilicate can be
synthesized using tailings. This provides initial
confirmation that mixtures of Yb, Tm, and Lu oxides can
be considered as potential substitutes for the
highpurity heavy rare-earth feedstock in EBC
applications, thus reducing the associated supply risk.
Further work is necessary to establish the effects of
variability in the tailings and to demonstrate the
ability of coatings made using tailings to meet all
four of the EBC requirements.
The authors acknowledge Denise Anderson,
Janell Crowder, and Eric Telfeyan for the analysis of
the chemical composition of the tailings samples,
James A. Brewer and Eleanor Gamble for
preparation of the disilicate samples, and Steven Duclos for
general discussions about rare-earth element use.
1. T. Graedel , R. Barr , C. Chandler , T. Chase , J. Choi , L. Christoffersen , E. Friedlander , C. Henly , C. Jun , N.T. Nassar , D. Schechner , S. Warren , M. Yang , and C. Zhu , Environ. Sci. Technol . 46 , 1063 ( 2012 ).
2. U.S. Department of Energy, 2011 Critical Materials Strategy, http://energy.gov/pi/office -policy-and-international-affairs/ downloads/2011-critical-materials-strategy . Accessed 10 Aug 2014 .
3. European Commission Enterprise and Industry, Critical Raw Materials for the EU . Report of the Ad-Hoc Working Group on Defining Critical Raw Materials , 2010 , http:// ec.europa.eu/enterprise/policies/raw-materials/files/docs/re port-b _en.pdf. Accessed 10 Aug 2014 .
4. S.J. Duclos , J.P. Otto , and D.G. Konitzer , Mech. Eng. 132 , 36 ( 2010 ).
5. U.S. Geological Survey , Rare Earth Statistics, 2014 , http:// minerals.usgs.gov/minerals/pubs/historical-statistics/ds140- raree. pdf. Accessed 10 Aug 2014 .
6. A.M. Srivastava and T.J. Sommerer , Interface 7 , 28 ( 1998 ).
7. M.C. Halbig , M.H. Jaskowiak , J.D. Kiser , and D. Zhu (Paper presented at the 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition , Dallas, TX, 7 - 10 January 2013 ).
8. K Wood. High Perform . Comp. http://www.compositesworld. com/articles/ceramic-matrix -composites-heat-up/ . Accessed November 1 , 2013 .
9. K.N. Lee , Surf. Coat. Technol. 133 - 134 , 1 ( 2000 ).
10. K.N. Lee , D.S. Fox , and N.P. Bansal , J. Eur . Ceram. Soc . 25 , 1705 ( 2005 ).
11. N. Maier , K.G. Nickel , and G. Rixecker , J. Eur . Ceram. Soc . 27 , 2705 ( 2007 ).
12. A.J. Fernandez-Carrion , M.D. Alba , A. Escudero , and A.I. Becerro , J. Solid State Chem . 184 , 1882 ( 2011 ).
13. R.D. Shannon and C.T. Prewitt , Acta Cryst. B25 , 925 ( 1969 ).
14. R.D. Shannon and C.T. Prewitt , Acta Cryst. B26 , 1046 ( 1970 ).
15. A.I. Becerro and A. Escudero , J. Eur . Ceram. Soc . 26 , 2293 ( 2005 ).
16. N. Maier , G. Rixecker, and K.G. Nickel , J. Solid State Chem . 179 , 1630 ( 2006 ).
17. A.I. Becerro and A. Escudero , Chem. Mater. 17 , 112 ( 2005 ).
18. N. Krishnamurthy and C.K. Gupta , Extractive Metallurgy of Rare Earths (Boca Raton , FL: CRC Press, 2004 ).