Shrinking from the Heat
ZrW2O8: A Negative Thermal Expansion Material
Synthesis, Processing, and Characterization
Glen R. Kowach
Bell Labs
Murray Hill, New Jersey
Collaborators
Department Head
David Johnson
Crystal Growth
Lynn Schneemeyer
Bob Laudise
Christian Kloc
X-ray Diffraction
Theo Siegrist
Neutron Scattering
Art Ramirez
Gabriele Ernst
Collin Broholm (NIST)
Ceramic Composites
Debra Fleming
Paul Lemaire
Jon Engelberth
Core of Optical Network
Dense Wavelength Division Multiplexing (DWDM) allows the
transmission of data over several channels each at a specific
wavelength near 1550 nm.
Filters (fiber Bragg gratings) needed for routing channels.
Fiber Bragg Grating Reflection Wavelength
Shifts with Temperature
λgrating = 2 neff Λ
DWDM systems utilize optical channels separated by 0.4 nm.
-A temperature change of 27°C puts the signal into another channel!
Thermal Expansion Comparison
Materials with a Negative CTE


Anisotropic
– β-eucryptite (LiAlSiO4) CTE = -5.5 ppm/K
– NZP (NaZr2P3O12) CTE ~ 0
– Cordierite (Mg2Al2Si5O18) CTE ~ 0
–
Isotropic
– AgI (4 to 100 K) CTE = -8 ppm/K
– ZrV2O7 (400 to 900 K) CTE = -7 ppm/K
– ZrW2O8 (0.3 to 400 K) CTE = -11 ppm/K
– Hf analogs demonstrate similar behavior.
Monolith Preparation



Need specific value of CTE (-9.0 ppm/˚C)
– ceramic composites with positive CTE material
Challenges with direct sintering
– metastability
Reactive sintering
– particle size (~ 1 micron mean diameter)
– additives (sintering aids, 0.1 wt% Y2O3)
– atmosphere (O2)
– metastability (quench to room temperature)
Chemistry of ZrW2O8

Phase Stability
Metastable
Obtained by quenching
in oxygen - ZrW2O8 is
kinetically trapped.
ZrW2O8 · 9.5 wt.% ZrO2
Microstructure
Typical ZrW2O8
grain size = 10 µm
Inclusions < 2 µm
Density = 98%
Thermal Expansion for
Various Compositions
CTE = -5
CTE = -11
Weight Percent and
Volume Fraction of
Excess ZrO2
a - 37.0%, 0.337
b - 19.5%, 0.174
c - 9.5%, 0.084
d - 0%, 0
Negative CTE Can Be Tailored
0
-1
-2
CTE (ppm/°C)
-3
-4
-5
-6
-7
-8
-9
-10
-11
0.00
0.05
0.10
0.15
0.20
0.25
Volume Fraction of Second Phase
0.30
0.35
Tubular FBG Package
Performance of FBG Package
Challenge: To produce a mechanically robust ceramic with
an isotropic CTE of -9.0 ppm/°C.
Package performance: ±0.025 nm over ambient temperatures
Large Single Crystal of ZrW2O8
Crystal Morphology of ZrW2O8
Single Crystal Synthesis
Origins of Thermal Expansion

Anharmonic potential
Diatomic molecule
Close packed structures
-bonds elongating
-positive CTE
Open framework structures
-rigid units
-possible negative CTE
ZrW2O8 Unit Cell (T < 420 K)
WO4
Tetrahedron
ZrO6
Octahedron
Phonon Density of States
Summary

Zirconium tungstate (ZrW2O8)
– Compound synthesized and sintered.
– Control of negative thermal expansion achieved.
– Prototype devices for temperature compensation of
fiber Bragg gratings were made which demonstrate
wavelength stability within DWDM optical network
specifications.
– Single crystal growth was discovered.
– Inelastic neutron diffraction was used to determine
mechanism for negative thermal expansion.