Energy and Thermodynamics: Marvelous Materials and Phascinating Phenomena

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27 Οκτ 2013 (πριν από 4 χρόνια και 14 μέρες)

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Energy

and Thermodynamics:
Marvelous Materials and
Phascinating

Phenomena

Alexandra Navrotsky

UC Davis

Energy, Environment, Resources,
Climate


Mineralogical, solid state chemical
and thermodynamic
aspects of


CO
2

management


Nuclear energy


Water


Metals


No free lunch


Science


policy


politics


Thermodynamics wins in the long run


Materials


Electronic, optical,
multiferroic
,
catalytic


Energy storage and release
-

fuel
cells, batteries


The nuclear fuel cycle


CO2 management and
sequestration


Why
I Count Calories

for a Living



They are fascinating


Energetics whisper secrets of the strength of chemical bonds


Entropies sing of vibrating atoms, moving electrons, and
structural disorder


Systematics have predictive power


They pay


thermodynamic data are essential to good materials processing


Environmental science needs thermodynamics, both for issues of
stability and as a starting point for kinetics


Mineralogy, petrology, and deep Earth geophysics need
thermodynamic data.

Thermodynamics



Is a policeman


Eliminates the impossible


Identifies the improbable


Simplifies your life


Links what can be measured to what you really want to know


Limits the number of independent
variables


Tells you if materials are compatible and under what conditions
they might be made


Provides a
macroscopic formalism that links to
microscopic insights


Quantum mechanics


statistical 浥chanics


ther浯dyna浩cs


Links directly to structure and dynamics in
solids


“Spectroscopy without selection rules”
-

everything contributes


Commercial
Setaram

AlexSYS

Calorimeter

Calorimetric Measurement of Surface
Enthalpies (Energies)


Measure enthalpy of solution versus surface
area, slope of line will give surface energy
Complications


Particles are hydrated and hold water strongly


Particles may be agglomerated, twinned, etc. so
sizes estimated by
Xray
, TEM, BET may differ and
interfacial energies may play a role

Water Adsorption
Calorimetry


0.3 J


Relaxor

Ferroelectrics


B site substitution Ti
4+
= 1/3Mg
2+
+ 2/3 Nb
5+


PMNPT is lead magnesium
niobate



lead
titanate


Ordering,
d
omain structure, symmetry
changes (
morphotropic

transition)


Difficulties in synthesis and processing


Project with Al
Migliore
, Frances Hellman,
Shiv Varma, and postdoc Gustavo Costa now
at LANL

Enthalpies of drop solution for (
1
-
x)PMN
-
xPT


perovskites

as
a function of composition
.

Enthalpies of
mixing for
(1
-
x)PMN
-
xPT

perovskites

as
a function of composition
.

PMNPT

Thermodynamic Constraints

on PMN Synthesis

Fluorite


Homovalent
: M
4
+

=
N
4+

Heterovalent
: M
4
+

= Ln
3+

+ 0.5

Vacancy

Navrotsky and
Asta

groups at UC Davis and Berkeley

theory and experiment

Energetics of doped ceria and
thoria

Nanomaterials
: Main Thermodynamic
Issue


Synthetic and natural
nanomaterials

are often forced,
by low temperature aqueous conditions, to remain
fine grained, with particle sizes of 1
-
100 nm.


How does this constraint alter thermodynamics,
phase
equilibria
, and the occurrence of specific crystal
structures?


Different phases have different surface energies, thus
their stability is affected differently by grain size
diminution


OXIDES AND OXYHYDROXIDES OF Ti,
Mn
, Fe, Co, Zn,
Al,
Zr
,
Hf
,
Ce
, U…..

SIZE EFECTS ON CHEMICAL REACTIONS


Polymorphism


Dehydration


Redox


General principle: phase assemblage with
higher surface energy is destabilized with
respect to that with lower surface energy


Important for ceramic, catalytic, geological,
and environmental applications

Magnitudes



Effect on free energy of reaction:



Surface energies range from 0.5 to 5 J/m
2
. Take
D
⡳urface energy⤠= 2 g⽭
2



Take surface area = 100 m
2
/g



Take molecular weight = 150



D
G =2 砠x00 砠x50 = 30 kg/
mol


General principle
-

small grain size
thermodynamically

stabilizes phase assemblage
with lower surface energy

Calorimetric Measurement of Surface
Enthalpies (Energies)


Measure enthalpy of solution versus surface
area, slope of line will give surface energy
Complications


Particles are hydrated and hold water strongly


Particles may be agglomerated, twinned, etc. so
sizes estimated by
Xray
, TEM, BET may differ and
interfacial energies play a role

Alumina

Enthalpy of Iron Oxides Relative to Bulk
Hematite plus Water

Goethite = Hematite + Water

Surface Energy Systematics


Spinels

(
g
-
Al2O3,
g
-
Fe2O3, 䵧Al2O4, 䍯3O4, Fe3O4, 䵮3O4⤠
all have lower surface energies than
rocksalt

oxides (
CoO
,
NiO
), metals, or
trivalnet

non
-
spinel oxides


Metals (Fe, Co, Ni) have lower surface energies than
rocksalt

oxides


So phase field (in pO2
-
T space) of
rocksalt

oxides shrinks at
nanoscale

and that of spinel expands.


This appears general

Co
-
O Phase Diagram

Oxidation
-
Reduction
Equilibria

among Transition Metal Oxides Change
Dramatically at the
Nanoscale

Because of Differences in Surface Energies


Relevant to materials processing, environmental science, geology, and even biology


For example, for10 nm iron oxides,
wustite

FeO

has no stability field at all, with iron
coexisting with magnetite


Spinels
, M3O4 have lower surface energies than divalent oxides MO and trivalent oxides
M3O4, expanding the spinel stability field.


Navrotsky et al. Science 330, 199
-
201 (2010)



Birkner and Navrotsky (
2012)
Am. Min.

MANGANESE OXIDES

Mn



O

The Path Forward


Rigorous

-

include surface energy as a
variable for all phases and calculate
equilibria

for given particle sizes


Practical



Choose particle size of 10 and 100
nm and add constant free energy terms to
each phase (estimating when necessary) and
calculate phase diagrams for “small” and
“very small” particle systems

Implications of Redox Shifts


Catalysis, hydrogen production, water
splitting,
batteries,sensors


Environmental redox of Fe, Cr, U….


Biology, origin of life, interpretation of data
from Mars


THERMODYNAMICS AS WELL AS KINETICS

Catalysis, sensors, batteries:

some recent studies




CoO
” catalysts for CO oxidation probably are
Co3O4, low surface energy may be important
both thermodynamically and catalytically


SnO2 a better gas sensor than TiO2



CaMnO
” catalyst for water splitting, a
biomimetic of Photosystem II in
photosynthesis


Li battery materials


nanoscale

and surface
effects




SnO
2




TiO
2




Energy of anhydrous surface (J/m
2
)




1.72



2.22


Energy of hydrous surface (J/m
2
)






1.49



1.89


Coverage below which differential heat of adsorption <
-
125 kJ/
mol

(molecules/nm
2
)




0.2



0.5



So SnO
2

holds on to water less strongly and
gases to be sensed can compete better for
surface sites

Why is S
n
O
2

such a good gas sensor?

CaMnO

water splitting catalysts



Nominally CaMn2O4.nH2O and
CaMn3O6.nH2O but actually more
oxidaized

so there is Mn3+ and Mn4+


Nanophase

layered structure


Relatively low surface energy (higher than
Mn3O4 spinel but lower than Mn2O3 and
MnO2

Battery materials


LixMO2
rocksalt

type vs. LixM2O4 spinel
type
-

predict spinel has lower surface energy


Redox
equilibria

and therefore
electrochemical potential may depend on
particle size


Thermodynamics of other materials, e.g.
triplite
-
tavorite

LiCoO
2

: Layered
rocksalt
-
derived structure


Hexagonal, R
-
3m

a = 2.82 Å ; c = 14.08 Å


Cubic close packing of oxide ions


octahedral
sites of alternate layers are occupied by Li &
Co respectively


LiCoO
2
/ Li battery: Li
+

ions
intercalate/
deintercalate


Co formal oxidation state goes from 3+ to 4+


Cell voltage is 4 V, related to free energy of
reaction. Does it change with particle size?

c

b

a

Surface energy

Energy of hydrous surface





2.10
±

0.35 J m
-
2

Energy of anhydrous surface
-



2.29
±

0.35 J m
-
2
almost no stabilization by hydration



Compare to
CoO


Energy of hydrous surface





2.82
±

0.20 J m
-
2

Energy of anhydrous surface
-



3.57

±

0.30 J m
-
2



DFT calculations, Shirley
Meng

group (2012), give 2.1 J/m
2

for
anhydours

surface, influenced
by coordination geometry and
spin state of Co
3+

Transformation and Crystallization Energetics of

Synthetic and Biogenic Amorphous

Calcium Carbonate (ACC)

Center for
Nanoscale

Control of Geologic CO
2

The transformation/crystallization enthalpies were measured using isothermal acid solution
calorimetry and differential scanning calorimetry (DSC)

Synthetic ACC


chemical precipitation and Biogenic ACC
-

extraction from California purple sea urchin

PNAS, (2010)
107, 16438

16443



ACC is a highly metastable phase compared to all
crystalline CaCO
3

polymorphs


Dehydrated synthetic ACC produced by heating is
energetically similar to biogenic ACC


The formation of anhydrous ACC from hydrated
ACC is exothermic



ACC crystallization is energetically downhill
through stepwise evolution of series of phases as:


More metastable hydrated ACC


L
ess metastable
hydrated
ACC


Anhydrous ACC
~

B
iogenic anhydrous ACC


Vaterite




Aragonite


Calcite

Major findings

Energetics of Amorphous

Ca
1
-
x
Mg
x
CO
3
∙nH
2
O


Two distinct regions of amorphous
Ca
1
-
x
Mg
x
CO
3
∙nH
2
O (0<x<1) phases


Homogeneous single phase (x < 0.47) and heterogeneous two phases (x > 0.47)

Two distinct amorphous precursors


x = 0
-
0.2
-

less metastable single phase
is frequently found in biogenic carbonates



x ~ 0.5
-

least metastable
phase could possibly be
dolomite precursor

AMC is more metastable than ACC but more persistent









NSF and DOE and UC