21 Application of Time Resolved Mass Spec trometry to Problems in High Temperature
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Chemistry 1
R. T. MEYER and L. L. AMES
Sandia Laboratory, Albuquerque, Ν. M.
Pulse
heating
and
time
applied
to the Langmuir
ZrO N
at temperatures
y
z
of "action" integrals
resolved
up to 2100°K. permitted
relative
temperature-time
measure
ZrO,
ZrO , 2
for ZrO + and ZrO
+
2
for Zr.
from
coefficient
was explored
gas contained reaction.
by analysis
in zirconia
Nitrogen
-release mechanism
is
lower +
O
quantities
of
in N
2
are a product
was the only gas detected. is
air-oxidized
for ZrO
of picomole
sacs, which
The
2
of Zr combustion
The mechanism
Vapor
Ν and N .
than for ZrO . mixtures
Time
evaluation
Zr suggest that the Langmuir 2
and
x
Numerical
were Ag, Z r ,
ion intensities
were
of Ag, Zr, ZrO
of the a to β transition
ments and identification species identified
mass spectrometry
vaporization
A
of
2
the
nitrogen
suggested.
r e s o l v e d mass s p e c t r o m e t r y is b e c o m i n g
increasingly important
as a research p r o b e of c h e m i c a l a n d p h y s i c a l p h e n o m e n a .
It has b e e n
w i d e l y a p p l i e d to k i n e t i c studies of shock w a v e i n i t i a t e d gaseous d e c o m positions (6, 7, 9, 23, 33).
R e c e n t l y , i t has b e e n d e v e l o p e d i n o u r l a b o r a
t o r y for the d e t e c t i o n a n d analysis of fast gas phase reactions i n d u c e d b y flash photolysis ( 18,19
). S e v e r a l investigators are also d e v e l o p i n g the
t e c h n i q u e for the s t u d y of flash a n d laser p y r o l y s i s of elements (37, plastics (8),
a n d celluloses (15).
39),
W e are n o w e x p a n d i n g t h e t e c h n i q u e
to studies of the h i g h t e m p e r a t u r e c h e m i s t r y of the r e f r a c t o r y metals a n d t h e i r b i n a r y a n d t e r n a r y alloys a n d c o m p o u n d s w i t h o x y g e n a n d n i t r o g e n . 'Permanent Address: Department of Chemistry, New Mexico State University, Las Cruces, Ν. M .
301
In Mass Spectrometry in Inorganic Chemistry; Margrave, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
302
MASS S P E C T R O M E T R Y IN INORGANIC C H E M I S T R Y
T h e a p p l i c a t i o n s i n c l u d e : ( 1 ) t h e r m o d y n a m i c measurements at t e m p e r a tures greater t h a n 2000°K., ( 2 ) v a p o r i z a t i o n k i n e t i c s , a n d ( 3 ) m e t a l c o m b u s t i o n studies.
refractory
I n their present f o r m , the first t w o
involve
v a p o r species i d e n t i f i c a t i o n a n d v a p o r pressure measurements i n transient h i g h t e m p e r a t u r e experiments, i n w h i c h v a p o r d e n s i t y , t e m p e r a t u r e , a n d t i m e are m e a s u r e d s i m u l t a n e o u s l y . T h e t h i r d a p p l i c a t i o n i n v o l v e s the analysis of p i c o m o l e q u a n t i t i e s of gas c o n t a i n e d i n t h i n - w a l l e d o x i d e sacs, w h i c h are a p r o d u c t of flash h e a t i n g m e t a l droplets i n o x i d i z i n g atmospheres
(21).
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T h e p u r p o s e of this p a p e r is to d e s c r i b e the e x p e r i m e n t a l t e c h n i q u e s that are b e i n g d e v e l o p e d a n d the p r e l i m i n a r y results a n d i m p l i c a t i o n s . T h e latter are necessarily q u a l i t a t i v e since o n l y e x p l o r a t o r y experiments have been performed.
T h i s is p a r t i c u l a r l y t r u e of the v a p o r i z a t i o n t h e r -
m o d y n a m i c s a n d k i n e t i c s research.
T h e gas analyses for the r e f r a c t o r y
m e t a l c o m b u s t i o n studies are m o r e a d v a n c e d since the present
mass
s p e c t r o m e t r y a p p l i c a t i o n has b e e n p r e c e d e d b y several other d i r e c t l y r e l a t e d investigations. T h e f o l l o w i n g d i s c u s s i o n is d i v i d e d into t w o p r i n c i p a l sections.
The
first section ( V a p o r i z a t i o n T h e r m o d y n a m i c s a n d K i n e t i c s ) c o m b i n e s the procedures a n d results for a p p l i c a t i o n s ( 1 ) a n d ( 2 ) .
T h e s e c o n d section
( R e f r a c t o r y M e t a l C o m b u s t i o n ) describes a p p l i c a t i o n ( 3 ) .
T h e first t w o
a p p l i c a t i o n s a c t u a l l y m a k e use of the t i m e r e s o l v i n g c a p a b i l i t y of
the
mass spectrometer d u r i n g a transient h i g h t e m p e r a t u r e e x p e r i m e n t .
The
t h i r d a p p l i c a t i o n relies u p o n the fast response of the spectrometer for a n a l y z i n g s m a l l b u t k i n e t i c a l l y stable samples of gas at a m b i e n t t e m p e r a t u r e .
Vaporization
Thermodynamics
and Kinetics
T h e p r i n c i p a l advantages offered b y t i m e r e s o l v e d mass s p e c t r o m e t r y a n d p u l s e h e a t i n g t e c h n i q u e s are as f o l l o w s : ( 1 ) b o t h p u l s e d resistive a n d laser h e a t i n g p e r m i t the a t t a i n m e n t of temperatures greater t h a n c a n b e p r a c t i c a l l y a c h i e v e d b y steady-state methods s u c h as electron b o m b a r d m e n t , d i r e c t c u r r e n t resistive, or r a d i o - f r e q u e n c y i n d u c t i o n ; ( 2 )
reaction
of the s a m p l e substance w i t h s u p p o r t i n g or c o n t a i n i n g materials is m i n i m i z e d or e l i m i n a t e d ; ( 3 ) heat losses d u r i n g h e a t i n g are not a p p r e c i a b l e ; ( 4 ) v a p o r densities c a n be r e c o r d e d as a f u n c t i o n of t e m p e r a t u r e d u r i n g a s i n g l e transient e x p e r i m e n t ; a n d ( 5 )
the a p p e a r a n c e of various v a p o r
species, s u c h as m o n o m e r s , d i m e r s , etc., c a n be o b s e r v e d as a f u n c t i o n of t i m e . O t h e r s p e c i a l features of the p u l s e h e a t i n g m e t h o d h a v e b e e n cited b y Baxter ( I ) , Parker (32), and Cezairliyan (2). It s h o u l d b e possible to o b t a i n i n f o r m a t i o n o n b o t h n o n e q u i l i b r i u m a n d e q u i l i b r i u m v a p o r i z a t i o n at temperatures a p p r o p r i a t e to p r a c t i c a l p r o b l e m s i n a b l a t i o n a n d r e e n t r y h e a t i n g . T h e k i n e t i c studies m a y p r o v i d e
In Mass Spectrometry in Inorganic Chemistry; Margrave, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
21.
Time
M E Y E R AND A M E S
Resolved
Mass
303
Spectrometry
f u n d a m e n t a l d a t a p e r t i n e n t to a s u r f a c e - d i f f u s i o n - c o n t r o l l e d v a p o r i z a t i o n mechanism. A t present, o u r e x p e r i m e n t a l efforts i n h i g h t e m p e r a t u r e c h e m i s t r y are d i r e c t e d t o w a r d the t h e r m o d y n a m i c s a n d kinetics of the r e f r a c t o r y metals, a n d t h e i r alloys w i t h o x y g e n a n d n i t r o g e n . T h e w o r k to b e d e s c r i b e d h e r e i n consists of p r e l i m i n a r y studies o n the v a p o r i z a t i o n of Z r , Z r O , a n d Z r O N . S o m e q u a l i t a t i v e observations o n A g are also r e p o r t e d . x
v
z
Principles of O p e r a t i o n . L a n g m u i r vaporization conditions, pulsed resistive h e a t i n g of w i r e samples, fast response t e m p e r a t u r e measurement,
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a n d t i m e r e s o l v e d mass s p e c t r o m e t r y h a v e b e e n e m p l o y e d i n the present i n v e s t i g a t i o n . T h e L a n g m u i r v a p o r pressure P
where
L
is g i v e n b y
is the steady-state rate of e v a p o r a t i o n
(weight
loss) of
a
species of m o l e c u l a r w e i g h t M i n t o a v a c u u m f r o m a surface area S at temperature T.
T h e L a n g m u i r pressure is r e l a t e d to the e q u i l i b r i u m
v a p o r pressure P
e q
t h r o u g h the L a n g m u i r s u b l i m a t i o n coefficient
C h u p k a and Inghram (3)
« : L
h a v e s h o w n that the mass spectrometer
ion
i n t e n s i t y Γ is r e l a t e d to the p a r t i a l pressure of a v a p o r i z i n g species b y
r
^ t
=
(3)
w h e r e k is the c a l i b r a t i o n constant for the a p p a r a t u s ; f u r t h e r , a p l o t of In (/* · Γ ) vs. l/T y i e l d s — AH /R
f r o m the s l o p e w h e r e AH
V
of v a p o r i z a t i o n , i f «
L
V
is i n d e p e n d e n t of t e m p e r a t u r e . I f «
d e p e n d e n t , the v a l u e of AH
L
is the heat
is t e m p e r a t u r e
o b t a i n e d w i l l b e different f r o m the t r u e
V
thermodynamic equilibrium value: AH„ w h e r e AH * V
= ΔΗ„
f L
e q
+ ΔΗ„*
(4)
is a n a c t i v a t i o n e n t h a l p y for v a p o r i z a t i o n . It c a n b e s h o w n
t h a t t h e L a n g m u i r coefficient is a f u n c t i o n of b o t h a n e n t h a l p y a n d a n e n t r o p y of a c t i v a t i o n : ΔΗ* otL.T
=
e
RT
T e m p e r a t u r e d e p e n d e n t studies of «
AS* ' e
L
R
( 5 )
.
w i l l p r o v i d e values of AH*
and
AS*, w h i c h s h o u l d b e u s e f u l i n p r o b i n g the m e c h a n i s m of v a p o r i z a t i o n f r o m a surface.
In Mass Spectrometry in Inorganic Chemistry; Margrave, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
304
MASS S P E C T R O M E T R Y IN INORGANIC C H E M I S T R Y
M o r e direct information on vaporization kinetics m a y possibly
be
o b t a i n e d f r o m t i m e r e s o l v e d measurements of the g r o w t h of the v a p o r i z a t i o n rate to a steady state v a l u e . I n p r i n c i p l e this w o u l d b e a c c o m p l i s h e d b y s u b j e c t i n g the s a m p l e to a r e c t a n g u l a r t e m p e r a t u r e p u l s e a n d o b s e r v i n g the v a p o r d e n s i t y η as a f u n c t i o n of t i m e to g i v e : ^
= *(T,S,C),
(6)
w h e r e k m a y b e a f u n c t i o n of t e m p e r a t u r e T, surface structure S, a n d
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composition
C.
T w o m e t h o d s of t e m p e r a t u r e m e a s u r e m e n t are b e i n g u t i l i z e d : i n t e g r a t i o n of the e n e r g y d i s s i p a t e d d u r i n g the resistive h e a t i n g p u l s e ; a n d fast response o p t i c a l p y r o m e t r y . T h e f o r m e r m e t h o d i n v o l v e s e v a l u a t i o n of e x p e r i m e n t a l a n d t h e o r e t i c a l " a c t i o n " integrals, w h i c h are
defined,
r e s p e c t i v e l y , as: t G = fed*,
(7)
e
ο and
T where i =
x
current, C
p
=
heat c a p a c i t y , ρ =
of i s o t h e r m a l t r a n s i t i o n , p 0
d e n s i t y at i n i t i a l t e m p e r a t u r e T
t i o n a l area at T , δ = 0
=
t
enthalpy
0
(usually 300°K.), A
0
=
cross sec
t h e r m a l e x p a n s i o n c o r r e c t i o n factor =
of l i n e a r d i m e n s i o n at Τ a n d T ) , a n d M = 0
T u c k e r (36)
r e s i s t i v i t y , AH
— a v e r a g e d r e s i s t i v i t y for t r a n s i t i o n state,
t
d =
(8)
T
0
and Cnare (5)
(ratio
molecular weight.
h a v e a p p l i e d the a c t i o n integrals s u c
cessfully to the analysis of e x p l o d i n g m e t a l w i r e s . I f the heat c a p a c i t y , resistivity, a n d t h e r m a l e x p a n s i o n are k n o w n as a f u n c t i o n of t e m p e r a t u r e , one needs o n l y to integrate E q u a t i o n 8 n u m e r i c a l l y to o b t a i n G
th
T h e n a n e x p e r i m e n t a l m e a s u r e m e n t of time provides G
e
vs. t.
filament
vs.
T.
c u r r e n t as a f u n c t i o n of
C o r r e l a t i o n of e q u i v a l e n t values of G
e
and
G
th
gives the d e s i r e d t e m p e r a t u r e vs. t i m e d e p e n d e n c e for the p u l s e h e a t e d filament.
R a d i a t i v e , c o n d u c t i v e , a n d v a p o r i z a t i o n heat losses are n e g l i
g i b l e for e n e r g y pulses shorter t h a n 1 msec, d u r a t i o n ( J , 2, 32). calculated action integral G
th
The
is i n d e p e n d e n t of the w i r e l e n g t h b u t is
d e p e n d e n t u p o n the cross-sectional area. T h e r e f o r e , a m e a s u r e m e n t the a c t u a l l e n g t h of w i r e b e i n g h e a t e d is n o t r e q u i r e d .
In Mass Spectrometry in Inorganic Chemistry; Margrave, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.
of
21.
M E Y E R AND ÂMES
Time
Resolved Mass
305
Spectrometry
T h e fast response p y r o m e t r y is b a s e d o n m u l t i c o l o r r a t i o m e t h o d s r e p o r t e d b y M a y f i e l d (16)
a n d Kottenstette (14).
for m o n o c h r o m a t i c emissive p o w e r e
x
e = x
Planck's radiation l a w
is
cUJCxA-Cexp ( - C / A T ) - l ] " *
(9)
2
w h e r e c ( A ) is the s p e c t r a l e m i s s i v i t y , C
=
x
4wc h a n d C 2
2
=
ch/k.
For
λ Τ p r o d u c t s e q u a l to or less t h a n 4 X 1 0 μ * °K., the a b o v e e q u a t i o n is s
well approximated by Wien's radiation formula: e Downloaded by UNIV OF ARIZONA on December 17, 2012 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0072.ch021
x
= c i A j C i A " exp (-C /\T). 5
(10)
2
T h e r a t i o of p o w e r e m i t t e d at a n y t w o w a v e l e n g t h s is t h e n g i v e n b y
«=£-&^-[*(s-ir)]-
