Titanium nitride and titanium carbide deposited on tungsten wires were exposed to hydrogen atoms (10(exp -4) atm pressure) produced by the action of microwave radiation on molecular hydrogen. The results of these experiments in the temperature range 298 to 1950 K indicate that no appreciable reaction takes place between atomic hydrogen and TiN or TiC. The formation of reaction products (NH3, CH4, C2H2) should be favored at lower temperatures. However, because of the high catalytic activity of Ti for H atom recombination, the rate of such reactions with H atoms is controlled by the rate of evaporation of Ti from the surface, this rate being low at temperatures below 1200 K. In order to interpret the stability of TiN and TiC in H atoms more fully, the stability of TiN and TiC in vacuum and H2 gas was also studied. The thermodynamic computations conform in order of magnitude to the experimentally found rates of decomposition of TiN and TiC in vacuum and are also consistent with the fact that no appreciable reaction is found with these compounds in molecular H2 at a pressure of 10(exp -3) atmosphere in the temperature range 2980 to 2060 K. When TiN or TiC was heated in atomic H or molecular H2, no reaction products other than those obtained from the simple decomposition of the nitride and carbide were observed. The gaseous products were analyzed in a mass spectrometer

Philipp, Warren H.

English

NASA Technical Reports Server

NASA TN D-1053
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TECHNICAL
D-1055
NOTE
STABILITY OF TITANIUM NITRIDE AND TITANIUM CARBIDE WHEN
EXPOSED TO HYDROGEN ATOMS FROM 298 ° TO 1950 ° K
By Warren H. Philipp
Lewis Research Center
Cleveland, Ohio
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON August 1961
https://ntrs.nasa.gov/search.jsp?R=19980228260 2020-06-15T23:25:17+00:00Z

|J NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
TECHNICAL NOTE D-lOS3
I
STABILITY OF TITANIUM NITRIDE AND TITANI-CM CARBIDE WHEN
EXPOSED TO HYDROGEN ATOMS FROM 29S ° TO 19S0 ° K
By Warren H. Philipp
SUMMARY
Titanium nitride and titanium carbide deposited on tungsten wires
were exposed to hydrogen atoms (10 -4 atm pressure) produced by the ac-
tion of microwave radiation on molecular hydrogen. The results of these
experiments in the temperature range 298 ° to 1950 ° K indicate that no
appreciable reaction takes place between atomic hydrogen and TiN or TiC.
The formation of reaction products (NHs, CH4, C2H2) should be favored at
lower temperatures. However_ because of the high catalytic activity of
Ti for H atom recombination_ the rate of such reactions with H atoms is
controlled by the rate of evaporation of Ti from the surface_ this rate
being low at temperatures below 1200 ° K.
In order to interpret the stability of TiN and TiC in H atoms more
fully, the stability of TiN and TiC in vacuum and H 2 gas was also stud-
ied. The thermodynamic computations conform in order of magnitude to
the experimentally found rates of decomposition of TiN and TiC im vacuum
and are also consistent with the fact that no appreciable reaction is
found with these compounds in molecular H2 at a pressure of i0 -S atmos-
phere in the temperature range 298 ° to 2000 ° K. When TiN or TiC was
heated in atomic H or molecular H2_ no reaction products other than
those obtained from the simple decomposition of the nitride and carbide
were observed. The gaseous products were analyzed in a mass spectrom-
eter.
INTRODUCTION
Because of the growing importance of rocket propulsion and space
technology_ interest in the chemical and physical properties of high-
melting-point materials has increased in recent years. In the search
for materials useful at high temperature_ one must be concerned with_ in
addition to the mechanical properties_ the stability of these materials
in various gases that are present in the exhaust of missiles and in the
upper atmosphere. With this in mind; a great deal of research has been
done on the stability of refractory compoundssuch as the borides_ car-
bides 3 and nitrides of titanium_ hafnium_ and tantalum.
This paper is concerned with the decomposition of titanium nitride
and titanium carbide in an environment of hydrogen atoms. The stability
of these compoundswas experimentally determi led between 298° and 1950° K
and substantiated by calculations using recen_ thermodynamic data. An
interesting aspect of this study is the thermc)dynamiccalculation and ex-
perimental determination of the rates of decomposition of TiN and TiC in
vacuumand in H2 gas at 10-3 atmosphere.
EXPERIMENTAL PROCEDURE
Preparation of Materisi_s
The samples of TiN and of TiC were in th_ form of crystalline de-
posits grown onto tungsten filaments from the gaseous phase. Tungsten
wires (O.Ol-in. about 22 in. long) were U-shaped and attacheddiam. and
at each end to heavy molybdenum leads. The m_thod used for the prepara-
tion of TiN was similar to that used by Agte Imd Moers (ref. i). A mix-
ture of H2 and N 2 in the volume ratio 1:5 was saturated with TiCI 4 vapor
at 30 ° C and passed over the tungsten wire at about 1400 ° C. The tita-
nium carbide deposits were made in a similar ;'ashion (ref. i) by passing
a mixture of H 2 and CH 4 in the volume ratio iO:i saturated with TiCl 4
vapor at 30 ° C over tungsten wire at about 1500 ° C. After about 0.i
gram of material had been depositedj the filanent together with the two
Mo leads was removed from the apparatus 3 immersed in water several times
to clean the surfacej and allowed to dry in a: r.
!
_O
Apparatus and Procedm'e
A diagram of the test apparatus is shown in figure i. Pure H 2 after
passing through a Deoxo unit to remove traces of 02 entered the system
through a flowmeter and passed over water at _0° C in the trap. (Water
vapor must be present if H atoms are to be fo_med.) The flow rate was
adjusted by the needle valve so as to maintair the desired pressure in
the test chamber, the system being open to a _acuum pump (284 liters/min).
The H atoms were produced by exciting the H 2 (flowing through a lO-mm-
diam. quartz tube) with microwave radiation (]000 w at 2450 megacycles).
The lO-millimeter quartz tube was positioned Jn a slot of the tapered
matching section of the wave guide.
In order to determine the number of H atoms that would strike the
sample, a platinum filament (2 cm long_ 0.011 in. wide_ and 0.005 in.
thick) was substituted for the TiN or TiC sample. The temperature of
the Pt filament in contact with H atoms was determined by measuring accu-
rately the electrical resistance of the filament. The numberof moles
of H atoms striking and recombining on the filament per minute N was
calculated by the following equation (see table I):
P
where P is power in watts required to heat the filament electrically
to the sametemperature attained when the filament was heated by the re-
combining H atoms_ and 2_ is the heat of recombination of the H atoms
(52,089 cal/mole). Using the Knudsenequation discussed later in this
report_ the numberof H atoms can also be described in terms of pressure
(table I)_ given the area A of the object being struck by them.
For a typical run involving H atoms_ the quartz reaction tube was
first cleaned with 5 percent HF followed by a washwith dilute phosphoric
acid. The excess acid was poured out, and the remainder was allowed to
dry in the tube. The presence of a thin layer of phosphoric acid on the
tube is claimed to render the quartz inactive as a catalyst for H atom
recombination_ according to J. W. Linnett_ of QueensCollege_ Oxford.
The previously weighed samplewas then inserted into the reaction tube
as shownin figure i. The system was evacuated and then filled with H2,
the process being repeated. Next_ the H2 flow rate was adjusted with
the needle valve to correspond to a pressure of about 0.7 millimeter of
mercury in the reaction tube. The pressure that corresponds to a given
reading on the flowmeter was obtained from a calibration of the flowmeter
against a McLeodgage. With the H2 flowing through the system_ the
microwave discharge was turned on and the sample was raised to the de-
sired temperature by passing an electric current through the coated wire.
This temperature was measuredby viewing with an optical pyrometer of
the disappearing filament type_ corrections being madefor the viewing
system (ref. 2) and for the emissivity of the TiN and TiC (the emissivity
for both the nitride and the carbide being taken as 0.50, an estimate
from the emissivity of similar materials). In general_ the duration of
each experiment ranged from 20 to 30 minutes_ after which time the fila-
ment current was turned off. Whenthe filament containing the coated
sample had cooled, the discharge was also turned off.
The activated carbon trap immersedin liquid nitrogen was then
closed off and the remainder of the system was filled with argon. This
trap was then removedand connected to a Consolidated Model 21-620 mass
spectrometer. As the trap was warmed_the gas evolved was passed into
the mass spectrometer for analysis.
Next_ the filament was removedfrom the reaction chamberand exam-
ined, and the weight changewas determined. The rate of evaporation of
Ti (g/(sq cm)(sec)) was then calculated (table II). The surface area of
the TiN or TiC deposit was determined from tile total weight of TiN or
TiC, its density_ and the geometry of the s_ple. The assumption was
madethat the sample had a smooth surface. Deposits_ if any_ on the
cooled wall of the reaction tube were dissolIed in aqua regia and ana-
lyzed for titanium. In somecases when a change in color of the crystal-
line deposit on tungsten wire was observed_ X-ray and electron-diffraction
patterns were obtained_ but these gave no in_ication of reaction products.
Experiments designed to determine the s_ability of TiN and TiC in
molecular H2 were carried out in a way simil:_r to those with H atoms,
with the exception that the H2_ instead of b_ing passed over water be-
fore entering the reaction tube_ was dried im a liquid-N 2 trap. The
weight-change data are given in table III. En the experiments used to
determine the stability of TiN and TiC in va_uum_the sameapparatus pre-
viously described was used. For these runs_ no activated carbon was used
in the trap between the reaction tube and th_ pump. The system was
plumpeddown until the reading of the thermocc_uplegage indicated that
the pressure of the system was a mffcron of m_rcury or less. The sample
was heated electrically to the desired temperature. The rapidity at
which the material sublimed and condensedonto the cooled wall of the
reaction tube determined the duration of the experiment (from 2 min to
several hours). The rate of decomposition w:_scalculated from the weight
loss in the sample (table IV) and comparedw:th the amountof Ti con-
densed on the wall of the reaction tube.
!
DO
_D
THERMODYNAMIC CALCULAT_ IONS
In order to discuss to better advantage the most probable behavior
of TiN and TiC when exposed at high temperat_Lres to H atoms_ H2_ or
vacuum_ thermodynamic calculations were made to determine the equilibrium
partial pressures of possible reaction produ{ts under the conditions of
the experiments. By application of the Knud_en equation_ the rates of
evaporation of Ti from TiN and TiC were also estimated. These calcula-
tions were made possible with the aid of a r_cent compilation of high-
temperature thermodynamic data published by the Bureau of Mines (ref. 3).
The first step was to calculate from th_se data the values of free-
energy changes A_ for the various reaction_ as listed in table V. The
equilibrium constants Kf were derived from the free-energy values using
the following well-known equation:
_F
in K_ -- -
R-T
where R is the gas constant and T is absolute temperature. The par-
tial pressures p of the reaction products were calculated in the usual
manner from the values of the equilibrium corstants. In these computa-
tions it was assumed _hat solid components were at unit activity and that
SI
the zctivities of g'a.seous products _muder the conditions of the experiment
were equal to their partial pressures.
The rate of evaporation of TiN and TiC expressed in temns of the
rate of evaporation of Ti from the sample was then calc,mlated from the
partial pressure of Ti above its nitride or carbide at equilibrium by
using the Knudsen equation (ref. %):
MG = ap 2_RT
where O is the rate of evaporation in vacuum (g/(sq cm)(sec)), M is
molecuAar weight of the species in the gas phase_ and a is the %ccom-
modation coefficient. This mccommodation coefficient a represents the
ratio between the rate at which molecules actually condense on the sur-
face and the rate at which they strike the surface. Langmuir has shown
that for metal atoms condensing on the surface of a metal the factor a
may be assumed to be equal to i (ref. %). Hoch, Blackburn; Dingledy, and
Johnston have shown that the same assumption is justified for the evapo-
ration of carbon from TaC and WC (ref. S). The values of the rates of
evaporation of titanium as given in table V were determined using a value
of unity for the factor a_ thereby setting a maximum value for the evap-
oration rate.
DISCUSSION
Based on the experimental results (tables II, III, and IV) and sub-
stantiated to some extent by the calculated values (table V), it is con-
cluded that neither TiN nor TiC reacts appreciably with atom:_e or molecu-
lar hydrogen at 10 -% and 10 -3 atmosphere; respectively_ in the tempera-
ture range 298 ° to about 2000 ° K. The probable reaction products of such
reactions (NH3, N2H%; CH%, and C2H2) could not be detected by means of a
mass spectrometer. A comparison of the experimental results with calcu-
lated values is shown on a plot of rate of evaporation of Ti against
temperature (fig. 2). According to thermodynamic calculations (table V
reaction of TiN and TiC with H atoms is favored at lower temperatures
where the probable reaction products are not dissociated to any great
extent_ while at higher temperatures simple decomposition is the predomi-
nant process. The fact that no reaction of TiN or T_C with H atoms was
detected at lower temperatures may be attributed to the fact that a thin
film of Ti metal; which probably forms on the surface during such a reac-
tion, is a good catalyst for H atom recombination (ref. 6); this metal
film would simply promote the recombination of the atoms before they had
a chance to react further with the nitride or carbide underneath. In
view of thisj the rate of reaction would be controlled by the rate of
evaporation of Ti from the surface. As shown in table V(n) and figure 2_
this rate is quite low at the lower temperature.
The experiments with H atoms are complicated by the fact that the
presence of a small quantity of water is required for the production of
satisfactory yields of atoms. Ti, TiN_ and TiC have strong affinities
for oxygen; thus, there is the possibility that oxidation of these mate-
rials by a small amountof water (2.3 percent) in the gas stream exerted
some influence on the results of these experiments.
The calculated rate of evaporation of FiN and TiC in a vacuumrepre-
sents a maximumvalue of this parameter. T_e actual rate could be some-
what less. In the calculations_ the .accommDdationcoefficient in the
Knudsenequation was taken as umity_ whereas in actuality the accommo-
dation coefficient mayhave a value less than one.
With regard to the accuracy of the experimental values_ the results
represent an order of magnitude rather than an exact value. It should
be noted that the values of the loss of Ti _romthe sample depend essen-
tially on the weight loss of the sample that could be obtained to only
two significant figures. Furthermore_ the _xperimentally determined
rates of evaporation depend upon an estimation of the exposed surface
area of the sample. There maybe somediscrepancy between the estimated
value and the true surface area.
Lewis Research Center
National Aeronautics and Space Administration
Cleveland_ Ohio_ June 7_ 1961
REFERENCES
i. Agte_ C._ and Moers_ K.: Methods for th_ Preparation of Pure High
Melting Carbides; Nitrides and Borides and a Description of a Few
of Their Properties. Zs. anorg, allgen. Chem.; vol. 198_ 1931; pp.
233-243.
2. May; Charles E.; Koneval, Donald_ and Frfburg_ George C.: Stability
of Ceramics in Hydrogen Between 4OO0 ° and 4500 ° F. NASA MEMO
3-5-59E_ 1959.
3. Kelley_ K. K.: Contributions to the Data on Theoretical Metallurgy.
XIII - High-Temperature Heat-Content, {eat Capacity, and Entropy
Data for the Elements and Inorganic Co apounds. Bull. 584_ Bur.
Mines_ 1960.
4. Dushman; Saul: Scientific Foundations of Vacuum Technique. John
Wiley & Sons_ Inc., 1949_ pp. 20-21.
©b
5. Hoch, Michael_ Blackburn_ Paul E., Dingledy 3 David P'3 and Johnston 3
Herrick L.: The Heat of Sublimation of Carbon. Jour. Phys. Chem.,
vol. 59, no. 2, Feb. 1955, pp. 97-99.
6. Wood, Bernard J., and Wise, Henry: Diffusion and Heterogeneous
Reaction. II - Catalytic Activity of Solids for Hydrogen-Atom Re-
combination. Jour. Chem. Phys., vol. 29_ no. 7, July 1958, pp.
i&16-1417. (See errata, vol. 30, no. 4, Apr. 1959, p. 1104.)
TABLEI. HYDROGENATOMSRECOMBINENGONPLATINUM
FILAMENTASFUNCTIONOFH2 PRESSURE
[H2 saturated with water at 20° C.]
Pressure of H2, mm Hg H ato_, moles/min
0.8
1.0
2.0
a4. i_6XlO- 4
4.. _2xlo- 4
5.09×10- 4
aH atom press, on Pt filament, about 10 -4 arm.
TABLE I!. - RATE OF DECOMPOSITION IN H ATOMS
[H atoms, 4.16×10 -4 mole/min e_uivalent to
0.07 mm Hg; 2.3 percent H20 in gas stream_
I
LO
F_
Material
TiN
TiC
Temp.; Time,
OK min
298 50
!585 50
164% 50
1962 20
298 180
1216 IS0
1894 20
1940 20
Weight
ioss;
g
0
0
0
0
0
0
0
.OO7O
{ate of weight
loss of Ti_
_/(sq cm)(sec)
0
0
0
2.86_<i0 -6
0
0
0
0
9TABLEIII. - RATEOFDECOMPOSITIONIN DRYH2
[H2 pressure, 0.7 mmHg.]
GO
I
Material
TiN
TiC
Temp.
oK
1645
1966
2120
1650
1970
Time_
min
12
i0
i0
3O
3O
Weight
loss_
g
0
.0064
.0299
Rate of weight
loss of Ti,
g/(sq cm)(sec)
0
S. 2xlO- 6
2.44><10 -5
TABLE IV. RATE OF DECOMPOSITION IN VACUUM
Material
TiN
TiC
Temp.;
oK
1245
1428
1567
1588
1779
1891
2012
2209
1650
1903
2139
2389
Time;
min
Weight
loss;
g
Rate of weight
loss of Ti;
g/(sq cm)(sec)
0
0
2.9x!O- 7
3. lxlO- 6
1080
120
70
9O
i0
i0
0
0
0
.0004
.0007
.0038
i0 .0072
2 .0133
25 0
25 0
20 .0007
20 .0073
0
0
0
4XlO -8
6><10-7
3. ixlO- 6
S. 9xlO- 6
5.44><10 -5
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Calculated (in vacuum)
Ti
TiN
TiC
Experimental
TiN in vacuum
TiN in dry H 2
TiN in H atoms
TiC in vacuum
I000 1200 1400 1600 1800 2000 2200
Temperature, OK
2400 2600
Flgure 2. - Rates of evaporation and decomposition.
N^s*-L,_,yFi,m,w. E-1291



Stability of Titanium Nitride and Titanium Carbide When Exposed to Hydrogen Atoms from 298 to 1950 K

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