ELECTRICAL RESISTANCE OF SOME REFRACTORY OXIDES AND THEIR MIXTURES IN THETEMPERATURE RANGE 600 DEGREES C. TO 1500 DEGREES C.

60 Pages·02.178 MB·English

by  HENSLER, JOSEPH RAYMOND

THE PENNSYLVANIA STATE COLLEGE
The Graduate School
Division of Ceramics
ELECTRICAL RESISTANCE OF SOME REFRACTORY OXIDES
AND THEIR MIXTURES IN THE TEMPERATURE
RANGE 600°C TO 1500°C
A. Thesis
by
Joseph Raymond Hensler
Submitted in p a rtial fulfillm ent
of the requirements
for the degree of
Doctor of Philosophy
January 195>1
Table of Contents
Page
I* In tro d u c tio n ....................................................................................... 1
II* Experimental Procedure................................. 5
A* A p p aratu s..............................................................................................5
F u rn a c e .............................................................................................5
Furnace C o n tro l.................................................................. 6
Measurement Circuit ............................................................ 6
Accuracy of M easurem ents................................. 9
B. Preparation of Specimens........................................................ 10
F a b ric a tio n ......................................................................... 10
F irin g ...................................................................................... 11
Electrode C ontacts............................................................ 12
C* Preparation of M aterials.............................................. 12
Si02 - A1s03 S e rie s...................................................................12
SiOa - TiOg S e rie s...................................................................13
Cr203 - A1203 S e r i e s ................................................... Ill
MgO - NiO S e r ie s ............................................................ 15
D* Test P ro ced u re......................................................................... 15
III. R e s u lts .................................. 20
A. A1203 ........................... 21
B. A1203 - Si02 Series 21
C* S i02 - Ti02 Series 22
D. A1203 - Cr203 Series 22
E* MgO - NiO Series . 23
Table of Contents (Cant'd)
Page
XV* Discussion of R e su lts.......................................................................kS
A. Ala03 ...............................................................................................U6
B. A1203 - Si02 S e rie s ........................................................
C. Si02 — Ti02 S e r i e s ........................................................
D. A1203 - Cr203 S e rie s .............................................................I4.8
E. MgO — NiO S e r i e s ......................................... . . . 50
F. General Remarks ...............................51
V* Sum m ary............................................................................................ 52
References C i te d ...................................................................... 5U
Acknowledgements $6
L ist of Figures
T itle Page
Figure 1 Test F u rn a c e .....................................................................7
Figure 2 D etail of Sample P o sitio n ..........................................8
Figure 3 Sintered A1203, 1500PC - 10 hours . . 17
Figure U Sintered A1203 25
Figure 5 Sintered A1203 - S102 Mixtures, 150CPC- 10 hrs. 27
Figure 6 Sintered A1«0, - SiO, Mixtures
150CPC - 10 hours .................................................28
Figure 7 Sintered Al-0, - S10_ Mixtures
1500°C - 10 h o u r s ...................................................2?
Figure 8 Resistance vs* Composition . . . . . . 30
Sintered Ala03-S102, 150CPC - 10 hours
Figure 9 Sintered S102 - T102 Mixtures . . . . . 32
1U50°C - 21* hours
Figure 10 Resistance vs. Composition . . . . . . . 33
Sintered Si02 - Ti02, U*50° C - 21* hours
Figure 11 Sintered A1203 - Cr203 M ixtures..................................36
15 OOPC - 10 hours
Figure 12 Sintered A1203 - Cr203 Mixtures . . . . 37
1500PC - 10 hours
Figure 13 Sintered A1203 - Cr203 Mixtures . . . . 38
150CPC - 10 hours
Figure 1h Resistance vs. Composition. .......................................39
Sintered A1203 - Cr203, 1500°C - 10 hours
Figure 15 Effect of MgO Addition to 35$ A1203 . . . . 1*0
65$ Cr203, Sintered 1500°C - 10 hours
Figure 16 Sintered MgO - N10 M ixtures...........................................1*2
1500°C - 10 hours
Figure 17 Resistance vs. C om position............................................1*3
Sintered MgO - NiO, 150CPC - 10 hours
List of Tables
T itle Page
Table I £L203 - siOa Sample.........................................................
Sintered 1*>00°C - 10 hours
Table II Si02 - Ti02 S am ples......................................... 18
1150° C - 21* hours
Table III A1203 - Cr203 S am ples.......................................19
1^00°C - 10 hours
Table IV MgO - NiO Samples.........................................................19
l£00°C - 10 hours
Table V Effect of Heat Treatment on R esistivity . 21*
of A1203
Table VI R esistivities for A1203 - Si0a Series . . 26
Sintered l£OOPC - 10 hours
Table VII R esistivities for Si02 - Ti02 Series . . 31
Sintered U*5CPC - 21* hours
Table VIII R esistivities for A1203 - Cr203 Series . . 31*
Sintered 1500°C - 10 hours
Table IX R esistivities for MgO - NiO Series . . . 1*1
Sintered 1$OOPC - 10 hours
1
I. INTRODUCTION
The Investigation of the electrical resistance of refractory
m aterials was suggested to the American Refractories In stitu te, under
whose sponsorship th is work was begun. The electrical resistance of
these m aterials, although high at low temperatures, decreases rapidly
with increasing temperature*
The Nernst filam ent, which preceded the tungsten filam ent
in electric lig h ts, was made essentially of ThOa. The decrease of
resistance with increasing temperature is so pronounced with ThOa th at
th is filam ent could be caused to glow, producing a lig h t source of good
intensity, by heating i t with a gas flame and applying a voltage across
it* G eller has constructed a furnace capable of temperatures up to
2000°C using resisto rs composed chiefly of ThOa* In both of these
applications the advantage is that an oxidizing atmosphere can be used*
This same effect of temperature on resistance is present,
though to a lesser extent, in the more coranon refractory m aterials*
I t becomes important in such applications as electrical melting
operations* A low resistance in the refractories could cause not
only a lowering of efficiency because of current losses but also
possible overheating of the refractories and even eventual destruction
of the lining.
The resistance-tem perature relationship of refractory
m aterials bears a striking resemblance to that of semi-conductors
whoso conductivity has been shown to be the resu lt of a deviation
from stoichiom etric proportions. Some commercial refractory m aterials
2
contain oxides capable of existence in non-stoiehiom etric ratios;
these are present either as major constituents or as im purities of
significant proportions* I t is possible that other oxides may exhibit
this tendency at elevated temperatures*
Thus, a study of this property is of practical importance
from two points of view: what of obtaining high resistances for appli­
cation as refractory linings and that of obtaining low resistances for
possible application as resistors or electrodes* Moreover, such data
should be valuable in learning mare of the nature of these m aterials
and the effect of temperature upon them*
Most refractory m aterials exhibit a realstance-tem perature
relationship approaching that proposed by Rasch and Hinrichson^^ which
may be expressed as follows:
/ - i e B/T
where f is the specific resistance, T is the absolute temperature,
and A and B are constants* Although th is rule is generally valid,
Ferguson^) pointed out that the constants A and B for any m aterial
are often constant only w ithin a given temperature range*
The actual values given far the same refractories by various
investigators usually do not agree* Diepschlag and W ulfesteig(3)
studied the influence of im purities, the pressure used in fabrication,
and the heat treatm ent upon the resistance of pressed and sintered
samples of magnesite, MgO, and i l 203. Im purities of as little as 2%
were found to change the resistance by 300J6* The influences of forming
pressure and heat treatm ent, while not so pronounced, were, nevertheless,
significant* They showed that the resistance of magnesite increased
with the grain size of the material*
Henryk) and K in g ^ pointed out that the presence of a
reducing atmosphere affects the resistance values obtained for refrac­
tory m aterials.
W a g n e r h a s pointed out that electronic conduction can
result from defects induced in a polar crystal by surrounding i t at
high temperatures with a gas of the electronegative component* Cu20,
for example, can incorporate excess oxygen into its lattice* The pre­
sence of excess oxygen ions causes a deviation from electroneutrality
which must be compensated by the donation of electrons by some of the
copper ions in the vicinity of the defect, these copper ions thus
assuming a higher positive valence* The presence of copper ions in two
valence states permits the exchange of electrons among these ions and,
under the influence of an external field , increases conduction*
Electronic conduction can be increased in other m aterials by causing
a loss of oxygen; e.g ., by heating in a reducing atmosphere* In this
case some of the cations must accept electrons and the conduction pro­
cess is analogous to that of the previous case* ZnO behaves in this
way* There also exists the possibility that the lattice defects
caused by the excess or deficiency of anions may migrate through the
crystal and act as current carriers* Wagner states that oxygen-excess
conduction is to be expected only when i t is energetically easy for the
cation to go to a higher valence*
F oS x^ found th is kind of behavior at high temperatures for
Th02, Ce02, and Ti02* Ce02 and Ti02 behaved like reduction semiconductors;
k
their resistance at high temperatures was loner in a reducing atmosphere
than in an oxidizing atmosphere* Th02, on the other hand, behaved like
an oxidation semiconductor, showing the opposite dependence upon the
surrounding gas*
Hartmann^ ^ found that the dependence of the resistance of
A1z03 upon the surrounding gas was that of a reduction semiconductor
at high temperatures*
Johnson and Weyl^ ^ showed that defects can be produced in
Ti02 by introducing small amounts of ions haring a valence greater
than +U* They postulated that Ti+^ ions in the region of an impurity
ion compensated for the deviation from electroneutrality by taking on
additional electrons. This gave rise to the presence of Ti+^ ions and
the conductivity was increased as in the case of Ti02 heated in a
reducing atmosphere*
Gellor^10^ found that the addition of T203 decreased the
resistance of Th02 at high temperature.
In view of the sensitivity of the electrical resistance to
small amounts of im purities and to differences in physical properties,
it would be im practical at present to attempt to obtain values which
would be specific for a given material* The values obtained can be
considered specific at best only to the extent of order of magnitude*
The course of action followed in this work was rather to seek
trends, starting with the m aterials available and adding controlled
amounts of impurities* The m aterials measured were selected both on
the basis of their application as refractories and far the knowledge
which could be gained from them which might be applied to refractories*
5
II. EXPERIMENTAL PROCEDURE
Furnace
The furnace used for the experimental work is shown in
Figure 1. I t was heated by two wire windings on concentric alumina
m uffles. The in terio r muffle* 2 inches in diameter and 6 inches
high, was wound with O.OU inch diameter platinum w ire, 8 turns per inch*
* The exterior muffle, 8 inches in diameter and 8 l/U inches high, was
wound with 0*07 inch diameter Kanthal A wire, U turns per inch. The
purpose of the la tte r winding was to reduce the temperature gradient
from the center of the furnace; its maximum temperature was 900°C*
The muffles were surrounded with insulating brick capable of
withstanding temperatures up to 1600^0. Alnnrl.na plugs (C) were inserted
in the top and bottom of the furnace* These were rammed from a mixture
of fine— and coarse-grained pure alumina and fired to lpOO°C for 1 hour.
Each had a 3/8 inch diameter hole in the center through which passed a
sintered alumina tube^^ containing the platinum-wire leads to the
electrodes. A therm ocouple) also was passed through the top one. A
200 gm. weight(F) was set on the top tube to hold the electrode against
the sample.
Figure 2 shows in more d etail the position of the sample with
respect to the electrodes and the thermocouple. The s a m p l e w a s set
upon a pressed and sintered alumina disc(H) covered with platinum foil^1^.
A platinum w ire, welded onto the bottom of the fo il, passed through
a hole in the disc and through the alumina tube to the exterior of the
furnace. Sim ilarly, the bottom of the top tube was covered with a
platinum s l e e v e t o which was welded a platinum w ire^ ) which passed