I,VDUXTRIAL AND ENGINEERING CHEMISTRY
1098
This brief article is written to acquaint chemists and textile workers with this interesting subject and to bring to their attention material which they might not otherwise encounter. Acknowledgment
Acknowledgment is due to my predecessors at the Bureau of Fisheries for the use of unpublished material, and especially to Lewis Radcliffe, deputy commissioner, and TT. T. Conn, technologist, of the U. S. Bureau of Fisheries for their aid and interest in the problem of net preservation. Nr. Kolbe of Erie, Pa., rendered valuable aid in making the practical fishing tests described in the article.
Vol. 23, S o . 10
Literature Cited ( 1 ) Conn, U. S. Bur. Fisheries, Documenf 1076 (1930). ( 2 ) Cunningham, Fish T r a d e s Gasetle and Poultry, Game, and R a b b i t Chronicle, Nos. 993-1009 (1902). (3) Doree, Biochem. J . , 14, 709 (1920). (4) Radcliffe, Chapter 13, “Marine Products of Commerce,” by D. K . Tressler, Chemical Catalog, h’ew York, 1923. ( 5 ) Robertson and Wright. U. S . Bur. Fisheries, Document 1083 (1930). (6) Taylor, I b i d . , 898 (1921). ( i ) Taylor and Wells, I h i d . , 947 (1923). ( 8 ) Taylor and Wells, I O i d , 998 (1926). (9) U. S. Bur. Fisheries, Unpublished Researches. (lo) U. S. Bur. Fisheries, Stabistical Bull. 833 (1924). (11) U. S. Tariff Commission, Survey S o . 36 (1924).
Electrical Resistivity of Refractory Materials at Elevated Temperatures’ Hobart M . Kraner WESTISGXOUSE ELECTRIC ASD ?I~ANUPACTURIIGCOYPAKY, DERRY,P A .
T
H E r e s i s t i v i t y of
The effect of impurities, particularly, upon the elec-
(1)
The chemical composi-
tion is important. Rare!y, if trical conductivity of ceramic material at elevated ceramic materials a t ever, are ceramic materials pure temperatures is treated. Generally speaking, the elecelevated temperatures compounds. trical conductivity of a ceramic mass depends on its is important in many kinds ( 2 ) The mineral constituionic mobility, viscosity, etc. The term “impurity,” of el e c t r i c a1 a p p a r a t u s , tion affects the electrical propas used, is not limited to those materials which occur erties and firing characteristics either because the equipof a composition. The minnaturally with the principal constituent under conment niay be subjected to eral constitution is affected by sideration, but is used also as it applies to materials abnormal temperatures due firing treatment. intentionally added to the principal constituents for t o faulty operation or because (3) The method of formbonding purposes in the firing process. it forms a part of heating ing the test piece or product Ceramic refractory compositions are herein regarded contributes materially to the equipment. values obtained. as similar materials, a glass being the bond in most Space l i m i t a t i o n s , time ( 4 ) The grain size of the cases and the various compositions varying in the factors, and heat capacity particles used is important, amount of this bond. of the i n s u l a t i o n hare re( 5 ) The f o r e g o i n g deterThe glass bond usually contains the larger number of sulted in m i n i m i z i n g the mine the bond and its charthe components present, and therefore its composiamount used. Consequently, acter, which in turn seriously tion to a large degree determines the electrical character the best refractory is sought affect the electrical, thermal, and mechanical properties of of the ceramic body. and that refractory is taxed ceramic materials. A review of the electrical characteristics of various to the limit. (6) I n a finidled product glasses a t elevated temperatures and the works of The limitations of space the thermal conductivity and several well-known authors are discussed to illustrate and time are stressed t o diffusivity may indirectly affect the generalizations made. greatest extent in heating a p a ceramic material in its operation as an electrical insulator diances. owing to the fact that the user does not wish to have a cumbersome piece of ap- a t elevated temperatures. paratus nor t o wait long to bring the appliance to operating Chemical Composition temperature, This being the case, critical temperatures are often attained, and the electrical properties of the refractory There are several types of insulating materials used in become of paramount interest. electrical equipment. Each has its distinct field. ImperI n many electric furnace linings, the choice of material is vious molded compositions are suitable to only approxiboth a chemical and a mechanical problem. It is chemical mately 120” C. These are usually organic in nature and in so far as reactions with the melt may be concerned, and are, of course, not resistant to high temperatures. The next mechanical in so far as the resistance to abrasion, spalling, and type of impervious insulation is a molded product consisting other thermal and dimensional properties of the lining are of minerals and glass. While this is heat resistant, it is concerned. not suitable for a wide use, owing to its high electrical conAn acquaintance with the electrical characteristics of many ductivity a t elevated temperatures. Glass is somewhat simiceramic materials a t elevated temperatures may be obtained lar in this respect. Porcelain of various sorts has a wide from the many tests which are recorded in the literature. application for higher temperatures where a dielectric is Peculiarly enough, there are as many resistivity values re- required. Ordinary porcelain is made from clay, flint, and corded for certain materials as there have been investigators, feldspar. owing to the fact that the methods of determinations and Spark-plug porcelain consists usually of only clay and a sources of materials are also as numerous. Certain generali- suitable artificial or natural refractory mineral such as zations may, however, be made regarding these materials. mullite (3A1203.2Si02),or sillimanite (A1203.Si02). This porThere are many things responsible for apparently discord- celain gives higher resistivities at high temperatures owing to ant results. The following facts explain some of these: the absence of feldspar in its composition. Ceramic compositions, such as these, have been vitrified; that is, a certain 1 Received May 28, 1931. Y
INDUSTRIAL, A Y D E,VGINEERING CHE,TfIXTRY
October. 1931
1099
0
Soda-Silica Series (2) G1B G1C G1D
%
%
%
GIF
%
77 57 72 95 65.6: 5S 87 0.17 0 17 0 25 0 24 0 63 1 . 4 5 2 09 0 38 A1213 NalO 2 1 . 8 9 2 6 . 2 1 31 64 38 .i3 h Z ~ 0 , , ... . , ... 0 58 0 50
Si02
Fe9'33
Soda-Silica-Maenesia Series (2) ?!A G2C G,2D /O
7%
80.05 0.09 0.57 16.23 2.70
75.95 0.08 1.27 15 38 6.49
/O
74 0 0 15 8
Soda-Silica-Alumina Series (2) GZA G5C ?E i0
60
75 7
81 25 76
2 29 21.21 020
Oi
0 11
"c
73.45 0.13 8 47 17 81 0.20
,P
i2.00 0.14 13.01 14.07
.....
Figure 1
degree of melting has taken place in and between the constituents so as to cause them to coalesce into a dcme impervious mass. This melting or coalescing a t a reasonable temperature requires fluxes (additions to the mass which react with the principal constituents) for the purpose of bonding them together. Thehe fluxes and the glass resulting are detrimental t o high resistivities a t elevated temperatures. Where higher resistivities are required, a low dielectric material of a porous nature usually results, on-ing to the higher refractoriness and smaller proportion of glass bond which would result from fluxing action in the mass. I n many cases where high resistivity is required in the insulating material of electrical apparatus, gas-tightness may also be required. In such a case only glass or porcelain can be used and the apparatus must be designed accordingly. For this purpose a glass or porcelain having a minimum of those materials giving low resistance is chosen. If gastightness is not required, the porous refractory may, of course, be used to give higher resistivity. The porous insulating material is usually of lower mechanical strength because of the fact that there is less glass bond holding the individual particles. Fundamentally, therefore, all fired refractories or other ceramic materials are somewhat the same. I n addition to the principal constituent, they contain suitable quantities of a glass bond. A refractory pure ceramic material which is porous contains little glass, whereas glass might be considered as being all bond. The glass phase of any refractory may be formed entirely from incidental impurities, or it may have been formed from intentionally added materials. A bond naturally fuses at a lower temperature than does the principal constituent and a t a given temperature has a lower viscosity than the more refractory portion. From this it mill be plainly sem that the glassy bond of a porous refractory or of a standard or special porcelain is an important factor in its electrical characteristics. A glass is generally a complete solution of two or more inorganic compounds, usually silicates, formed at elevated temperatures by the solubility of one of these compounds in the other. In most cases silicates do not crystallize readily upon cooling from regular firing processes. The properties of these supercooled liquids vary with the rate of cooling as well as with their chemical compositions. So it is that
annealing affects all the properties of a glass within usual annealing rates. Prolonged annealing of a glass a t elevated temperatures may, therefore, greatly modify the properties of the material of which the glass is a part. In fact, articles made entirely of glass revert to crystalline form if held long enough a t crystallizing temperatures. Pure crystal compositions with no glass bond are not used, although the amount of bond in some is extremely small. I n such extreme cases, the properties of the principal constituent are exhibited quite fully, but in most refractories it is the properties of the glass which determine the behavior of the materials in service. Study of such refractories, therefore, resolves itself very largely into an investigation of the bond. The minor constituents are usually found dissolved in the first liquid phase which forms, while the major constituent is present in excess, usually in the form in which it was added. This means that the minor constituent, being present in the glass, determines the characteristics of the fired product. The electrical conductivity of a glass a t a given temperature is dependent upon two things-namely, the inherent mobility of the ions and the viscosity of the melt in which the ion must move in order to carry current. The ionie mobility is roughly dependent upon the size of the ions, the larger ones moving with less speed than the smaller, This is illustrated by the fact that those glasses which have potassium as the predominating flux have higher resistance than those with sodium, while lithium glasses have still lower resistances. It is interesting to note that soda and magnesia decrease the electrical resistance of a glass, while alumina, which increases the viscosity of the melt, also increases the resistance. Bryson ( 2 ) and Sutton and Silverman (6)2 have shown that the electrical resistance of a glass drops almost in direct proportion t o the decrease of viscosity with rising temperature. Their works are also interesting in that they show how rapidly small additions of S a 2 0to a normal glass increase the electrical conductivity as compared with similar additions of XgO and Alto3. Additions of S a a O and JIgO increase the conductivity, the former more rapidly than the latter, whereas A1203 lowers the conductirity, partly owing, perhaps, to its hardening effect or to its refractoriness and its increase of the viscosity of the glass (see Figure 1). Contains a very complete biblioqraphy on glass.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1100
Mineral Composition
While the chemical composition of a mixture is important, it is little more so than the mineral composition. It is well known that, if a crystal is melted, the electrical conductivity increases rapidly as it passes into the liquid phase, and a solid glass of a certain chemical composition has a higher electrical conductivity than a crystal of the same composition. It would be expected, therefore, that the firing treatment of a composition would affect the mineral constitution and, therefore, the electrical properties.
1
5
Katurally, higher temperatures favor this reaction. Undoubtedly, a finer crystallization and a thin-layer glass bond between the crystals will result when firing to a temperature only slightly above 1545" C. If fired to a temperature a t which the entire mass is liquid and then cooled, corundum first crystallizes until the composition of the liquid is represented by the point a t C (Figure 2). Here mullite begins to crystallize, forming from some of the already crystallized corundum in the formation of a glass-free mixture of mullite and corundum. Here again there is theoretically no glass bond, but, practically, there probably exists a thin layer of glass owing to the high viscosity of such a layer and its inability to crystallize. In the system MgO-Al203-Si02, Figure 3, the attempt is made to bond magnesia with about 10 per cent of clay. If the mixture is fired to a temperature below 1345" C., there is theoretically no reaction between the clay and the magnesia. As a matter of fact, a product consisting of periclase (MgO), amorphous silica, and finely divided mullite probably results. ZOO
\
C ~ S T O B 4 L l 7 6U r d U U l T C 1
si02
A Figure 2-The
4 34/flJ-2s;02
T-o!. 23, xo. 10
\
4 0 3
S y s t e m A l ~ O a - S i O(1) ~
The case of clay in the alumina-silica system, composition
A , Figure 2, is taken as an example. After dehydrating a t 700" C., the composition is A1203.2Si02and probably consists of amorphous silica and alumina intimately mixed, If heated above that temperature, the mineral mullite (3Al2Os.2Si02) and silica, in one of its forms, develop. Equilibrium is attained only by extremely slow heat treatment. If the f h g treatment is arrested below 1545' C., extremely fine crystals of mullite, and probably amorphous silica, remain as such. If fired above 1545' C., the eutectic forms and carries the excess mullite into solution until a temperature is reached where it has all dissolved. Upon cooling from this final temperature, larger crystals of mullite form until a t 1545" C. the eutectic composition, which was the final liquid composition to remain, solidifies. Theoretically, it crystallizes, but, as is the case in firing most ceramic products, owing t o the relatively rapid cooling treatment, it solidifies as a glass. From this it is seen how important the firing treatment is upon the mineral constitution obtained.
40 *O:\ 20
-
eo0
Lc?o
'oa, T€flf€RWURE
Figure 4-Electrical
-
OrGRhrJ
c
Resistivity of Refractories (3)
Compositions such as this have been fired to temperatures even slightly above 1345" C. with the result that incompatible phases have existed together owing to the high viscosity of the melt wherein equilibrium was not obtained. Such conditions are easily possible in ceramic mixtures because of the fact that the batches are only coarsely ground and that the firing processes result in only partial melting. Where a ceramic mixture is compounded from incompatible phases and the temperature of the mixture raised, the first liquid phase to form is that of the lowest eutectic of the system from which compatible crystals separate. It is evident that with increase of temperature above this, the incompatible phases go into solution 'and the compatible phases separate rapidly. If the heating process is arrested during this process, it may be readily seen how complex the mineral constitution or chemical composition of the glass may be. Hence, in the case of MgO bonded with 10 per cent clay and fired to temperatures a t which equilibrium is attained, only periclase spinel, and perhaps forsterite and a glass would be expected, but actually the phases mentioned may exist together (composition X , Figure 3).
Figure 3 -The S y s t e m MgO-AhOs-SiOn ( 5 )
Effect of Alkalies
Composition B, Figure 2, may be formed from clay and corundum. If fired t o temperatures below 1545" C., a heterogeneous mass exists. Fine mullite crystals, in the clay, and excess silica, also from the clay, exist together. Jf fired slightly above 1545" C. where the liquid eutectic is formed, mullite will probably separate from the liquid as soon as formed until all the excess silica is consumed in the formation of the new stable phase, and this will continue as long as liquid and fluidity permit crystallization.
It has been shown by Bryson and is generally recognized that the alkalies rapidly decrease the resistivities of ceramic material a t high temperatures. Small quantities of these also rapidly decrease the refractoriness and increase the fluidity a t elevated temperatures. The alkali which enters porcelain through the feldspar is present in the porcelain to the extent of only 5 per cent, yet it is responsible for various lowering of electrical resistance. Lithia is more active as a flux than either soda or potash. Its molecular weight is less, and its
INDUSTRIAL, AND ENGINEERING CHEMISTRY
October, 1931
TEMP.
F L I N TFIRE CLAY
KAOLIN
300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500
6,600 729 140 42.2 15.3 7.01 3.53 2.12 1.47 1.05 0.864 0.721 0 582
L3,300 1,450 269 74.7 30.2 14.6 7.44 3.91 2.35 1.46 1.06 0 846 0.681
c.
Table I--.Electrical Resistivity of Refractories ( 4 ) ,Ohms per cc. X 10') MARYLAND ITALIAN SILLIXASITE Si02 MgO TALC TALC
....
35,800 6,070 1,430 467 179.0 73.9 36.3 17.9 11.50 5.72 4.29 3.15
....
82,800 11,300 2,360 775 300 117 49 25.9 15.7 11.1 8.05 5.66
fluxing-action per unit neiglit i s mudl greater m-tien used in most ceramic materials. Its effect upon electrical conductivity is correspondingly great. It has been used advantageously, however, by utilizing its great fluxing power through the use of smaller amounts and therebg. maintaining suitable electrical properties in the resulting mixture. Modern spark-plug porcelain differs from ordinary porcelain in that it contains little or no alkali and little or no intentionally added fluxes which would lower the resistivity a t elevated temperatures. Magnesia is found to be less apt to lower the resistivity of a mixture a t elevated temperature than would the alkalies (see Figure 1). It is therefore often used in ceramic materials where a flux is required to mature the product a t a reasonable firing temperature, and a t the same time retain suitable electrical properties. Pure magnesia is generally acknowledged to be the best individual material for high resistivity a t elevated temperatures. Small percentages of impurities are naturally detrimental. The works of Henry ( d ) , Tables I and 11,and of Hartmann, Sullivan, and Allen ( 3 ), Figure 4 and Table 111,are admirable in showing the relative positions of the various common ceramic materials in the ceramic insulation field. Henry's work shows very clearly how well MgO and high MgO compositions stand out above other ceramic materials in insulating value a t elevated temperatures. It also shows how much better kaolin is than ordinary flint fire-clay refractories from this standpoint. Table 11-Chemical Analyses (4) FLINTFIRECLAY KAOLIN Moisture Loss on ignition Si02 Alios FeiOa CaO MRO Ti02 K10
M no Total carbon Inorganic carbon FZO, Sulfur
%
%
1.35 12.90 45.20 36.77 1.30 0.50 0.08 1.80 0.52
3.07 16.83 44,50 37,55 0.54
Si02 Silica brick Magnesia brick Natural zirconia Bauxite Fire clay
SILICA
%
... 99: 81 0.17 0.014
None None
Trace Trace
0.03 0.75
Trace 0 25 0 03
0 02
Trace
Table 111-Analysis
of Refractories (3)
Fez02 A1208
%
%
94.8 6,O 21.5 40.4 50.4
0.2 0.3 0.7 0.9
0.8
% 1.8 6.3 4.7 55.5 45.5
1101
CaO
MgO
%
%
2.4 3.1 0.0 0.2 0.5
0.3 84.1 0.0 0.0 0.8
Ti02
% ...
.., 0.5 2.7 1.9
MISCELLANEOUS
%
..
72.6'(2rOn)
..
..
Upon examination of the chemical analyses of the two latter materials, i t will be seen that the impurities of the flint fire clay are responsible for this. These lower the refractoriness as well, and it can also be said that they increase the glassy portion of the flint fire clay above that which is obtained in the kaolin a t similar firing temperatures, Figure 5 gives compositions, including a vitreous porcelain, investigated by the author and shows how rapidly the resistivity decreases with temperature, a t relatively low temperatures. Here also the magnesia compositions show higher re-
....
.... . . . 33:OOO 11,600 4,730 848 94.7 14.1 2.79 0.615
.
.
....
28,200 7,190 2,190 86 1 339 238
....
....
....
15,900 3,500 970 353
157
74.9
....
....
.... *. .
....
.... ...
... ,
INDIAN TALC
.
67.000 17,800 5.440 1,790 760 340
.... ....
DIASPORE
....
3,370 462 10.3 35.0 12.7 6.07 2.92 1.5
0.886
.... ....
....
yistivities than porcelain. The latter, it is t o Le remembered, contained 5 to 6 per cent of alkalies from the feldspar. The humps in curves 1 and 2, Figure 5 , show the effect of moisture upon porous compositions. Such a porous body with all its 'exposed surface naturally absorbs moisture from the air rapidly. The increase of resistivity of such compositions with increase of temperature represents the evolution of absorbed water by the heating process, This characteristic of a porous material absorbing moisture with consequent reduction of resistance is particularly noticeable in clay-bearing unfired or lightly fired materials, such as those represented in Figure 6. This chart refers to an assembly whose insulation is an unfired mixture of clay, corundum, and zircon pressed together. The clay contains hygroscopic and chemical water. Until
IA'DUSTRIAL A N D EA'GIA'EERING CHEMIST& Y
1102
Tal. 23, No, 10
\
2
0
I
I
I
I
I
I
l
l
IO0
200
30C
400
6W
b0(1
700
800
Figure 6-Resistivity of Clay-Corundum. Zircon Insulation
The thermal diffusivity and conductivity of a refractory predetermines the temperature a t which it will operate. A refractory having too great a thermal insulating power will not dissipate heat rapidly enough to keep it below critical temperatures. This is important in many heating appliances, and in many cases a building up of temperature in the insulation causes it to leak sufficientcurrent to add to the temperatures already existing. Thus it is that, while a porous and poorly fired clay brick may have suitable and desirable thermal insulating properties, it may be entirely unsuitable for electrical insulators for this very reason, as a result of which it soon vitrifies and, with a high electrical conductivity of the melted surface, becomes a relatively good electrical conductor. Conclusion
I n view of the foregoing, it might be well to point out the probable reasons for there being such a small amount of really fundamental data on the electrical conductivities of pure
8
+w
.
so0
030
Temperature, C. Figure 7-Effect of B e n t o n i t e on Resist i v i t y of Zircon
ceramic compounds: there are few really pure ceramic products; the ceramic materials used vary in the quantity and kind of impurities contained; and the treatment that the materials receive may seriously alter their properties, Consequently, it would appear that the generalizations which exist will help to limit the field of considerations, but that analysis and measurements made on individual specimens will be necessary for closer discrimination. It is hoped that this paper will suggest points for consideration in the selection of suitable electrical insulation. Literature Cited (1) Bowen and Grieg, J . A m . Ceram. Soc., 7, 238 (1924). (2) Bryson, J . Soc. Glass Tech., 11, 331-46 (1927). (3) H a r t m a n n , Sullivan, and Allen, Trans. A m . Electrockem. Soc., 38, preprint (1920). (4) Henry, J. .Am. Ceram. Soc., 7, 764 (1924). (5) Rankin a n d Merwin, A m . J . Sci., 46, 301-25 (1918). (6) Suttonland~Silverman,J . A m . Ceram. Soc., 7, 86 (1922).
Freezing Points of Mixtures of Sulfuric and Nitric Acids' W. C. Holmes, G. F. Hutchison, and Barton Zieber E. I. DU PONTDE NEMOURS ASO COMPANY, WILMINGTON, DEL.
IGH-acidity mixtures of sulfuric and nitric acids are in use on a very extensive scale for nitration purposes in industry, particularly in the explosives and dye industries. For this reason a knowledge of the freezing points of such mixtures is of considerable industrial importance, since the unexpected freezing of a mixed acid in storage may be the cause of much inconvenience and delay. A further reason for knowing definitely the freezing properties and ranges of mixtures of sulfuric and nitric acids comes in the fact that, in plant operations, nitric acid is commonly added to fuming sulfuric acid in order t o lower the freezing point, since 100 per cent sulfuric acid freezes at $10" C., and 110 per cent sulfuric acid (H2SO*-S03)a t f34" C. It has been known that the addition of a small amount of nitric acid depresses the freezing point of the concentrated acid, and it
H
1
Received May 21, 1931.
has sometimes been erroneously assumed that the addition of larger amounts would necessarily cause further depression. The freezing properties of mixtures of sulfuric and nitric acids of lorn-water content have already been investigated by Holmes ( 2 ) . It was found that the addition of nitric acid t o 100 per cent sulfuric acid did depress the freezing point a t first. On the addition of further amounts of nitric acid, however, a rapid rise in the freezing point occurred, until a maximum was reached. This maximum was found to exist a t the point where the acids were present in the proportion 5H2SO4-HNO3,and it was assumed that the formation of a definite chemical compound was indicated when the acids were present in the above proportions. I n this work, freezing-point determinations were made on three sets of mixtures of sulfuric and nitric acids, having total acidities of 100, 95,
