Sept., 1957
CHEMICAL EFFECTS OF PLASTIC DEFORMATION AT HIGH PRESSURE
1240
CHEMICAL EFFECTS OF PLASTIC DEFORMATION AT HIGH PRESSURE’ BY H. A. LARSEN AND H. G. DRICKAMER Department of ChemistTy and Chemical Engineering, University of I&hois, UTbana, Illinois Recsivsd June 1% 1067
During plastic deformation at pressures from 3000 to 50,000 atm. solid potassium hexacyanoferrate( 111) is rapidly converted to potassium hexacyanofekrate(I1) and other products. Pressure alone without plastic deformation causes no a p preciable reaction in this system. Although the same reaction can be induced thermally the mechanism of the conversion during shear at high pressure does not involve bulk heating or localized heating in any important way. Two possible mechanisms for the specific influence of lattice distortion are postulated. Extreme distortion near an anion could cause it to release electrons directly to neighboring anions thus reducing the size of the donor anion and alleviating the distortion. Alternately, regions having abnormally high concentrations of potassium ions could act as temporary trapping sites for electrons thus facilitating their transfer from one anion to another.
More than twenty year? ago Bridgman2 first studied the plastic shearing of solid materials while confining them with hydrostatic pressures as high as 50,000 kg. cm. -2. His study of some 300 materials dealt principally with a measurement of the pressure dependence of the shear stress at which plastic deformation took place. Little systematic in\ estigation was made of irreversible phenomena induced by such a treatment. Equipment similar to Bridgman’s capable of applying continuous shear t o samples confined at 100,000 atm. was constructed and operated in a study of irreversible effects of pressure and shear. Experimental Procedure Each high pressure shear experiment involved the compression of two IO-mggowdered sampIes between the mating surfaces of three arboloy platens as shown in Fig. 1. The Carboloy cylinders were 1 in. in diameter by 1 in. long and were supported by steel jackets shrunk around them. The top and bottom platens were ground as truncated cones with inch diameter flats and a cone half angle of 84’. The central platen with its steel jacket was pinned into the hub of a 10 inch diameter sprocket wheel (Boston Gear Works, Quincy, Mass., BKSBGO). During the compression some of the sample extrudes from between the platen faces until an equilibrium thickness is attained. Torque was then applied to the sprocket holding the central platen by means of a mechanical drive forcing it to rotate about the common axis at 0.38 r.p.m. with respect to the other two platens. At high pressure where the sample- laten friction exceeded the ultimate shear stress of the sampye this rotation caused plastic deformation of the compressed sample. After shearing the samples for the desired time, pressure was released, and the samples were recovered for analysis. The shear stress applied to the sample was calculated from the measured torque by assuming a uniform value of shear stress throughout the sample. Samples of sheared potassium hexacyanoferrate( 111) containing potassium hexacyanoferrate( 11) were analyzed by comparing the intensity of the infrared C=N bands of these two compounds using a Perkin-Elmer model 112 single beam, double-pass, infrared spectrometer equipped with a lithium fluoride prism. The sheared sample was first dissolved in water, and the solution was evaporated to dryness over a steam cone and then dried for 10 min. in a 110’ oven to assure elimination of the water of hydration of potassium hexacyanoferrate( 11). This recrystallization process was necessary because the potassium hexacyanoferrate(I1) produced during the shearing action was almost completely amorphous as shown by X-ray analysis, and it gives an infrared spectrum quite different from that of the crystalline material. These differences were probably the result of combination bands between the C=N vibration and the vibration of the lattice evident only in the crystalline material. The infrared spectrum of a Nujol mull was then ex(1) This work was supported in part by the U. 8. Atomio Energy Commission. (2) P. W. Bridgman, Phya. Revs. 48, 825 (1935).
amined between 2145 and 1980 cm.-l. The ratios of the intensities of the potassium hexacyanoferrate(I1) bands at 2093,2073,2063,2043 and 2026 cm.-I to the intensity of the 2115 cm.-1 band of potassium hexacyanoferrate(II1) were then compared with similar data obtained from samples of known composition to determine the ratio of hexacyanoferrate(I1) to hexacyanoferrate( 111) in the unknown sample. The potassium hexacyanoferrate( 111) used in these experiments was “Baker Analyzed” reagent from the J. T. Baker Chemical Company, Phillipsburg, New Jersey.
Results Potassium hexacyanoferrate(II1) is rapidly converted by plastic deformation at pressures of 3000 atm. and higher to potassium hexacyanoferrate(I1). The other products which form are probably paracyanogen and iron according to eq. 1 4KsFe(CN)6+3K4Fe(CN)s
+ Fe + p (CN),
(1)
This reaction does not take place rapidly a t high pressure without simultaneous plastic deformation. The same reaction can be induced thermally, however. A quantitative study of this reaction has been made by shearing samples for one minute at each of several pressures from 3000 atm. t o 50,000 atm. (Fig. 2) and by varying the amount of shear a t 10,000atm. (Fig. 3). All of these experiments were conducted at a single rate of shear except for two runs at 10,000 atm. which were sheared at five times the normal rate. Comparison of the data for the two rates of shear shows that the total amount of shear rather than total time of shear or shear speed is the determining factor in this reaction. The results of several experiments conducted in a laboratory oven to determine the approximate rate of the atmospheric pressure thermal degradation .of potassium hexacyanoferrate(II1) are shown in Flg. 4. The rate of thermal conversion of a sample which had been partially converted by shear is also shown in Fig. 4. Corrections for warm up were applied to the measured times on the basis of an Arrhenius equation using the crudely measured activation energy of 40 kcal./mole-l. The results of these experiments also are presented in Fig. 4. Potassium tetracyanoaurate(II1).1.5 hydrate is also reduced by shearing at 50,000 atm. for ?ne minute, probably according to eq. 2. No quantitative study of this reaction was made. KAU(CN)~.I.~€?~O +KAu(CN)*
+
z (CN), + 1.5Hz0 (2) 2
H. A. LARSENAND H. G. DRICKAMER
1250
1
4KaFe(C20&
+3FeCzOr 4-
Vol. 61
+
6XzC2O4 6C02(s) (3)
After shearing at 50,000 atm. for one minute, potassium hexacyanochromate(III), KaCr(CN)6, shows in addition to its original infrared spectrum a new band at 2166 cm.-I. A similar effect is SAMPLE l M CARBOLOY noted for potassium hexacyanocobaltate(II), &Co(CN)a, where the new band is at 2170 em.-'. Evidently the conversions represented by these changes are slight, and the matter was not pursued. Shear at 50,000 atm. shows no effect detectable by infrared analysis for potassium hexacyanomanganate(III), KoMn(CN)s, or for potassium hexacyanoSPROCKET '883 TOOL STEEL cobaltate(II1) K&O(CN)~. Complex cyanides of nickel, silver and gold(1) seem to be formed by shearing under high pressure mixtures of potassium cyanide with simple salts of these compounds. The BRONZE COLLAR powdered mixtures react so easily upon standing, however, probably due to the hygroscopicity of the potassium cyanide, that it is not possible to say with SAMPLE certainty that the results are due to the high presCARBOLOY sure shearing. 6150 TOOL Discussion STEEL Rapid conversion of solid potassium hexacyanoferrate(II1) to potassium hexacyanoferrate(I1) and FULL SCALE other products by means of high pressure shear can be explained in terms of the influence of extensive Fig. 1.-Exploded view of platens, samples, and collar for lattice distortion introduced by the plastic deformashear experiment. tion. Although the same reaction does take place upon heating without shear or increased pressure, 100 I I I I 1 several pieces of evidence show that the temperature rise due to plastic deformation is insufficient to explain the observed reaction. Let us consider this # evidence at the outset. z Heat loss considerations mentioned during the P UI K discussion of experimental procedure show that the W > average temperature rise of the solid is less than 14". z 0 Since a bulk temperature close t o 350" would be 0 required to explain the observed reaction the possibility of extensive reaction due to a general temperature rise can be dismissed at once. The prob0 10 20 30 40 50 lem of localized heating requires more careful conPRESSURE ATM x 10-3 sideration. Seitza has estimated 200" as an upper f?* limit to the temperature rise at an isolated moving 0 dislocation and has estimated that temperatures X 8 I between 1000 and 10,000"K. may be reached at the If point of encounter between a moving dislocation u) u) and an obstacle such as another dislocation which w a impedes this motion. Temperatures of 1000" have 6 been measured by Bowden and Yoffe4 during slida ng friction. Calculation of the effect of these high U W temperatures which exist for times as short as lo-" z see. seems useless due to the many uncertainties in0 1 0 20 30 40 50 volved. There are several experiments, however, PRESSURE ATM x 10-3 Fig. 2.-Conversion and shear stress of KaFe(CN)Bsheared which provide convincing arguments against any important role of local heating. First there is the for 1 min. a t various pressures. fact that several compounds, both organic and inSeveral samples of an impure potassium tris-(oxa- organic which are more sensitive to heat than potas1ato)-ferrate(III), KsFe(C20.&, were sheared at sium hexacyanoferrate(III), e.g., potassium tris50,000 atm. for one minute. Comparison of the (oxahto)-ferrate(II1), have been sheared under analysis of the combined samples with that of the identical conditions without any effect of note. starting material showed that no significant reac- Experiments conducted at five times the normal tion had taken place. Based upon repeatability of rate of shear also show that the reaction mechathe analysis, it is estimated that a 10% conversion (3) P. Seitz, Aduances an Phys., 1, 43 (1952). of the starting material according to the stoichiome(4) F. P. Bowden and A. D. Yoffe, "Initiation and Growth of Explutry of eq. 3 would have been detected. rlon in Liquid. and SoiidB," Cambridge Univerritp PreUr, 10624 6150 TOOL STEEL
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9
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Sept., 1957
CHEMICAL EFFECTS OF PLASTIC DEFORMATION AT HIGHPRESSURE
nism does not involve local heating. A fivefold power increase would cause a fivefold increase in the temperature rise. If heating were the important mechanism such an increase in the temperature rise would be expected to increase the rate of reaction by a factor greater than five, the observed figure. Still another indication that local heating is not the important reaction mechanism is the fact that potassium hexacyanoferrate(II1) which has been partially converted by shear undergoes thermal decomposition at 20-100 times the rate of unsheared material. If the reaction during shear weie a thermal process, such a phenomenon would not be expected. The absence of any important thermal effects and the specific influence of plastic deformation both indicate a mechanism for this reaction which is intimately associated with extensive lattice distortion. The general picture of slip during high pressure shear is one in which there are many intersecting slip planes and extensive lattice distortions. The most common sort of distortion is probably that found near dislocations where the disordered ions have neighbors different in kind, number and distance than they would have in a perfect crystal. Interstitials and vacancies must also be present in large numbers as a result both of diffusion near dislocation jogs and as a result of the intersection of moving dislocations. When chemical reaction takes place on an active slip plane it is highly likely that the reaction products will contribute toward the cessation of slip on this particular plane in a manner similar to the process by which precipitation hardening takes place in alloys. As the conversion with shear of potassium hexacyanoferrate(II1) goes essentidy to completion such a mechanism of slip propagation helps to account for lattice distortion near every molecule. During the conversion of potassium hexacyanoferrate(II1) the basic reaction step to be explained involves the transfer of electrons from one anion t o three others as the result of lattice distortions. As is the case for most chemical reactions it is difficult to ascertain the physical details by which the crucial electron transfer takes place. The suggestions offered below for this step in the process must be regarded as speculative. Basically there are two ways in which lattice distortions can aid the process of electron transfer. One method involves a sufficient perturbation of some of the anions that their stability is reduced. The other process involves the provision of electron traps which have sufficient electron affinity to aid in the removal of electrons from anions subjected to less severe perturbations. Severe crowding of anions during shear may lead to the direct transfer of an electron to an anion which has more room. The release of negative charge would decrease the size and hence the crowding of the perturbed anion. Since the hexacyanoferrate(I1) ion formed in this way is more stable than hexacyanoferrate(II1) ions the electron deficient anion will have no means of regaining its lost electron. Instead it will probably split off cyanide radicals and release two other electrons. There is evidence which indicates that the cyanide radicals formed react to form polymeric paracyanogen at high pressure. It is inter-
loot
I
I
I
0
a
“y O 20 r
-
’
2
0
u o 0
I
i
I
NORMAL SHEAR RATE 5 X NORMAL SHEAR RATE PLOTTED AT 5 X ACTUAL TIME
I
1251
I
3
2
5
4
TIME, MIN.
Fig. 3.-Conversion of KsFe(CN)awith shear at 10,000 atm. 100
I
o ORIGINAL
POWDER
n POWDER SHEARED
FOR 0.15 MIN AT 10,000 ATM
0
1000
2000
TIME, MIN.
Fig. 4.-Thermal conversion of KaFe(CN)a at 250”.
esting to note that Tyler6 has postulated relatively free electrons near slip planes to explain the short lived (0.2 sec.) enhancement in electrical conductivity observed by Gyulai and Boross during plastic deformation of potassium chloride and potassium bromide. Although potassium hexacyanoferrate(111) does not normally triboluminesce,’ triboluminescence is another process by which plastic deformation gives rise to free electrons.* The very absence of triboluminescence in this compound may be due to a rapid capturing of any freed electrons to form hexacyanoferrate(I1). Regions in the lattice having abnormally high concentrations of potassium ions could serve as trapping sites in the second type of mechanism. As in the F-centers of the alkali halidesg such trapped electrons would probably be shared among many neighbors so that escape from the trap to other anions would be easy. Some of the trapped electrons would return to the donor anions; others would complete successful reactions. Here again the electron deficient anion would be expected to decompose further. The fact that some potassium metal is found during thermal decomposition of potassium hexacyanoferrate(II1) l o may be of significance here. The marked influence of shear on the rate of thermal degradation of potassium hexacyanoferrate(111) (Fig. 4) has already been cited as evidence that the mechanism of conversion during shear is different from the mechanism of thermal conversion. The residual strains in the sheared material (6) W. W. Tyler, Phys. Reus., 86, 801 (1952). (6) F. Gyulai and J. Boras, Math. Naturw. A n i . Ungar. Aiead. Wiss., 69, 116 (1940).
(7) “Gmelins Handbuah der Anorganisohen Chemie,” 8th ed., Val. 69B, 1932, p . 980. (8) G. Wolff, I. Schonewald and I. N. Stronski, 2. Krist., 106, 146 (1964). (9) B. 8. Gourary and F. J. Adrian, Phyr. Rev., 106, 1180 (1967).
(lo)
B. Ormont and B. A. Petrow, Monatsh.. 68, 171 (1983).
1252
NOTES
are felt to be responsible for the increased rate at which it is converted thermally. These strains are undoubtedly much less than those which prevail during plastic deformation. As illustration of this fact it is pointed out that preliminary shearing enhances the rate of thermal conversion at 250' by a factor near 100 whereas the rate of conversion during plastic deformation is about 14 orders of magnitude faster than the estimated rate of thermal conversion at the temperature employed during shear. This interpretation is consistent with a number of related facts. Cold worked metals require high temperatures and appreciable times for complete annealing. Heating a solid sample of sheared potassium hexacyanoferrate(II1) for 1.5 hours a t 315" induced only a partial recrystallization of the potassium hexacyanoferrate(I1) as determined by the crystal sensitive bands in the infrared spectrum. The factor by which the rate of thermal decomposition of potassium hexacyanoferrate(II1) is enhanced by preliminary shearing falls from 100 to 20 during the course of the thermal decomposition, a fact which is explained by thermal annealing of some of the residual shear strains. The mere presence of the products of decomposition of potassium hexacyanoferrate(II1) does not accelerate its rate of thermal decomposition.ll As pressure is increased the rate of conversion with shear of potassium hexacyanoferrate(II1) rises (11) P. I. Byal'kevich, Vslrtnik Akad. Nauk, Beloruss S.S.R., 1, 71 (1953); abo C. A., 49, 50891 (1955).
Vol. 61
more rapidly than the shear stress rises (Fig. 2). With a pressure increase from 10,000 to 40,000 atm. for example, the rate of reaction increases by a factor of 5.0 while the shear stress increases by a factor of 2.3. This effect of pressure is interpreted as follows: As pressure is increased the amount of slip taking place on a given plane is decreased thuc; causing more slip planes to operate and increasing the likelihood that each molecule will be in a disordered region. Greater distort,ions of the lattice are also a result of increased pressure but this factor already has been accounted for in terms of the increase in the shear stress. The data obtained with shear at 10,000 atm. are not of sufficient accuracy to make an unambiguous determination of the order of the reaction. As expected, the data for the thermal reaction are best fitted by first-order kinetics. The fact that the oxalate complex does not react under shear is attributed to a shift in equilibrium with pressure. At a pressure of 50 atm. of carbon dioxide ferrous oxalate in potassium oxalate solution is partially converted to potassium tris-(oxalato)-ferrate(III).12 The necessity of estimating several physical properties made impossible a reliable estimate of the sign of the volume change involved in the reaction expressed by eq. 3. We are obliged to J. C. Bailar, Jr., for lending us some of the samples and for useful discussions. H. A. Larsen wishes t o express thanks to the Shell Fellowship Committee for financial assistance.
\
r
(12) C. Schaper, Z. physik. Chem., 73, 308 (1910).
NOTES ENERGY INTERACTIONS IN T H E FLUOROCHLOROMETHANES BY DONALD E. PETERSEN AND KENNETH S. PITZER Department of Chemistry, Univerdtu of California, Bsrkeleu, California Received March 1 , 1067
It is a common approximation to assume constant properties for a bond between a given pair of kinds of atoms regardless of their surroundings. Many examples of constant bond distances could be cited. If bond energies are constant. for the C-X and C-Y bonds, then the redistribution reaction for the various compounds CX,Y4 - n is governed in good approximation by purely random statistics. Calingaert2has found several examples of this type, and Forbes and Anderson3 showed that the chlorobromomethane system was random. Of course, this can only be an approximation-even isotope redistribution reactions are known to deviate minutely from randomness. In contrast, however, t,he reported values of the (1) This reaearch was asaisted by the American Petroleum Institute through Researoh Project 50. (2) a. Calingaert, st d., J . Am. Cham. Soa., 61,2748,2765,2758,3300 1939); 63, 1099, 1104 (1940). (3) G. 8. Forbw and E. H. Anderson, Mi., 66, 931 (1944).
C-F bond length vary rather systematically over an appreciable range. Table I shows several examples. Where there is but a single fluorine, the C-F distance is about 1.42 A., but with three or four fluorines present the distance is near 1.32 8. Brockway4 explained this effect by the double bond-no bond resonance. Ff F/" XaC
F '
-
X d
F-
f3
?
XIC
\F+
This resonance will make the C-F bond energy greater in compounds with two or more fluorines. The quantitative magnitude of the effect is not readily predicted, however. In the lighter paraffin hydrocarbons there are significant deviations from constant bond energies but these have been explained by non-bonded interactions arising from electron ~orrelation.~ These were treated by the same methods that were developed for intermolecular forces. It is interesting to apply this method to the fluorochloromethanes and (4) L. 0. Brockway, THISJOURNAL,41, 185 (1937). (5) R. S. Pitmr and E. Catalano, J . Am. Cham. Soc., 78, 4565, 4844 (1956).
7
