Exoeiectron Emission from Ground Aluminum Powder
1329
Exoelectron Emission from Ground Aluminum Powder and Its Relationship to the Adsorption of Oxygen, Water, and Some Organic Compounds Yoshihiro Momose,. Yasuo Iguchi, Sakae Ishli, and Kazuei Komatsuzaki Department of industrial Chemistry, Faculty of Engineering, lbaraki University, Hitachi, ibaraki, Japan
(Received December 3, 1975)
Publication costs assisted by lbaraki University
The surface properties of mechanochemically altered aluminum powder have been studied by means of optically stimulated exoelectron emission (EEE). T h e “glow-curve” characteristics of E E E from the powder ground in air were strongly influenced by the kind of organic vapor contained in the Geiger-counter gas. The vapors are listed in the descending order of the emission as: (n-C3H,)zNH > n-C3H,NH2 > CH&OOC2H:, > C6H6 > C2H50H > (CH3)2CO > CH&N > n-C4H&1. T h e emission is correlated with the dielectric constant or acidity constant of the organic compounds, by which the electric field formed on the surface may be influenced. When the powder ground in air was exposed to air for a longer time, the emission became weaker and decreased with a n increase of the air humidity. T h e decay of the emission may be associated with the growth of oxide film and the more amorphous layer formed by condensation of water molecules. The powder ground in various environments (vacuum, 0 2 , H20, CzHbOH, and n-CSH7NH2) gave a quite different “glow curve”. The emission for vacuum was weaker than that for other environments and the emission varied with increasing vapor pressure of H20, C ~ H B O Hand , n-C3H,NH2. The surface altered in these environments exhibited a different emission decay during exposure to air, which indicates that the modified surface has a specific chemical activity.
Introduction Exoelectron emission phenomena have been reviewed from time to From the viewpoint of whether the phenomena are due to processes inherent to metals or result from interaction with the environment, Ramsef has used the terms “intrinsic” E E E and “extrinsic” EEE, respectively. The latter means that adsorption, the presence of oxide, or some other external material factor is essential for E E E and the present paper is associated with this type of EEE. The surface phenomena of aluminum is of considerable interest in view of the wide-spread use of this metal because of its light weight, mechanical workability, and remarkable resistance to atmospheric corrosion. Therefore, there is great interest in the role of exoelectrons in mechanochemical rea c t i o n ~ in ~ - adhesion ~ or corrosion phenomena and further in the role of these electrons in heterogeneous reactions. Few reports are available concerning the effect of adsorption of gases other than water and oxygengJOon the emission from aluminum. However, Polyakov and Krotovall have reported that the emission rate upon detachment of polymers from glass is determined primarily by the type of functional groups in the polymer. The present work was undertaken to investigate the interaction between ground aluminum powder and some gaseous compounds by means of EEE: the effect of organic vapor in the Geiger-counter atmosphere on the exoelectron “glow curves” (the change of the emission intensity with temperature) for powder ground in air or in various environments containing the organic vapors, water vapor, and oxygen, and also the “glow curves” as a function of grinding time and exposure time to air after grinding. Experimental Section Maierials. Commercial aluminum powder (purity 99.5%, 115-200 mesh, Wako Chemicals) was used. Its surface would, of course, be covered by an oxide film. The surface of the powder received no special preparation other than storing in
air dried with phosphorus pentoxide for 1 or more days before use. The purity of the oxygen used was 99.99%. The water used was redistilled. The organic vapors were identical with those in earlier papers12J3 and we used the following compounds: di-n-propylamine, n-propylamine, ethyl acetate, benzene, ethanol, acetone, acetonitrile, and n-butyl chloride. Grinding. The grinding of the powder was performed by use of a magnetic stirrer in two experimental methods. In one method, the powder (0.50 g) was introduced into a glass vessel (16 mm in diameter and 55 mm deep) with an aluminum plate laid a t the bottom and then ground in air (18-24 “C, 32-62% relative humidity (RH)) for a given time (usually 10 min). The powder was broken between a rotator (a small iron bar, 350 rpm) and the aluminum plate to become a finer powder. The sample was kept in air for a given time (usually 1 to 1.5 min) after grinding (this time will be termed exposure time) and then spread evenly in a gold vessel (20 mm in diameter and 3 mm deep). This sample will be termed the A sample. In the other method, the powder (20.0 g), having been introduced in a larger glass vessel and outgassed under vacuum below Torr for 15 min, was ground between a magnet rotator (600 rpm) and an aluminum plate for 30 min in the presence of various gases ( 0 2 , H20, C ~ H S O Hand , n-C:rH,NH2) or in vacuo. The reaction vessel volume was 424 cm:’. The grinding was started the moment the gas was admitted into the vessel. T h e initial pressure of gas will be termed Po. The pressure change of the reaction vessel with time was measured manometrically. We note that some welding between the powder and the aluminum plate took place in vacuo and in oxygen and that in the presence of water or organic vapors, the plate surface became rough or glossy, respectively. The exposure time in air (15-23 OC, 35-59oh RH) was usually 10 min and 0.50 g of the sample was used for the E E E measurement. This sample will be termed the V sample. Exoelectron Counting. E E E was measured by use of a modified Geiger counter. T h e counter-gas composition was a mixture of the organic vapor (20 Torr) and argon (84 Torr) The Journal of Physical Chemistry, Vol. 80, No. 12, 1976
Momose et ai.
1330
TABLE 1: DeDendence of "Glow-Curve" Characteristics of Ground Aluminum Powder on Organic VaDors * Organic vapor
Intensity at 25 "C, countls 50 50 12 7 10 9
2 0.5
Peak intensity, count/s (temp, "C)
Intensity at 239 OC,countls
Total count (25-239 " C )
150(88)
30
61 900
100(106)
25
No peak 34(108)
83
45 400 29 200 15 800 8 300
16 11 1
19(72) 19(66)
9(80) 2.5( 110)
5 700 2 700 900
1
ca.0 (230 "C)
(25-230 "C)
Grinding time, 10 min; air exposure time, 1-1.5 min. except that of ( ~ - C ~ H T ) ~(14 NTorr) H and Ar (90 Torr). These counter gases gave the same counting rate for a radioisotope. An A or V sample in the gold vessel was mounted on t h e sample holder in the counter. The counter was then evacuated and flushed with argon, and t h e counter gas was admitted. After the emission had been measured a t 25 "C for 1 min, the "glow curves" were examined by heating the sample to 239 "C (230 "C only for n-C4H&I) a t the rate of 19 "C/min. In the case of the A sample, the sample was illuminated by a weak fluorescent light (wavelength of light > 295 nm) during measuring of the emission, because in this case the inside of the counter had been exposed to the light in the laboratory through quartz glass, and the number of the "glow curves" examined was 7 to 16 for each counter gas. In the case o f t h e V sample, a 20-W bulb was used as a light source (wavelength > 300 nm), and the number of the "glow curves" was 3 to 5 for each grinding environment. A counter gas of C2H50H-Ar was used for the samples treated in the vapors of 0 2 , H20, and C2H5OH, and both counter gases of C*H,iOH-Ar and n C:]HYNH2-Ar were used for those in n-CnH;NH2 vapor and in vacuo. In all experiments an accelerating voltage of 96 V was applied. Both the total count (25-239 "C) and the counting rate were recorded by a scaler and a ratemeter. The value of the total count given in the text does not contain the count a t 25 "C for 1 min.
Results (a) A Samples. T h e "glow-curve" behavior in different counter gases differed widely in spite of identical mechanical treatment. T h e "glow curves" except t h a t in t h e CH:3COOC2H:,-Ar counter gas exhibited an emission peak. T h e emission for the latter increased with temperature with a discernible shoulder near 150 "C. Table I shows the relationship between the median values of the "glow-curve" characteristics and the kind of organic vapor contained in the counter gas. The organic compounds are arranged in the descending order of the value of the total count. In all the experiments the mean deviation from the median value was less than f 2 8 % for the total count, &35%for the intensity o f t h e peak emission, and &14 "C for the peak temperature. The intensity of emission was strongly influenced by the kind of organic compounds and the order of the compounds was very similar to that for both sandblasted mild steel12 and iron surfaces exposed to a discharge from a Tesla coil.':' The range of the peak temperatures over all the compounds was much wider, compared with that for the sandblasted steel (55-66 "C) and the iron surface (152-173 "C for the main peak). Judging from the peak temperature deviations of rt14 "C, there are two groups of the peak temperatures (on the average The Journal of Physical Chemistry, Vol. 80, No. 12, 7976
Total count I
70 -
. -x U
60-
C
a 50-
-s U
'G 4 0 c +
.C 30-
0
50
100
203
150
a0
Temperature, 'C
Figure 1. Effect of grinding time on the "glow curve" (air exposure time, 1-1.5 min): (a) 60 min, (b) 10 min, (c) untreated powder.
I , ,
50
# , II
,
I
/
I
,
100 150 200 Temperature,T
I
I
,
250
Figure 2. Dependence of the "glow curve" of ground aluminum powder on time of exposure to air (22 "C, 50% RH) after grinding: (a)0.02 h; (b) 0.5 h; (c) 3 h; (d) 21 h: (e) 44 h.
108 "C for n-CdHyC1, CsHB, and n-C:3HjNH2, and 77 "C for (n-C.]H.;)yNH,CH&N, C2H:OH, and (CH:])yCO). The effect of the grinding time and exposure time to air on the emission was examined by use of a counter gas of C2H:OH-Ar. Figure 1 shows typical "glow curves" for two grinding times together with that of the untreated powder. In the case of 60-min grinding time, the ground powder became slightly black. The emission increased with an increase of the time of grinding. This indicates that the intensity of emission is regarded as a measurement of the extent of the mechanical damage, though the emission would be expected to reach a maximum and plateau with continued grinding. Figure 2 shows the "glow curves" as a function of the time of exposure to air (22 "C, 50% RH).The emission was gradually suppressed by exposure t o air, t h e temperature for t h e emission peak
1331
Exoelectron Emission from Ground Aluminum Powder
TABLE 11: Dependence of "Glow-Curve" Characteristics of Ground Aluminum Powder on Exposure Time to Air after Grinding a Temp and humidity of air
Exposure time, h
Intensity at 25 "C, countls
Peak intensity, countls (temp, " C )
Intensity at 239 O C , countls
Total count (25-239 " C )
22 "C ca. 0% RH
0.5 3 43
6.2 2.7 0.9 0.5
16.0(82) 12.8(95) b b
12.0 9.5 6.1 6.0
7900 5900 3200 2200
0.5 3 21 44
4.6 1.3 0.4 0.2
11.1(92)
7.6(97) 5.5(148) 3.4(182)
9.0 5.6 4.6 2.5
5600 3900 2800 1600
0.5 3 21 42
2.2
8.3(121) 6.0(140) 4.2(170) 3.4(193)
6.5 4.6 3.0 2.6
3700 3000 1800 1300
21
22 "C ca. 50% RH
22 "C ca. 100% RH
a
1.0 0.3 0.3
Grinding time, 10 min. The intensity of emission gradually increased with increasing temperature.
became increasingly higher, and the "glow curve" for 44-hr exposure time was quite similar to that of the untreated powder. Table I1 shows the relationship between the "glowcurve" characteristics and the time of exposure to air of different humidities in which the sample has been stored immediately after grinding in air (22 OC, 50% RH). Relative humidities of 0 and 100% refer to ambient air dried by phosphorus pentoxide and air saturated with water vapor, respectively. The variation of the characteristics with exposure time depended strongly on the moisture content of the atmosphere. Figure 3 shows plots of the total count ( T )vs. exposure time ( t ) in each ambient air on a semilogarithmic scale. The points fell on a straight line, represented by the equation T = B - h log t , in each case, and further the slope became steeper with decreasing humidity. The values ( k ) of the slope were 2910 (0% RH), 2000 (50% RH), and 1290 (100%RH). The "glow curves" in the absence of light illumination were also examined. Even a counter gas of (n-C3H7)2NH-Ar, giving the largest amount of electrons emitted under light illumination, gave rise to only very weak emission: the peak intensity was 2 counth a t 143 OC and the total count was 400; for the C2H50H-Ar counter gas the total count was about 200. This clearly indicates that photon energies are essential for producing emission from the ground aluminum powder. On the other hand, in the presence of illumination by much stronger light (wavelength > 300 nm), such as sunlight, the emission for a C2H50H-Ar counter gas was a factor of 60 larger over the entire temperature range, but the emission peak was located a t almost the same temperature. (b) V Samples. Table I11 shows the "glow-curve" characteristics (median values) of the optically stimulated emission from samples ground in various gaseous environments. I t is clear that the mechanochemically altered surface has still maintained an essentially different nature even under exposure to the same counter-gas atmosphere. In the environment of 0 2 , H20, n-C3H?NH2, and under vacuum, an emission peak was observed a t almost the same temperature (near 110 "C), but in the case of C ~ H S O Ha, broad emission curve with a slightly elevated intensity a t about 50 O C and sometimes about 170 "C, similar to curve b in Figure 1,was observed. It may be noted that the values of the total count both for the environments of n-CsH7NH2 and under vacuum and for outgassing alone were a factor of 6 or more larger in the case of the nC3H7NHpAr counter gas than in the case of C2H50H-Ar, the same as shown inTable I, but the emission peak for each case
O h ' '005"OI
'
"0.5 ""' 1 ' ' " '5" " Exposure time, hr
10
'
50
Figure 3. Plots of total count of ground aluminum powder vs. time of exposure to air (22 OC):(0)0% RH; (A) 50% RH; (0) 100% RH.
was located a t almost the same temperature. The emission for CzHsOH environment increased by grinding for 3 hr, but the pressure decrease for this time was almost the same as curve f i n Figure 5. Figure 4 shows the dependence of the total count on the initial pressure of the vapors. The values for both n CsH7NH2 and C ~ H B O were H largest a t 4.1 Torr and decreased, more rapidly in the latter case, with an increase of initial pressure, but those for H2O indicated an ascending tendency. Figure 5 shows the pressure change with the time of grinding in various environments. The curve for 0 2 (g), H2O (b,c), and CzHsOH (f) showed considerably large variations which are caused by mechanochemical reactions. The curve for nC3H7NH2 was almost independent of the grinding time in spite of a larger initial pressure range. Table IV shows the dependence of the representative "glow-curve" characteristics for various environments on the time of exposure to ordinary air. In the case of the samples ground both during outgassing and in HzO vapor, the characteristic values for 0.17-0.19-hr exposure time differed markedly from those for times longer than this time. In the case of C ~ H S O H and n-C:jH,NH2, the values varied gradually in the same manner as shown in Table 11. The peak temperatures for n-C:3HYNH:! was virtually independent of the exposure time, compared with those for C2H50H. The values ( h ) of the slope obtained from the plots of the total count vs. air exposure time were 1210 for C2HsOH and 16 700 for n-C3H7NH2. The former is approximately equal to the value for 100% R H air obtained from Table 11. Discussion (a) Effect of Organic Vapor in the Counter Gas on the The Journal of Physical Chemistry, Vol. 80, No. 12, 1976
Momose et al.
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TABLE 111: "Glow-Curve" Characteristics of Aluminum Powder Ground in Various Environments a Counter gas
Grinding environment
CZHSOH+ Ar
Vacuum
Pressure (PO), Torr
2.0 16.8 4.2 8.2 4.1 8.3 16.8 16.5 16.9
0 2
HzO HzO O H CzHjOH CzHbOH CzHsOH' n-C3HjNH2 Outgassingd n-CSH7NH2 + Ar
Intensity a t Intensity a t 25"C, Peak intensity, 23Qoc, countls countls (temp, "C) countls
Vacuume n-CSH7NHZ n-C3H7NH2 n-QH7NH2 Outgassing d
4.1 16.6 81.3
Total count (25-239 "C)
6.0 20 9.0 11
13 ca.0
8(109) 31(100) 11(107) 16(100) 25(93) 13(50) 11(60) 18(54) 26(111) 3.0(168)
13 7.0 15 15 2.0
3 800 15 700 5 200 7 700 17 500 8 500 5 100 10 100 12 600 900
18 55 65 20 2.0
65(121) 175(121) 190(111) 150(148) 32(161)
43 65 35 50 12
34 500 80 000 I 5 200 58 700 9 900
9.0
3.0 5.0 21 10 8.0 12
21
a Grinding time, 30 min; air exposure time, 10 min. This includes grinding during and after outgassing. Grinding time, 3 h. Only outgassing (15 min) without grinding. Grinding after outgassing.
!44 '"%pX, 1 53
I-
,
1
,
,
,
,
,
m/
0
10
20
30
40 50 P o , Torr
60
70
80
Figure 4. Plots of total count vs. initial pressure (Po) of vapor: (A,0, 0)C2H50H-Ar counter gas; (A,0 ) c-C3H7NH2-Ar counter gas.
Emission ( A Sample). According to Scharmann? EEE caused by mechanical treatment is based on the following three effects: (a) creation of fresh metal surface on which adsorption takes place, (b) imperfections in the metal, and (c) imperfections in the covering oxide. In this experiment t h e oxide film on t h e aluminum powder is broken even with very small loads by grinding to reveal the bare metal surface, perhaps because the substrate metal is softer than its oxide so that the latter cracks easily. T h e freshly exposed aluminum may interact with oxygen, water vapor, and other gases present in air immediately after grinding and oxide film may be formed again. Thus, the oxide film is thought to contain a great many defects, notably vacancies, which aid in the diffusion of metal ions and t h e growth of the oxide layer2 and to be partly hydroxylated, because under atmospheric conditions alumina contains physically adsorbed water, surface hydroxyl groups, and possihly hydroxyl groups incorporated in the bulk lattice; the adsorbed molecular water can be easily removed, whereas t h e hydroxyl groups can hardly be removed completely even under the most stringent conditions.'4 Ramseyg and Linke and Meyer"' have reported that the emission from abraded aluminum results from the adsorption of water molecules as a The Journal of Physical Chemistry, Vol. 80,No. 12, 1976
t
1
0
5
10 15 20 25 Grinding t i m e , rnin
30
Figure 5. Pressure change with grinding time in various environments: (a) after outgassing; ( b ) H 2 0 (Po,4.3 Torr): (c) H20 (Po, 8.4 Torr): (d) C~HSOH(Po, 4.1 Torr); (e)C2H50H(Po, 8.2 Torr): (f) C2HSOH(Po, 16.7 Torr);(9)O2 (Po, 17.1 Torr): (h) rbC3H7NH2 (Po, 4.1 Torr): (i) rbC3H7NH2 (PO,16.6 Torr): (j) n-C3H7NH2(Po, 81.5 Torr).
hydrogen outward orientation of either H20 molecules or OH radicals, which lowers the work function. We will explain the experimental results in terms of an electron trap model shown in Figure 6. Figure 7 shows the relationship between the total count shown in Table I and the reciprocal dielectric constant ( U t ) of each organic compound. I t is apparent that the amount of emitted electrons is closely correlated with the dielectric constant except for both n-CdH&I and CeHe. This suggests t h a t t h e emission occurs perhaps under the influence of the electric field ( E ) produced by the surface hydroxyl groups, namely, t h a t as a result of the introduction of the substances into the surface, the electric field strength reduces from the
Exoelectron Emission from Ground Aluminum Powder
1333
TABLE IV: Dependence of "Glow-Curve" Characteristics f o r E a c h G r i n d i n g Environment on Exposure T i m e to Air a f t e r Grinding
Grinding environment (Po, Torr)
Intensity a t 25OC, countls
Peak intensity countls (temp, "C)
Intensity a t 239 "C, countls
3.0 24.0 48.3
2.0 0.5 ca. 0 ca. 0 ca. 0
8.0(136) 3.5(93) 3.0(104) 3.0(114) 2.5(126)
6.0 3.0 2.0 1.5 1.5
3 800 1 500 1300 1100 1000
HzO" (8.2)
0.17 1.0 3.0 23.2
2.0 1.0 0.5 ca. 0
8.0(129) 3.5(95) 4.0(93) 3.0(95)
7.0 2.5 2.0 1.5
4 600 1600 1 500 800
CzH50H" (16.8)
0.20 1.0 3.0 24.0 48.0
9.0 6.0 4.0 2.5 2.0
12 (63) 9.0(72) 7.0(72), 6.5(181) 5.5(79), 5.5(165) 5.5(198)
7.0 7.0 6.0 5.0 5.0
5 800 4 900 4 200 3 200 3 000
n-C3H,NHzb (81.3)
0.17 1.0 3.0 24.1 47.7
Exposure time, h
During outgassing"
a
0.19 1.o
150(121) 140(126) QO(133) 66(144) 54(150)
30 22 16 9 8
50 55 45 41 34
Total count (25-239' C)
63 300 59 600 42 500 30 800 25 300
CZHsOH-Ar counter gas. n-CsH7NHz-Ar counter gas.
r/,//J////, I..
I o
AI
disturbed oxide film
.-
A[
.disturbed substrate metal
Figure 6. Electron trap model: (E) the vector of electric field strength due to adsorbed surface hydroxyl groups onto which the organic molecules may be adsorbed; (Eo)the vector of electric field strength across the oxide film, which forces aluminum ions from the metal through the oxide film to the surface, to react with oxygen gas.
70330r
I
h
0
Figure 7. Total count of emitted electrons as a function of reciprocal dielectric constant (1/c) of organic compounds.
value ( E )to the value ( E / € )In . order to expand and clarify the role of the simultaneous optical and thermal stimulation in relation t o both t h e proposed surface field a n d oxide imperfections, it would be useful to refer to the concept proposed
by Claytor and Brotzen,15 who used a similar technique on aluminum deformed in tension. They have associated thermal stimulation with vacancy diffusion to the surface, where then some of vacancies create preferential sites for optically stimulated emission. T h e electrons trapped in such sites are removed by light absorption a t photon energies lower than those for the original oxide-coated surface. The fact that hardly any emission was observed in the dark seems to be attributable to the diffusion of electrons trapped a t vacancies through the oxide film to adsorbed oxygen, giving rise to 0- ions instead of electron emission. I t should be noted that this electric field does not deal with a model of the transversal electric field across the fissures created in t h e oxide layer, as proposed by Gieroszyhki and Sujak.I6 Dear et al." have reported that substances such as C6H6 and n-C4H&1 fit a plot of t h e heat of immersion of aluminum powder against dipole moment, but t h a t n-CdHgOH and n CdHgNH2 show a much higher heat than t h a t corresponding to their dipole moments, which may be attributed to hydrogen-bond formation. The molecules on the straight line in Figure 7 may presumably be adsorbed through hydrogen bonding. In previous it has been proposed that the emission may be strongly associated with the proton attracting interaction between the functional groups of organic molecules adsorbed and the hydroxyl groups on the damaged solid surfaces. T h e dielectric constant of organic compounds used is correlated with the negative logarithm of the acidity constant, pK"+, for conjugated acids (BH+) of organic compounds as conjugated bases (B),as follows.18--20 B
(n-C3H7)2NH n-C:jH7NHz CHSCOOC~HF, (CHdzCO CH3CN
