X-ray and spectroscopic studies of mechanically treated or irradiated oxides

Reactivity of So&Is, 4 (1987) 1-21 Elsevier Science Publishers B.V., Amsterdam

- Printed

in The Netherlands

REVIEW X-RAY AND SPECTROSCOPIC STUDIES OF MECHANICALLY TREATED OR IRRADIATED OXIDES

U. STEINIKE

*, U. KRETZSCHMAR,

I. EBERT

and H.-P. HENNIG

Akademie der Wissenschaften der D. D. R., Zentralinstitut Rudower Chaussee 5, 1199 Berlin (G.D.R.)

ftir physikalische

Chemie,

and

L.I. BARSOVA

and T.K. JURIK

Academy of Sciences of the U.S.S.R., 117312 Moscow (U.S.S.R.)

Institute

(Received

March 23rd, 1987)

May 14th, 1986; accepted

of Physical Chemistry,

Lenin Avenue 31,

CONTENTS

Abstract ............................................................... 1. Introduction ......................................................... 2. Experimental ........................................................ 3.Results ............................................................. 3.1. Influence of pressure and shearing stress .................................. 3.1.1. Results for periclase ........................................... 3.1.1.1. Structural defects ....................................... 3.1.1.2. Dependence of structural defects on the surrounding atmosphere durmg treatment ............................................. 3.1.1.3. Distribution of structural defects ............................. .......................... 3.1.1.4. Influence of activation by irradiation 3.1.2. Results for quartz (SiO,) ........................................ 3.1.2.1. Structural defects ....................................... 3.1.2.2. Dependence of structural defects on the surrounding atmosphere during treatment ............................................. 3.1.2.3. Dtstribution of structural defects ............................. .......................... 3.1.2.4. Influence of activatton by irradiation 3.2. Influence of shock stress ............................................. 3.3. Thermostability of structural defects .................................... 4. Comparison of the influence of pressure and shear stress m periclase and quartz ......... References ..............................................................

0168-7336/87/$03.50

0 1987 Elsevier Science Publishers

B.V.

8 8 11 11 11 13 14 15 16 16 17 19

2 ABSTRACT Results on the kind and distribution of structural defects in quartz (SiO,) and periclase (MgO) after mechanical treatment have been presented. Zero-, one-, two- and three-dimensional defects are generated. Mainly surface near regions are distorted, where interactions of the induced defects with the atmosphere are possible. The mechanical energy causes a stronger structural distortion in quartz than in periclase, but the depth of penetration of the distortions is larger in the latter compound. The distribution of the defects depends mainly on the mechanism of treatment; kind, concentration and penetration depth depend on the material-specific features of the substances.

1. INTRODUCTION

The impact of mechanical energy on solids and its influence on reactivity have been intensively investigated for some time [l-12]. Mechanical energy is used in a great number of processes in the chemical industry for conditioning of raw materials and for processing of intermediate and final products. During the impact of mechanical energy on a solid, a large number of elementary, micro- and macro-processes are induced, leading to storage of energy as a result of modifications of the morphology and/or structure [13-171. These processes are very complex and dependent on the kind and intensity of the mechanical stress and on the specific nature of effects upon the lattice structure of the solid. The stress mechanism (shock, pressure, shearing stress) also determines the kind and distribution of changes in the structure [l&21]. The excitation energy supplied usually is not exactly calculable, because the mechanisms of the energy transfer are not yet known. The fact that structural defects mainly occur together and influence each other makes the situation still more complicated. Because a separation of different kinds of defects is difficult, an integral perturbation state or disordered state is considered. For many cases it is sufficient to correlate this integrally measured perturbation state with some experimentally measured parameters of reactivity. For a deeper interpretation of the connection between structure and solid-state reactivity and for an explanation of the mechanisms of solid-state reactions it is necessary to have some knowledge of the kind of induced defects and also their concentrations and distributions in the solid. The aim of this paper is to survey the results available on the kind and distribution of structural defects (including electronic disorder) in oxides and to discuss some comparative considerations of the range and mechanism of their generation. The selected oxides are quartz (SiO,) [20-291 and periclase (MgO) [30-351. Periclase is an ionic compound with an NaCl lattice. In contrast, quartz belongs to the framework silicates, the most stable and characteristic struc-

3

tural element of which is the SiO, tetrahedron. In quartz there are only Si-0 bonds, which are to 52% ionic according to Pauling [36]. The structural defects considered are those created in a vibratory or planetary mill by pressure and shearing under air. Moreover, the influence of shock (disintegrator) and variations in the surrounding gas atmosphere as well as comparative reflections between mechanical activation and activation by irradiation are discussed, in addition to the influence of an irradiation on the mechanically activated solid.

2. EXPERIMENTAL

The substances investigated were periclase (MgO), obtained by hydrothermal dissociation of MgCl,, contaminated with iron (1.2 - 10P2 wt.-%), manganese (2.2 - 10d3 wt.-%), chromium (1.1 . 10e3 wt.-%) and nickel (1.2. 10P3 wt.-%), and quartz sand from Hohenbocka (G.D.R.) with grain diameters of 0.2-0.4 mm. The equipment for activation under laboratory conditions consisted of a vibration mill and planetary mill for pressure and shearing stress and a jet mill and disintegrator for shock stress. y-Radiation (Y-~‘CO) at - 196 o C. Electron paramagnetic resonance (EPR) spectroscopy, X-ray diffraction and BET surface area measurements were used to obtain the results.

3. RESULTS

Each of the mechanical activations results in crushing or smashing of the initial agglomerates, and thereby in an increase in the specific surface and in structural changes that are determined by the primary particle diameter and lattice distortions (in mechanically activated quartz there are no lattice distortions). Surface and crystallinity are linearly related to each other (Fig. 1, Table 1). Fig. 1 shows remarkable difference in the changes, which are dependent on the stress mechanism. The effect of activation, characterized by surface and crystallinity, is increased by pressure and shearing stress. As a result of pressure and shearing, round crystallites are generated, whereas shock stress forms cornered crystallite fragments. 3.1. Injluence of pressure and shearing stress 3. I. 1. Results for periclase 3.1.1.1. Structural defects. Paramagnetic impurity ions such as Mn2+, V2+ and Cr3+ are located at cation sites of cubic symmetry and can be used as indicators for the detection of structural defects.

4

5o

x, %

100

Fig. 1. Correlation between specific surface area (S) and crystallinity mill: A, MgO; X , SiO,. D = Disintegrator: 0, MgO; 0, SiO,.

(X).

SM = Vibration

Fig. 2 shows the middle part of the EPR spectra of the initial material (1) and of material that has been mechanically treated (2 and 3). The former (1) shows EPR lines of the paramagnetic ions. On applying pressure or shearing the intensities of each line of the Mn2’ hyperfine sextet and of the Cr3+ line decrease and the linewidths increase. The decrease in the intensities of the EPR spectra can be explained by the creation of preferably distorted surface near ranges (ca. 30 nm). This supposition is supported by the fact of the

Mn”O-

--A--3

0;

F*

Mn2+,Cr3+

Fig. 2. Middle part of the EPR spectrum activated substance (2, 16 h; 3, 32 h).

of the initial

MgO (1) and of the mechanically

5

Fig. 3. Middle part of the EPR spectrum of the same substance y-irradiation.

as in Fig. 2, but after

growth of the specific surface during activation. The increase in the linewidths and the X-ray results also indicate the presence of distortions in the bulk of the crystallites. By mechanical activation a narrow EPR line with g = 2.0019-2.0022 is created by the formation of F+ centres [37] (F+ centres are the result of trapping of electrons by mechanically induced anion vacancies). Such F+ centres are found, e.g., after strong irradiation of MgO single crystals [38] and also after pulverization [39]. The mechanically generated F+ centres are in the bulk, as opposed to those found by Spicyn et al. [40] in the surface or near the surface of MgO powders. Subsequent y-irradiation after mechanical treatment leads to an increase in the concentration of F+ centres and to the formation of V- centres (cation vacancies filled with one hole) with g, = 2.036 and g,, = 2.002 [39-431. Fig. 3 shows the middle part of the EPR spectra after y-irradiation. The maximum attainable concentration of F+ centres after l-4 Mrad y-irradiation at - 196 o C depends on the mechanically generated concentration of anion vacancies, including the number of those from the initial material. On mechanical treatment the number of the anion vacancies is increased; the y-irradiation supplies the electrons necessary for the formation of F+ centres. Oxidation processes may also be a source of electrons to a small extent.

6

Fig. 4. Dependence of the maximum lattice distortion, Au/a.

of concentration

of F+ centres,

Z In,

on the relative

The maximum attainable concentrations of Ff centres (2,) and of Vcentres increase with increasing duration of treatment until they reach a stationary state, a kind of “equilibrium”. Obviously it is not possible to obtain even higher concentrations of anion and cation vacancies under the conditions chosen. Also with regard to other structural changes mechanical activation brings about such an equilibrium state. The generation of vacancies is connected with a deformation of the lattice. There is a nearly linear correlation between the maximum concentration of F+ centres and the relative lattice distortion, Au/a, in the investigated range (Fig. 4). A stricter consideration of the dependence of the lattice distortion and the concentrations of the centres on the period of treatment shows a more rapid increase of the lattice deformation than that of the concentrations of the centres during the first hour of mechanical treatment. The generation of vacancies is the decisive process in the destruction of the MgO lattice. From the results of high voltage electron microscopic (HVEM) measurements it was concluded that the treated single grains contain distortions in the bulk, which indicates a high dislocation density, that is no longer resolvable. The diffraction of these grains yields a typical texture diagram. Such texture diagrams have also been observed by Raether [44] after plastic deformation and by Willman and Agarwala [45] after mechanical surface treatment. Both suggested that these diagrams are caused by the decrease in the particle diameters and by twisting of the crystallites in the surface near region. However, such texture diagrams will also arise if a large number of small particles with statistically preferred orientations are attached to a relatively large grain. Experimental evidence exists for the occurrence of both of these effects. The dislocations are obviously located in the surface near region of the single grains.

-2

-4 Ils -

j/

02

014

Q6

,L:,

,

QE r,nm

1.0

Fig. 5. Differential FCDF of MgO. - - -, Initial sample; mechanically a planetary mill. r = Distance of an arbitrary atom from the or&h.

activated

in

Information about the short-range order, i.e., of the order of interatomic distances, has been obtained by X-ray wide angle diffraction [calculation of radial distribution functions (RDF)] [46]. In all the samples investigated the functions of the difference distribution agree well in their maximum positions. There are no significant deviations from those of the initial material. Each of the difference distribution functions shows well shaped maxima, which are typical of crystalline compounds and infer a long-range order for distances of r > 0.5 nm (r is the distance of an arbitrary atom from the origin in nm). However, the peak heights in the region of the long-range order decrease with increasing duration of the treatment (Fig. 5). This decrease can be explained, in a similar manner to quartz [47], by a demonstrable reduction in the diameters of the coherently scattering ranges and a distortion of the lattice. The mechanical treatment also influences the short-range order in the region between 0.35 and 0.55 nm (2nd and 3rd coordination spheres). The less peaked curvature can be explained by deviations of the orientations of two unit cells. Under the extreme circumstances of mechanical treatment as they exist in a planetary mill (Table l), changes of the 1st coordination sphere of Mg-0 may result. Distortions within a unit cell are created, for which a large dislocation density may be responsible. The distortion within the region of a unit cell causes a lattice expansion (from 0.4211 to 0.4218 nm, Table 1). The observed lattice dilatation means a supply of energy in the planetary mill that is high enough to increase the Mg-0 distance and to lower the lattice energy against the Coulombic attractive forces. Table 1 illustrates the considered structural defects.

8 TABLE 1 Structural defects in mechanically treated periclase t = Period of treatment; S = specific surface area; E, Au/a = lattice distortion; a, = lattice constant; Z, = maximum attainable concentration of F+ centres, equivalent to the concentration of anion vacancies. Sample *

t (h>

S*lO% Cm28-l)

EflO% Au/u

a, +2.10-4 wn>

Znl (8-l)

1 2 3 4 5 6 7

0 2 8 16 32 64 2.5

1.5 2.4 6.9 9.1 9.1 _

6.4.10-4 8.0.10-4 1.2.10-3 1.4.10-3 1.6.10-3 1.6.10-3

0.4211 _

5. 1o16 2.6. 1016 _ 1 *lo’* 1.6.1018 9.3.10” -

l

I _ 0.4218

Mechanical treatments l-6 in a vibration mill, 7 in a planetary mill.

3.1.1.2. Dependence of structural defects on the surrounding atmosphere during treatment. The mechanical treatment has mainly been performed under air,

but also under argon in some experiments. Under the latter conditions insignificantly stronger deformations (measured by X-ray diffraction) and concentrations of anion vacancies are observed. The atmosphere during treatment exerts a far stronger influence on the kind of defects created [48]. On mechanical treatment other groups of EPR spectra are generated: 1st group: g_L= 2.022, g,, = 2.002; 2nd group: g, = 2.007, g,, = 2.002. The first group can be assigned to special O- ions in the surface region, which are generated by the mechanical treatment [49,50]. The second group is regarded as due to 0; centres [50,51]. During treatment under air mainly 0; centres are created, whereas under argon preferably O- and V centres originate. The 0, centres are mainly located in or near the surface. The following mechanism for the formation of oxygen-containing defects is proposed. By mechanical activation O- and V defects are created, independently of the atmosphere of treatment. This formation of defects may occur by trapping of holes and by trapping of oxygen ions from the lattice. Under air the O- and V defects generated by the mechanical treatment react with the oxygen of the air, resulting in 0, defects [50,51]: o-+0,-,0, This reaction takes place preferably in the strongly distorted surface region. Such a reaction is not possible in an argon atmosphere. Therefore, only Oand V defects are found in argon. 3.1. I, 3. Distribution of structural defects. For structural defects to influence possible solid-state reactions, their distribution is essential. The distortion process runs from the surface to the interior. The one- and two-dimensional

9

Fig. 6. Dependence of the shape of the spectrum of the F+ centres treatment. Mechanical activation for . . . . . . , 0.5; -,2;and---,64h.

on the duration

of

defects are mainly concentrated in the surface region and their density decreases from the surface inwards. The localization of the zero-dimensional defects and that of oxygen-containing centres also depend on the intensity of the treatment. At first the anion vacancies are preferably located in the surface region, at sites of non-cubic symmetry (mechanically distorted). With increasing intensity of treatment the concentration of anion vacancies increases in the bulk. Conclusions of this kind could be drawn from the shape of the F+ centre spectrum. Initially the line anisotropy is striking, and becomes even more marked on y-irradiation, and the dependence of the shape of the spectrum on the duration of treatment (Fig. 6) also provides evidence. Information about the distribution of the mechanically created structural defects with respect to depth has also been obtained by stepwise dissolution of the differently treated samples and subsequent EPR measurements at room temperature (it is assumed that there are mono-dispersed sphere-like particles with a mean radius 7, which was calculated by means of the specific surface area, and symmetrical dissolution). Fig. 7 shows the depth distributions of the defects. The concentration of the 0; centres decreases strongly with increasing distance from the surface. In the interior no such centres are detected any longer. This can easily be understood, because according to the reaction above the formation of 0, centres is possible only in the strongly distorted

100

Fig. 7. Depth distribution of the structural distortions and of the F+ and oxygen-defect electron 0; (ODE centre, obtained by intermittent dissolution). N = Number of centres; A = amplitude of the Mn*+ lines (relative measure of the state of order); ? = calculated mean particle radius; * , @ = values of the untreated initial material.

near-surface region in the presence of air. In contrast, the F+ centres, for the formation of which no interaction with a gas is necessary, can still be detected at F/2. The concentration of the F+ centres passes through a maximum, which lies outside the strongly distorted layers in the direction of the grain centre. This fact supports the observation, that the concentration of F+ centres in the bulk increases with increasing duration of treatment and

Vacancies I” thermodynamic equ~l~bwm.deformatton of the lattw (heterogeneous lattice deformation )

I

GeneratIon

of catlon and anion vacancies I” the near- surface region at sites of non-cubic symmetry)

( preferably

Deformation of the lattice (d~slocat~ons,crushmg of coherently scattering reqons ) Cation and annon vacancies ( preferably I” the bulk 1 Deformed

1

Saturation

lattice

Dlslocatlons

state

Begmnlng of the destruction of the lattice in the ranges of a lattice unit ( deformation of the 1st coordlnatlon sphere

)

Scheme 1. Mechanism of the generation of structural defects in the MgO lattice under pressure and shearing stress.

11

exceeds that of the surface layers. The decrease in the concentration of F+ centres in the near-surface region with increasing duration of treatment can be explained by the strong distortion of this sphere, e.g., a high dislocation density with a destruction of the short-range order. In such surroundings zero-dimensional lattice defects do not exist, as they are associated with a well formed short-range order. The thickness of the near-surface layer is about 30 nm. The model in Fig. 11 shows the distribution of the defects, and Scheme 1 presents the mechanism for the creation of the different defects. Irradiation with y-rays (maxi3. I. I. 4. Influence of activation by irradiation. mum dose 100 Mrad) does not result in changes detectable by X-ray diffraction, and only electron processes occur. Electrons appear which can form F+ centres by recombination with anion vacancies. These processes lead to saturation after a dose of 2-4 Mrad. 3.1.2. Results for quartz (SiO,) 3. I. 2. I. Structural defects. Quartz is one of the most frequently used solids with respect to mechanical treatment [52-581, because of its technological importance. In addition to the effects already mentioned (reduction to small pieces, specific surface area, primary particle diameter and crystallinity mechanical activation leads to long-range changes and [20-23,55,57,59-611, short-range order caused by the broken bonds. The mechanically treated quartz is in a partially crystalline state. It still contains typical elements of the crystalline quartz structure and is characterized by the absence of a long-range order (Fig. 8).

Fig. 8. Radial distribution function after 70 h of mechanical treatment.

(RDF). 1 = a-quartz; 2 = amorphous SiO,, 3 = a-quartz r = Distance of an arbitrary atom from the origin.

12 TABLE 2 Structural

defects in mechanically

treated

quartz

t = Period of treatment; S = specific surface concentrations of the E’ and ODE centres.

x (%>

S

t (h)

Cm*

area;

8-l)

X= crystallinity:

Z,

-G*

Z ODE

and l

=

*

cc’)

K’)

2

1 -

100 84

_ 0.1’ 1o19

_ _

4 8 65

18 25 31

45 22 9

0.6. 1019 1.7.1019 3.0. 1o19

2.2.2019 4.4. 1or9 4.5. 1o19

0

Z,,,

* Activation under an argon atmosphere. ** Activation in air.

The short-range order is preserved within a region of a SiO, tetrahedron, i.e., r < 0.30 nm. The region of short-range order (0.30 < Y < 0.65 nm), which results from the arrangement of the SiO, tetrahedron in the non-planar six-atom rings, is partially destroyed by treatment for longer than 4 h. Altogether about 50% of the regular arrangement of the tetrahedra is destroyed. Complete removal of this arrangement cannot be achieved under the chosen experimental conditions (vibration and planetary mill). Only outside a region of radius Y> 0.65 nm of the SiO, tetrahedra rings is the long-range order removed. Overall the partially crystalline structure shows a lower packing density than the undistorted quartz. The density of the quartz decreases with increasing duration of treatment (Table 2). The effect of the mechanical energy will also include the region of the first coordination sphere if the SiO, tetrahedra are regarded as the most stable unit and primary lattice unit. During mechanical treatment of quartz, Si-0 bonds are broken, creating radicals. Of the two possible radicals,

Models

of the E’cenln

rmodel

ccg

Fig. 9. E’ centre prototypes.

2 model

.%o.

($g&

s

13

I -Si’

I and - SiO ’

I

I

only the former was detected and termed an E’ centre (Fig. 9 shows models of E’ centres which are discussed in the literature) [62,63]. During mechanical activation, a statistical average of about 5 . 10” Si-0 bonds per m2 of freshly produced surface are broken [64,65]. The concentration of these centres strongly increases with increasing duration of activation and approaches a threshold. The measured maximum concentration of E’ centres is 3 . lOI g-‘. According to the literature [64,65], based only on the increase in surface area, 1.5 . 10” E’ centres should be detected. One order of magnitude fewer centres are actually observed. A linear correlation is found between the concentration of E’ centres and the crystallinity of quartz, which represents an integrated measure of the distortion state. In Table 2 the structural defects including the E’ centres and the centres that will be discussed in Section 3.1.2.2. are summarized. 3.1.2.2. Dependence of structural defects on the surrounding atmosphere during treatment. The structural defects and the specific surface area, the crystallinity and the primary particle radius are not significantly influenced by the surrounding atmosphere during the treatment in an argon, hydrogen or oxygen atmosphere. In contrast, in carbon dioxide quartz shows stronger distortions of its structure (up to 40% less crystallinity). However, the atmosphere during activation decisively influences the formation of paramagnetic centres, which result from Si-0 bond cleavages. The broken Si-0 bonds are at the site of a chemical interaction with gas and form special centres depending on the existing atmosphere. Accordingly, E’ centres will only result if there are no reactions of the broken Si-0 bonds with the gas atmosphere, e.g., by activation in an inert gas atmosphere (argon) or if the number of adsorbed gas molecules does not correspond to the number of freshly created chemisorption centres and some of the centres remain unsaturated. Under the presence of oxygen, oxygen-defect electron centres (ODE) are formed:

I I - SiOO’ G+- Si’O; I

I

The ODE centre originates from the reaction of the primarily radicals with oxygen according to the following scheme:

created

14

I

I

I

-Si’+‘OSi-+50,+2-Si+O;

I

-

I

I

Si’ + ‘0 : O’-+ Si-O-O’+

Si’O,

I

I

I 2- SiO’ + ‘0 : O’-,

I

2- Si -O-O’+

I

2Si+O,

Under a sufficient oxygen pressure all the generated chemisorption centres react with oxygen, and a limiting number of ODE centres will be attained. This threshold is about twice the initial value of Z,, obtained by treatment of quartz under argon, where no saturation of the chemisorption centres can result. The maximum concentration of ODE centres is 4.5 . 1019 g-i. Hydrogen also reacts with both the radicals that are created by bond fracture as follows [24-261:

I

I -Si’

I

+‘OSi-

I

I + H2+

-SiH

I

I + HO-Si-

I

Hence, the generated EPR active centres permit conclusions to be drawn about the kind of the chemical interaction of the mechanically treated solids with the surrounding gas atmosphere. Their respective concentrations can be regarded as a measure of the state of distortion in the solid. 3.1.2.3. Distribution of structural defects. By pressure or shearing stress, preferably a distortion in the surface region of the quartz grains is produced. The thickness of the distorted zone is about lo-30 nm (this thickness has been determined by means of dissolution and subsequent recording of the concentration of centres and the grain-size spectra, and from the ratio of the specific surface area to crystallinity). The surface layer is partially crystalline. It contains electronic defects, which are generated via the Si-0 bond fracture. The centres are detectable up to a distance of one fifth of the grain radius from the outer surface to the grain centre. The decrease in the concentration of ODE centres is stronger than that of the E’ centres (Fig. 10). This difference in the distributions of the concentrations is evident because of the necessity for the ODE centres to acquire oxygen from the surrounding atmosphere for their formation. It is essential that both the centres are concentrated in the near-surface region. The quartz nuclei below the strongly distorted layer are nearly unaffected with regard to the observed defect centres. Fig. 11 shows a model of the distribution of the structural defects.

15

gram edge

quartz gram

0

5

Fig. 10. Depth

10

distribution

gram middle

a_15

nm

of E’ and ODE centres

20

in quartz.

3.1.2.4. Influence of activation by irradiation.

X-ray and y-irradiation (up to 2800 Mrad) do not lead to defects detectable by X-ray diffraction. Only a small concentration of E’ centre-like defects is found, which suggests released electron processes [29,66]. The concentrations increase with increasing irradiation dose. On subsequent mechanical activation, these centres recombine. The similarly created changes in the density (p = 2.64 g cm-3 for unirradiated quartz and p = 2.56 g cm -3 for quartz irradiated with a 2000 Mrad dose) are regarded as being connected with the electronic defects generated [51]. Application of y-irradiation after the mechanical activation leads to additional structural defects, which are detected by means of differential RDF and which are comparable to those observed after mechanical activation. The number of E’ centres increases by a factor of 3 . lo3 on y-irradiation of mechanically activated quartz. In a lattice deformed by the

starting

gram vacant sites and electram defects perlclase \, ,’ quartz strong dlsarder of the near-surface reglan wth electranlc defects

artmary particle sze

grams after mechamcal

pracessmg ---strong

@ perlclase

impact crushing disorder range

, quartz

Fig. 11. Model of a distribution of structural defects after mechanical treatment under pressure and shearing (a) in the MgO grain, (b) in the quartz grain and (c) after shock stress.

16

impact of mechanical energy Si-0 bond fissions can be generated by y-irradiation. The formation of SDE centres is not initiated by the y-irradiation, although the latter takes place under air, i.e., in the presence of oxygen. A combination of mechanical and irradiation energy also does not convert quartz into an amorphous state. A saturated state is again attained with regard to the distortion. When this state is already obtained by mechanical activation. the y-irradiation has no detectable influence in differential RDF. 3.2. Influence of shock stress Under the chosen conditions treatment by shock of both quartz and periclase results in only relatively unimportant effects similar to crushing or structural changes. Overall it can be calculated that the concentration and distribution of the defects differ from those generated by pressure and shearing stress. The surface regions are not preferentially distorted. A shock treatment mainly creates distortions at corners and edges, which penetrate into the interior of the grain (Fig. 11). 3.3. Thermostability of structural defects The defects which are detectable by X-ray diffraction are annealed out from 400 o C upwards in both MgO and quartz. Complete elimination could not be observed in this temperature region until 1000 and 570” C, respectively (the measurements were not extended above 570” C in the latter instance because of the a-j3 quartz transition at this temperature). Possible deformations derived from linewidth changes in the EPR lines of Mn2+ ions have been observed in the temperature range - 196 to + 600 o C. Decreases in the linewidths were not detected. The concentration of F+ centres decreases with increasing annealing temperature ( - 196 to + 600’ C). At 600’ C F+ centres are no longer detectable. The measured T~,~ values (i.e., the temperature at which the concentration of F+ centres decreases to half of its initial value) are dependent on the degree of distortion of the samples (Table 3). The decrease in the concentration of F+ centres with increasing annealing temperature might be caused by the recombination of F+ centres and not so

TABLE

3

T,,~ values for samples 4 and 5 in Table 1 Sample *

t @*I

71/2

4 5

16 32

100 160

* See Table 1.

(“(3

17

much by elimination of vacancies. Such an elimination of vacancies should operate on the defects which are detectable~by X-ray diffraction. The E’ and ODE centres in quartz recombine at 400” C. At room temperature all the structural defects are stable for at least 2 years.

4. COMPARISON OF THE INFLUENCE PERICLASE AND QUARTZ

OF PRESSURE

AND SHEAR

STRESS IN

The results of pressure and shearing stress were investigated by means of two oxides with considerable differences in their crystal-chemical properties {periclase (MgO): ionic compound; lattice energy, 4000 kJ mol-’ [67]. Quartz (SiO,): atomic bond with ionic portion; lattice energy, 444 kJ mol-’ [68]}. In both substances morphological changes and structural defects are generated. Crushing of MgO is mainly caused by breaking of the primary agglomerates. In quartz, on the other hand, crushing is caused by a reduction in the grain diameters due to bond cleavages. The increase in surface area is explained analogously. The generation of atomic defects (anion vacancies and bond cleavage) is connected with lattice deformation. For the concentrations of all mechanically generated defects, a quasi-equilibrium value exists. The impact of mechanical energy affects the solid from the outside into the interior. Preferably the surface regions are distorted (about 30 nm thick), where interactions between the generated defects and the atmosphere are also possible. Below this strongly distorted range there is a less influenced core. In quartz a higher concentration of defects is generated. Under comparable conditions (60-70 h of mechanical activation) about one order of

quartz

r. nm

CrystallIne 0

crystallinedisorder

M

DolthA crystollme

m

armrptnJs

Fig. 12. Different penetration depths of the mechanically created distortions in the lattices of MgO and quartz. r = Distance of an arbitrary atom from the origin.

18

magnitude more centres are observed than in periclase (quartz, 4. 1019 gg’; periclase, 1.6 *1Ol8 g-l). As a result of the impact of mechanical energy or the extreme stress in a planetary mill, partially crystalline quartz is created, whereas periclase remains in a crystalline state. Fig. 12 shows the different depths of penetration of the mechanical energy into the lattice. In quartz the long-range order is destroyed within the region Y> 0.65 nm, whereas within the range r -C0.35 nm no influence on the short-range order exists. In contrast, for periclase the long-range order is preserved within the region Y> 0.5 nm, whereas the short-range order is influenced over the whole range of r < 0.5 nm. The different behaviour concerning the intensity of structural distortions within the first coordination sphere of the oxides under discussion is explicable by considering the structures of the oxides as coordination structures with ion bonding and by looking at the coordination conditions from the viewpoint of energy. Quartz consists of $0, tetrahedra and periclase of MgO, octahedra. The coordination polyhedra are connected via a common corner by one oxygen ion. The strength of the electronic bonding, s, in one coordination polyhedron is s = z/n, where z is the valency and n the coordination number:

SiO, tetrahedron MgO, octahedron

Z

n

s

4 2

4 6

1 I/3

In the SiO, tetrahedron s = 1 and in periclase s = l/3. These values demonstrate that under comparable experimental conditions within the SiO, tetrahedron structural distortions do not exist, but distortions within the MgO, octahedra are observed.

TABLE 4 Influence

of mechanically

changed

structure

on the reactivity

Solid

Influence

Periclase and quartz

Increase in solubilization [22,52] Increase in dissolution [22] Interactions between the generated

Quartz

defects

of solids

and the atmosphere

[24-26,341

Increase in the formation of CSH with Ca(OH), [69] Decrease in the temperature of CaTiSiO, formation [70] Decrease in the temperature and energy of activation of quartz --) cristobalite transformation [70,71] Mechanically induced sorption during activation of solids in the presence of gases takes place on interaction between the solid and gas [23]

19

In quartz the mechanical energy causes a stronger structural distortion than in periclase. One of the reasons for the stronger distortion in quartz may be its much lower lattice energy. However, the penetration depth is larger in periclase than in quartz. In contrast to periclase, no defects could be detected in the bulk of quartz. The strongly distorted surface region of quartz may have a shielding effect, which hinders spreading into the interior. A limit of experimental sensitivity may also be a factor. In summary, it can be stated that the distribution of defects depends primarily on the stress mechanism, as had been found by Heegn [19]. However, with regard to the kind, concentration and penetration depth of the defects there are substantial specific differences. The kind, concentration and distribution of the defects decisively influence the solid-chemical reactions and can considerably increase the reactivity. Hence solubilization reactions and decomposition reactions are readily influenced (Table 4).

REFERENCES 1 G. Heinicke. Tribochemistry, Akademie-Verlag, Berlin, 1984. 2 V.V. Boldyrev, Eksperimentalnye Metody v Mechanochimii Tverdych Neorganiceskich Vescestv, Izdatel’stvo Nat&a, Sibirskoe Otdelenie, Novosibirsk, 1983. 3 V.V. Boldyrev, Zerkleinerung und Tribochemie, Preprints Fachtagung, Freiberg, 1981. 4 ES. Kapteva, T.S. Jusupov and A.S. Berger, Fiziko-chimiceskie Izmenenija Sloistych Sikilatov v Processe Mechaniceskoj Aktivacii, Izdatel’stvo Nat&a, Sibirskoe Otdelenie, Novosibirsk, 1981. 5 T.S. Jusupov, V.E. Istomin and T.A. Komeva, Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Khim. Nauk, 6 (1983) 3. 6 Z. Juhacz, Epitonyag, 27 (1975) 9, 37. 7 G. Jimbo, Proceedings of the International Symposium on Powder Technology 81, Kyoto, September 27-October 1, 1981, p. 365. 8 E.G. Awakumov, Mechaniceskie Metody Aktivacii Chimiceskich Processov, Izdatel’stvo Nauka, Sibirskoe Otdelenie, Novosibirsk, 1979. 9 V.V. Boldyrev, Kinet. Katal., 13 (1972) 1411. 10 V.V. Boldyrev and K. Meyer, Festkoerperchemie, (1973) 497-560. 11 R. Schrader and B. Hoffmann, Festkoerperchemie, (1973) 522. 12 H. Heegn and C. Bernhardt, Freiberg. Forschungsh. A, 553 (1976) 57. 13 K.-P. Thiessen and K. Sieber, Z. Phys. Chem. (Leipzig), 260 (1979) 403. 14 M. Senna, Proceedings of the International Symposium on Powder Technology 81. Kyoto, September 27-October 1, 1981, p. 457. 15 M. Senna and K. Schtinert, Powder Technol., 31 (982) 269. 16 K. Sigrist and H. Fichtner, Phys. Status Solidi, 95 (1986) 473. 17 H. Heegn, C. Bemhardt, G. Gottschalk and K. Husemann, Chem. Tech. (Leipzig), 26 (1974) 699. 18 C. Bernhardt and H. Heegn, 4th Eur. Symp, Zerkleinern, Ntirnberg, September 15-17, 1975, Preprints Part 1, p. 213; DECHEMA - Monogr., 79 (1976) 213. 19 H. Heegn, Veranderung der Festkijrpereigenschaften bei mechanischer Aktivierung und Feinzerkleinerung, Dissertation, Akademie der Wissenschaften der D.D.R., Berlin, 1986.

20 20 U. Steinike, I. Ebert, H. Geissler, H.-P. Hennig and U. Kretzschmar, Cryst, Res. Technol., 13 (1978) 597. 21 U. Steinike, I. Ebert, H.-P. Hennig and B. Mtiller, Cryst. Res. Technol., 14 (1979) 1469. 22 U. Steinike, H.-P. Hennig and U. Kretzschmar, Cryst. Res. Technol., 17 (1982) 1585. 23 U. Steinike, I. Ebert, and H.-P. Hennig, Z. Phys. Chem., 266 (1985) 302. 24 H. Geissler, H.-P. Hennig, D. Kunath, I. Ebert, V. Hoppe and U. Steinike. Cryst. Res. Technol., 17 (1982) 1259. 25 H.-P. Hermig, I. Ebert, U. Steinike, H. Geissler and U. Kretzschmar, Cryst. Res. Technol.. 15 (1980) 353. 26 H.-P. Hennig, I. Ebert, U. Steinike, H. Geissler and H. Jost, Z. Anorg. Allg. Chem., 484 (1982) 221. 27 H.-P. Hennig, I. Ebert, U. Steinike, H. Geissler, U. Kretzschmar and G. Boden, Z. Chem., 20 (1980) 388. 28 J. Jedamzik, G. Boden, H.-P. Hennig, I. Ebert, H. Geissler, U. Steinike, E. Richter and G. Scholz, Z. Phys. Chem. (Leipzig), 261 (1980) 258. 29 F. Neissendorfer, U. Steinike, H. Steil and G. Herms, Restocker Manuskripte, 7 (1985) 66. 30 U. Kretzschmar, I. Ebert, U. Steinike and H.-P. Hennig, Cryst. Res. Technol., 16 (1981) 949. 31 U. Kretzschmar, I. Ebert, U. Steinike and H.-P. Hennig, Cryst. Res. Technol., 17 (1982) 257. 32 U. Steinike, L.I. Barsova, T.K. Jurik and H.-P. Hennig, Cryst. Res. Technol., 16 (1981) 971. 33 U. Steinike, U. Kretzschmar and B. Tolochko, Cryst. Res. Technol., 18 (1983) 793. 34 U. Steinike, L.I. Barsova, T.K. Jurik, H.-P. Hennig, U. Kretzschmar and V.J. Spicyn, Cryst. Res. Technol., 21 (1986) K58. 35 V.I. Spicyn, U. Steinike, L.I. Barsova and T.K. Jurik, Izv. Akad. Nauk SSSR, Ser. Khim. Nauk, 4 (1982) 755. 36 L. Pauling, Am. Mineral., 65 (1980) 321. 37 B. Henderson and J.E. Wetz, Adv. Phys., 17 (1968) 745. 38 G.K. Walters and T.L. Estler, Appl. Phys., 32 (1961) 1854. 39 N.G. Kakazei and I.P. Bykov, Dopov. Akad. Nauk Ukr. RSR, 9 (1974) 844. 40 V.I. Spicyn, L.I. Barsova, G.L. Gromova and T.K. Jurik, Izv. Akad. Nauk SSSR, Ser. Khim. Nauk 5 (1976) 963. 41 V.I. Spicyn, L.I. Barsova, G.I. Gromova and T.K. Jurik, Inzv. Akad. Nauk, SSSR, Ser. Khim. Nauk, 4 (1982) 762. 42 A.J. Tenth, Solid State Phys., 6 (1973) 1134. 43 Ju.1. Aristov, A.I. Volkov, V.N. Parmon and K.I. Zamaraev, React. Kinet. Catal. Lett., 25 (1984) 329. 44 H. Raether, Ergeb. Exakten Naturwiss., 24 (1951) 54. 45 H. Willman and R. Agarwala, Acta Crystallogr., 7 (1954) 664. 46 U. Kretzschmar and F. Neissendorfer, Cryst. Res. Technol., (1987) in press. 47 G. Herms and H. Steil, Restocker Physikalische Manuskripte, 5(l) (1979) 79. 48 I.V. Berestechaja and P.Ju. Butjagin, Dokl. Akad, Nauk SSSR, 260 (1980) 361. 49 M. Iwamoto and I.H. Lunsford, Chem. Phys. Lett., 66 (1979) 48. 50 A.J. Tenth, T. Lawson and F.J. Kibblewhite, J. Chem. Sot., Faraday Trans I, (1972) 11, 69, 1181. 51 A.M. Volodin, React. Kinet. Catal. Lett., 28 (1985) 189. 52 W. Dusdorf, Krist. Tech., 1 (1966) 59. 53 I.J. Lin, S. Nadiv and D.J.M. Grogzian, Miner. Sci. Eng., 7 (1975) 313. 54 P.A. Rehbinder and G.S. Chodakov, Silikattechnik, 13 (1962) 200. 55 L. Lidstrom, Acta Polytech, Stand., 75 (1968) 1.

21 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

G.S. Chodakov, Fizika Izmel’cenijo, Nauka, Moscow, 1972. K.-H. Lindner, Dissertation, Berlin, 1966. E. Schlegel, Dissertation B, Freiberg, 1976. P.G. Kihlstedt and E. Froberg, Silic. Ind., 36 (1971) 45. H. Heegn, C. Bernhardt, J. Gottschalk and K. Husemann, Chem. Tech. (Leipzig), 26 (1974) 696. K. Steier and D.W. Fuerstenau, Chem.Ing.-Tech., 48 (1976) 569. A.V. Shendrik and D.M. Julin, Phys. Status Solidi B, 85 (1978) 343. L.L. Yip and W.B. Fowler, Phys. Rev., 11 (1975) 2327. G. Hochstrasser and I.F. Antonini, Surf. Sci., 32 (1972) 644, 665. A.N. Streleckij and P.Ju. Butjagin, Dokl. Akad. Nauk SSSR, 225 (1975) 1118. F. Neissendorfer, Cryst. Res. Technol., in preparation. Gmelins Handbuch der anorganischen Chemie, Magnesium, Verlag Chemie, Berlin, 1952. H.-J. Blankenburg, Quarzrohstoffe, Dt. Verlag fur Grundstoffindustrie, Leipzig, 1976. J. Jedamzik, Dissertation, Akademie der Wissenschaften der D.D.R., Berlin, 1982. U. Steinike, Wissenschaftliche Beitrtige der FSU, Jena, 1985, 136. U. Steinike, D.-Chr. Uecker, K. Sigrist, Th. Kohler and W. Plotner, Cryst. Res. Technol., (1987) in press.