Etching of MgO crystals in acids: kinetics and mechanism of dissolution

Kinetics of etching of MgO crystals have been studied in H₂SO₄, HNO₃ and HCl. The effects of etching time, acid concentration and temperature on the growth of hillocks, on the selective etch rate and on the rate of overall dissolution are demonstrated. It is observed that etch rates are independent of time, but are determined by the temperature and concentration of the acid. The etch rate-concentration curves show maxima which are characteristic of an acid. The values of activation energy for the processes of dissolution, selective etching and hillock growth and the corresponding frequency factors are computed. It is established that the process of dissolution in concentrated H₂SO₄ is diffusion controlled, while in H₂SO₄ with concentrations below 18 N and in HNO₃ and HCl it is reaction rate controlled. The pre-exponential factor is found to be a function of acid concentration. The results are discussed from the standpoint of chemistry. A comment on the data published on MgO by previous workers is made.     

On the kinetics and mechanism of dissolution of MgO crystals

The kinetics of dissolution of {1 0 0}, {1 1 0} and{1 1 1} surfaces of MgO crystals have been investigated in H₂SO₄ and H₃PO₄. It is observed that in concentrated H₂SO₄ and in concentrated and dilute H₃PO₄, the etch rates change in the sequence {1 1 1}>{1 1 0}> {1 0 0}; and in dilute H₂SO₄ in the sequence {1 0 0}>{1 1 1}>{1 1 0}. From the etch rates determined for 4N H₂SO₄, 36N H₂SO₄ and 4N H₂SO₄-glycerine solutions at different temperature, the values of activation parameters are calculated. It is found that the value of the activation energy in a particular acid is practically constant for different planes, but that of the pre-exponential factor changes. Increase in the viscosity of 4N H₂SO₄ due to the addition of glycerine lowers the etch rate but the value of activation energy remains unchanged. However, in highly viscous 4N H₂SO₄ etchant, the value of the activation energy of selective etching decreases somewhat above 60° C, and the etch pits formed at dislocations are deeper and larger than those at screw dislocations. The results are discussed and the important conclusions are enumerated.     

Etching of MgO crystals in acids: surface micromorphology

Several etchants based on simple acids are proposed for revealing dislocations on {100}, {110} and {111} faces of MgO single crystals. Some acids under different etching conditions are shown to produce etch pyramids at decorated dislocations, while etch hillocks, spherulites and dendrites are formed under different conditions. With increasing concentration of H₂SO₄, 110 pits, circular pits, 100 pits, 110 pyramids as well as 100 pits, dendrites and spherulites, and hillocks are formed in that order. Etching characteristics of other acids also showed a more or less identical trend. The observations are interpreted with the assumption that with increasing concentration of acid, an effective undersaturation and later supersaturation is developed very close to the dissolving crystal face. The chemistry of the dissolution process is also discussed.     

Wear damage of MgO single crystals

Mechanisms of surface and sub-surface wear damage in MgO single crystals were investigated by scratching with two sintered alumina sliders, having tip radii of 60 and 120m, using a simple scratching apparatus in a controlled atmosphere. The degree of surface and sub-surface cracking is dependent on the shape of the slider and the normal contact load, which are related to the penetration into the crystal. The chevron crack on the (001) plane in the [100] sliding direction consists of cracks intersecting at an angle of 90°, and with a spread angle of about 120°, and the normal crack. The nature of the sub-surface damage is investigated; a parallel crack develops in front of the slider and an oblique crack propagates towards the front of the slider. Then an internal normal crack is formed between the oblique crack and the parallel crack. In the [110] direction, the oblique crack initiates from the top of the normal crack under the surface, and the parallel crack continues from the oblique crack. This wear damage is explained by the dislocation interactions occurring due to the distribution of resolved shear stresses during sliding. The wear caused by the chevron crack is a factor of 10 higher than that with plastic flow. Internal cracks do not have a direct influence on the increase of wear.     

Dislocation motion in MgO crystals under plate impact

Dislocation dynamics in MgO single crystals is studied by means of plate impact experiments in which specimens are subjected to stress pulses of 214 to 376 MPa resolved shear stress with duration in the range of 0.09 to 0.23sec. Dislocation structures in the recovered crystals are observed by an etch pit technique and by transmission electron microscopy. Etch pit studies show that inclusions play a role in the generation of interior glide bands and that there is a 20 to 40-fold increase in dislocation densities outside the glide bands. Assuming the average dislocation velocity is approximately proportional to the applied stress the drag coefficient for MgO at dislocation velocities of 1 to 2 km sec^–1 is observed to be 5×10^–5 Nsec m^–2. Transmission electron microscopy determines the spacing and curvature of dislocations in glide bands.     

Cathodoluminescence at frictional damage in MgO single crystals

The frictional damage in single-crystal MgO was studied using the cathodoluminescent mode of a scanning microscope as well as selected-area electron channelling pattern analysis. With a hemispherical diamond slider, the distorted structured layer beneath the surface was noted, the cathodoluminescence of which was quenched, presumably due to the high density of defects. This non-luminescent layer was encased in the plastically deformed luminescent zone, which extended downwards to where there was a (0 0 1) cleavage crack. It was also found that with a larger ball slider, luminescent slip lines on both {1 1 0}₉₀ and {1 1 0}₄₅ were pronounced inside the track after a single to-and-fro traversal, and successive traversals were able to generate a non-luminescent, distorted structured layer inside the track.     

Exoelectron emission from ion-implanted MgO single crystals

Thermally stimulated exoelectron emission (TSEE) measurements were carried out for pure MgO single crystals, annealed and then irradiated to either ion beams of different doses and energies or X-rays. H⁺- and He⁺-ion implantation causes an increase of the TSEE yield compared to that of nonimplanted samples. The TSEE response is dependent on the dose and energy of the implanted ions.     

Effect of mechanical processing on the dissolution rate of MgO

The kinetics of MgO dissolution after attrition grinding have been studied using photometry and pH measurements. The dissolution rate constant, limiting magnesium ion concentration, and solution pH have been shown to pass through a maximum at a certain attrition time. The effect of attrition time on the dissolution rate is interpreted in terms of the surface condition and aggregation of the powder particles.     

Magnesioferrite formation in low-concentration Fe/MgO single crystals

Single crystals of MgO containing iron in concentrations from 310 to 12900 ppm have been examined by magnetic resonance and reflection electron diffraction before and after ageing in oxygen in the temperature range 600–800° C. Before ageing, only the e.s.r. spectrum of isolated Fe³⁺ ions in cubic sites was seen and the diffraction patterns revealed MgO alone. After ageing, a strong anisotropic line, centered near g=2.00 and exhibiting the characteristics of ferrimagnetic resonance, was found and its presence was coupled with the appearance of a spinel pattern superimposed on that of the MgO. Analysis of the variation of anisotropy field with ageing time gave estimates of the room-temperature magnetization, first anisotropy constant and Curie temperature of 258 emu cm^–3, –2.7× 10^–3 J cm^–3 and 500±10 K, respectively, which, in view of the close agreement with published data, suggested magnesioferrite formation even at very low iron concentrations. In the initial stages of ageing a strong isotropic resonance line was observed which appeared to be a precursor of the anisotropic ferrimagnetic line.     

Dissolution kinetics of MgO crystals in aqueous acidic salt solutions

The revelation and morphology of dislocation etch pits as well as the rates of macroscopic dissolution and selective etching on the {1 0 0} plane of MgO crystals in aqueous solutions of various inorganic salts are investigated in relation to the nature and concentration of salt in solution and the etching temperature. It is found that addition of a salt generally facilitates etch pit formation and that the rates of surface dissolution and selective etching increase with additive concentration, etchant temperature and character and ageing of dislocations, while the etch pit morphology depends on the concentration and chemical nature of an impurity, etching temperature and the ageing of the dislocations. It is also observed that some fast etching solutions produce very shallow etch pits at screw dislocations. The results are discussed from a consideration of solution pH, standard electrode potentials of metals and stability of complexes present in solution. The importance of the surface entropy factor in revealing etch pits at screw dislocations is pointed out.     

Selective etching and dissolution of BaF2 crystals

The selective etching and dissolution of BaF₂ crystals have been studied in aqueous solutions of inorganic salts, inorganic and organic acids. The effect of additives on selective etch rates is demonstrated. It is observed that chemical exchange reactions determine the selective etching and dissolution of BaF₂ crystals. The form of etch pits corresponds to the real dissolution form of BaF₂ crystals (rhombododecahedron). It has been established that the process of dissolution in HNO₃ and HCl is diffusion controlled. The activation energy is independent of acid concentration and has the same value for both HNO₃ and HCl. The pre-exponential factor is concentration dependent. An empirical equation for dissolution kinetics has been obtained.     

Thermoluminescence centres created selectively in MgO crystals by fast neutrons

A possibility of using the thermoluminescent detection of collision-created interstitial centres in MgO by the TL peak at 700 K, arising at the radiative recombination of anion interstitials with F+ centres, for the selective detection of fast neutrons in mixed neutron-gamma fields, is examined. Selectivity and sensitivity of such a detector are discussed. For the present time, the sensitivity of the thermoluminescent detection of anion interstitials created by neutron irradiation is comparable to that of the EPR detection of F+ centres, the main limitations of the thermoluminescence method being connected with the background thermoluminescence and absence of suitable luminescence centres in this temperature region, when undoped and untreated MgO crystals are used. Possible ways to overcome these shortcomings are discussed.     

The temperature dependence of permittivity in MgO and Fe-MgO single crystals

The permittivities (ε′) of undoped MgO and iron-doped Fe-MgO single crystals have been measured over the temperature range 20 to 650? C for frequencies between 500 Hz and 50 kHz. From 25 to around 200? C the temperature dependence of ε′ fits well with Havinga's formula and the value of [(ε′ ? 1) (ε′ + 2)]^?1 (?ε′/?T) = 1.02 × 10^?2K^?1 found for undoped MgO agrees closely with data published for lower temperature ranges; this increases considerably with the addition of iron, rising to 2.85×10^?5K^?1 for MgO single crystals doped with 12 900 p.p.m. iron. Above 200? C the permittivity changes much more rapidly than the Havinga formula predicts, the variation being greater in iron-doped specimens. The frequency dependence ofε′ is also temperature-dependent; below 200? Cε′ follows [ε′(Ω) ? ε′ _∞] ∫Ω ^(n?1) withn=0.98±0.02 for all samples, but above 200? C the value ofε′ falls more rapidly with frequency than would be expected from this law. The effect is more pronounced for MgO with 12 900 p.p.m. iron. The results are discussed in terms of a contribution to the measured permittivity arising from temperature-enhanced conductivity.     

Observations of contact damage in MgO and LiF crystals by cathodoluminescence

Cathodoluminescence (CL) studies in a scanning electron microscope of the static and dynamic contact damage in MgO and LiF crystals are described. The main luminescence for both MgO and LiF was found to be associated with the plastically deformed zone at and around the contact site, although there were differences of details in the CL behaviour of the two materials. It was also found that in MgO the intensity of luminescence from screw dislocations was markedly higher than that from the edge dislocations for all possible orientations of the specimen. It is proposed that this simple and rapid technique can be used for assessing the mechanical state of a surface.     

Thermally activated flow parameters in the deformation of MgO single crystals

Strain-rate cycling methods have been used to determine the thermally activated flow parameters in the compressive deformation of MgO single crystals. Activation volumes of the order of 100b ³ have been measured for deformation below room temperature, indicating dislocation-point defect interactions as the rate controlling mechanism for hardening at these temperatures, whileV ^* values of the order of 1000b ³ have been measured at and above room temperature. This, together with the fact that the effective stress remains constant with strain at these temperatures identifies the interaction of dislocations with dislocation dipoles as the rate controlling mechanism in this temperature range. The Gibbs free energy for deformation,G, equals the activation enthalpy,H, at low temperatures and the deviation ofG fromH above room temperature may be attributed to entropy effects.     

The formation of hydrogen-filled cavities in MgO crystals annealed in reducing atmospheres

Defects causing cloudiness in some MgO crystals annealed in reducing atmospheres are shown to be lenticular cavities up to 200 Μm diameter, containing hydrogen gas under high pressure. A close connection between the formation of cavities and the hydroxyl impurity content of the crystals is established, and a mechanism for their formation proposed, involving the reaction of oxygen vacancies and electrons, created at the surface, with V_OH centres in the interiors of the crystals. On the basis of these results and work by Amelinckxet al. and Barret al. a general principle is postulated, in which a special gaseous environment, by reacting with a crystal surface, causes a second reaction within the crystal which converts an anion impurity into a permanent gas and produces gas-filled cavities. This principle may be applicable to many ionic crystal/reducing gas systems.