EFFECT OF CRYSTALLIZATION WATER MOLECULES ON THE ELECTRICAL AND MECHANICAL STRENGTH OF CRYSTALLINE HYDRATES

K. M. KEVROLEVA

(Presented by the scientific seminar on dielectrics)

The question of the effect of crystallization water molecules on the electrical and mechanical strength of crystalline hydrates should be considered simultaneously with determining the relationship between the energy of the crystal lattice and the electrical and mechanical strength of crystalline hydrates, since the number of crystallization water molecules dictates the structure and the change in the forces of interaction between the elements of the crystalline-hydrate lattice. Applying a uniform method of growth, using only different temperature intervals of crystal growth, we obtained single crystals of $\mathrm{NiSO_4 \cdot 7H_2O}$, $\mathrm{NiSO_4 \cdot 6H_2O}$, $\mathrm{CoSO_4 \cdot 7H_2O}$, $\mathrm{CoSO_4 \cdot 6H_2O}$; their crystalline form was determined, the energy of the crystal lattice was calculated, X-ray structural analysis was carried out, and their breakdown voltage, microhardness, and refractive index $n_t$ were measured.

Determination of $U_{\mathrm{br}}$ was carried out under conditions of a nonuniform field (the shape of the specimen is shown in Fig. 1); the conical recess and the small thickness of the specimen in the tested region (200 microns) prevented the formation of surface discharges. The test, performed on specimens of the indicated shape, characterized the nature of the substance [1, 2]. Electrodes in the conical recess and on the bottom of the specimen were applied by rubbing on metallic foil (silver). Electrodes obtained by depositing a metal layer in vacuum, by coating with aquadag, and by a mechanical suspension of metallic powder in toluene or ether could not be used because of damage to the surface of the specimen (weathering, dissolution, cracking).

The refractive index of the specimens $n_t$ was determined by the method of Ansheles and Burakova [3].

Fig. 1. Shape of the specimen for breakdown of solid dielectrics in a nonuniform field.

It follows from Fig. 2 that when the structure of a compound changes, its electrical and optical properties also change; namely, with a decrease in the number of crystallization water molecules, the energy of the crystal lattice, the electrical strength, and the refractive index of the compound increase.

A similar relationship between the energy of the crystal lattice and electrical strength, as follows from Fig. 3, is also observed for layered crystalline hydrates (talc, phlogopite mica, muscovite mica). The electrical strength of the micas is taken from the data of Mantrov and Baev [4]; the electrical strength of talc was determined by us.

Fig. 2. Breakdown voltage and refractive index of crystalline hydrates as functions of the energy of the crystal lattice.
— NiSO₄·nH₂O
-- CoSO₄·nH₂O

Such a regularity is, possibly, characteristic of crystalline hydrates and characterizes

The determination of \(E_{\mathrm{br}}\) was carried out on talc specimens 20–50 microns thick under conditions of a uniform field at a pulse voltage of duration \(10^{-6}\) sec.

A number of authors [5, 6] have obtained higher values of \(E_{\mathrm{br}}\) for micas; moreover, according to the data of Hackett and Thomas [7], the electrical strength of phlogopite mica increases, as the specimen thickness decreases, in an interval much greater than is predicted by Fröhlich’s theory. With an increase in thickness from 20 to 60 microns, the breakdown voltage of the mica fell by 27%. A considerable dependence of \(E_{\mathrm{br}}\) on specimen thickness was also noted by us for Onega talc. Thus, when the specimen thickness was reduced from 150 to 50 microns under conditions of a uniform field at \(\tau_{\phi}=10^{-5}\) sec, the electrical strength of talc increased by 40% (Fig. 4). The dependence of \(E_{\mathrm{br}}\) of talc on specimen thickness was also observed at other times of voltage action (Fig. 4a); it is common for lamellar

Fig. 3. Dependence of the electrical strength of lamellar crystalline hydrates on the energy of the crystal lattice.
\(\Delta\) — muscovite mica
\(\times\) — phlogopite mica
\(\circ\) — talc

but inhomogeneity of structure. It is probable, however, that the value of the electrical strength of talc and micas, when a greater degree of uniformity of the electric field is created, should be higher.

values shown in Fig. 3, but the order of variation of \(E_{\mathrm{br}}\) (the decrease in the value of the electrical strength from muscovite mica to talc) must remain the same.

We have determined the value of the electrical strength of the crystallohydrates \(\mathrm{MgSO_4\cdot 7H_2O}\), \(\mathrm{ZnSO_4\cdot 7H_2O}\), \(\mathrm{NiSO_4\cdot 7H_2O}\), \(\mathrm{FeSO_4\cdot 7H_2O}\) under conditions of a homogeneous field at a pulse voltage of \(\tau = 10^{-6}\) sec. (Fig. 5).

Fig. 4. Dependence of \(E_{\mathrm{br}}\) of Onot talc on the thickness of the specimen. Homogeneous field, \(\tau_f = 10^{-6}\) sec.

Fig. 4a. Dependence of \(E_{\mathrm{br}}\) of Onot talc on the thickness of the specimen at different durations of voltage application. Homogeneous field.
1—thickness \(140 \pm 10\ \mu\); 2—thickness \(180 \pm 10\ \mu\).

Single crystals of the indicated compounds were grown in the TVN laboratory by the dynamic method.

The vitriols under consideration have similar chemical and structural properties. According to our calculations, the lattice energy of these compounds is \(429\text{--}433\ \mathrm{kcal/mol}\). The electrical strength,

of the indicated crystalline hydrates also differs only slightly (Fig. 5). For crystalline hydrates of the type \(RSO_4 \cdot nH_2O\), a dependence is observed between the electrical strength and the degree of fixation of the molecules of crystallization water (Fig. 6). The temperatures of final dehydration of the compounds are taken from the data of the State Institute of Experimental Chemistry.

Fig. 5. Electrical strength of crystalline hydrates of the type \(RSO_4 \cdot 7H_2O\), similar in chemical and structural properties. Uniform field. \(\tau_\phi = 10^{-6}\) sec. Breakdown in transformer oil.

Thus, the data considered make it possible to assert:

1. Molecules of crystallization water, entering the crystal lattice, weaken the bonding force and reduce the electrical strength of the compound.

Fig. 6. Electrical strength and degree of fixation of molecules of crystallization water.

1. The electrical strength of crystalline hydrates increases with an increase in the energy of the crystal lattice of the compound and varies only within small limits for crystalline hydrates having similar values of lattice energy.

Effect of Molecules of Crystallization Water on the Mechanical Properties of Crystalline Hydrates

There are conflicting points of view in the literature concerning the influence of molecules of crystallization water on the mechanical properties of compounds. I. I. Zaslavskii [8] considers that water produces a densifying effect in solid systems. A. E. Fersman [9] indicates that molecules of crystallization water, entering the crystal lattice, as it were push it apart, weakening the internal bonds and reducing the value of the hardness.

We measured the microhardness of the crystalline hydrates
$\mathrm{NiSO_4 \cdot nH_2O}$, $\mathrm{CoSO_4 \cdot nH_2O}$, $\mathrm{CuSO_4 \cdot nH_2O}$ as a function of the number of molecules of crystallization water. As follows from Fig. 7, an increase in the microhardness and surface energy of the crystalline hydrates considered occurs with a decrease in the number of water molecules in the lattice of the crystalline hydrate and with an increase in the energy of the crystal lattice.

Fig. 7. Mechanical properties of crystalline hydrates as a function of the energy of the crystal lattice and the number of molecules of crystallization water.
1—$\mathrm{NiSO_4 \cdot nH_2O}$, 2—$\mathrm{CoSO_4 \cdot nH_2O}$, 3—$\mathrm{CuSO_4 \cdot nH_2O}$

The change in surface energy is given according to data of P. A. Savintsev, kindly placed at our disposal. The specimens were tested on a PMT-3 instrument, according to the method developed by M. M. Khrushchev [10].

The minimum volume $(V)$ of homogeneous material whose microhardness is to be determined is equal to $V = 10d^3$, where $d$ is the diagonal of the impression. To eliminate the influence on the hardness number of the proximity of the edges of the specimen, the impression must be located from the latter at a distance exceeding or equal to the length of the diagonal, and the thickness of the specimen must be not less than ten times the depth of the impression. The magnitude of the microhardness was determined as the average of 15–30 measurements.

Figure 8 presents the change in hardness of the crystalline hydrates
$\mathrm{CaSO_4 \cdot nH_2O}$, $\mathrm{MgSO_4 \cdot nH_2O}$, $\mathrm{FeSO_4 \cdot nH_2O}$, $\mathrm{CuSO_4 \cdot nH_2O}$ as a function of the number of molecules of crystallization water in the lattice, according to Vinchell’s data [11].

The results of the investigation confirm Academician A. E. Fersman’s proposition concerning the weakening effect of molecules of crystallization water on the mechanical properties of crystalline hydrates.

From the experimental data (Fig. 7) it follows that the relationship between the lattice energy and microhardness, and between the lattice energy and the surface energy of crystalline hydrates, is analogous to that established earlier for alkali-halide crystals [12, 13]. This proposition remains valid only for crystalline hydrates of the same type of structure. Compounds with a layered structure, talc, possessing a considerable total lattice energy,

Fig. 8. Dependence of the hardness of crystalline hydrates on the number of molecules of crystallization water in the lattice (according to A. N. Winchell).
1—CaSO₄·nH₂O, 2—MgSO₄·nH₂O, 3—FeSO₄·nH₂O, 4—CuSO₄·nH₂O

has a low hardness, since the bonding forces between the individual components within and between packets are unequal, and the mechanical strength is determined by weak intermolecular bonds. For platy crystalline hydrates the relationship between the values of hardness and the energy of the crystal lattice remains analogous to that established earlier, namely: the hardness of the compound and the energy of the crystal lattice increase in the following sequence: talc, phlogopite mica, muscovite mica.

The weakening effect of molecules of crystallization water on the mechanical properties is also observed for platy crystalline hydrates. According to the experimental data of Lashev [19], who studied phlogopite mica with different amounts of crystallization water, the mica from the Pamir, which has a smaller percentage content of H₂O in the lattice, has, as follows from Table 1, the greater hardness, compressive strength, and shear strength.

Table 1

Characteristics of phlogopites

Changes in the electrical and mechanical properties of crystalline hydrates upon removal of molecules of crystallization water from the lattice of a given crystalline hydrate are connected with the position occupied by the water molecules in the lattice of the crystal under study.

A. E. Fersman [9] points out that water molecules, on entering the lattice of a crystalline hydrate, as it were expand it; when they are removed from the crystal lattice, according to A. F. Wells [14], the following changes may occur:

1. Removal of water from crystals leads to compression of the lattice, while the structure of the anhydrous substance is not directly related to the structure of the hydrate.

2. If the anhydrous crystal is a rigid lattice containing interstices or channels sufficiently large for $\mathrm{H_2O}$ molecules to fit into them, reversible hydration and dehydration occur without a noticeable change in the dimensions of the unit cell (“lattice” crystals—zeolites).

3. In the case of the incorporation of water between infinite chains or layers of compounds, dehydration results only in a bringing together of the structural elements, with continuous changes in one or several dimensions of the unit cell. As shown by the X-ray structural studies we have carried out, the crystalline hydrates $\mathrm{CuSO_4 \cdot 5H_2O}$, $\mathrm{NiSO_4 \cdot 7H_2O}$, $\mathrm{CaSO_4 \cdot 2H_2O}$, and talc belong to cases 1 and 3 under consideration. Laue patterns taken on single-crystal specimens of copper sulfate, gypsum, and crystalline talc indicate that, upon dehydration up to $200^\circ\mathrm{C}$, the single-crystal lattice of $\mathrm{CaSO_4 \cdot 2H_2O}$ and $\mathrm{CuSO_4 \cdot 5H_2O}$ is destroyed. A calculation we performed from Debye patterns of partially dehydrated specimens indicates that, upon removal of water molecules, the single-crystal lattice is rearranged into a polycrystalline lattice of a crystalline hydrate of lower water content.

Laue patterns taken on specimens of crystalline talc, without calcination and thermally treated at temperatures up to $800^\circ\mathrm{C}$, do not indicate significant changes in the crystal lattice. An analogous conclusion was drawn in the works of Boyarsky [15], Avgustinik [16], and others from X-ray and optical study of the structure of talc during dehydration.

We measured the electrical strength and microhardness of $\mathrm{CuSO_4 \cdot 5H_2O}$, $\mathrm{NiSO_4 \cdot 7H_2O}$, $\mathrm{CaSO_4 \cdot 2H_2O}$, and Onega talc as a function of the degree of dehydration.

As follows from Figs. 9, 10, and 11, when molecules of crystallization water are removed, there is a considerable decrease in the microhardness of single crystals of gypsum, copper sulfate, and seven-water nickel sulfate.

The electrical strength of the indicated compounds was determined on pressed specimens under conditions of a uniform field (specimen shape, Fig. 12) and on single crystals under conditions of a nonuniform field (Fig. 1). During breakdown in air, in order to eliminate flashover, in addition to the conical tip, a glass tube was glued to the specimen in the region under study (the thickness of the specimen at the thinnest point was equal to 200 microns) (Fig. 13).

The experimental data (curves $E_{\mathrm{pr}}$, mean, and $U_{\max}$, Figs. 9, 10, 11, and curve 2, Fig. 14) indicate that, in order to reveal the influence of dehydration on the electrical strength of crystalline hydrates, the study should be carried out in air, since when determining $E$ of dehydrated specimens in transformer oil or in a silicone-organic liquid, secondary phenomena (impregnation of the specimens) distort the true picture.

Fig. 9. Dependence of the electrical strength and microhardness of copper sulfate on the dehydration temperature. Breakdown in a silicon-organic liquid under impulse voltage, $\tau_{\mathrm{f}} = 10^{-6}$ sec.
1 — $\mathrm{CuSO_4 \cdot 5H_2O}$, pressed, uniform field;
2 — $\mathrm{CuSO_4 \cdot 5H_2O}$, single crystals, uniform field;
3 — microhardness of $\mathrm{CuSO_4 \cdot 5H_2O}$ single crystals.

Fig. 10. Dependence of the electrical strength (pressed specimens) and microhardness (single crystals) of $\mathrm{NiSO_4 \cdot 7H_2O}$ on the dehydration temperature. Breakdown in transformer oil under impulse voltage, $\tau_{\mathrm{f}} = 10^{-6}$ sec. Uniform field.

Fig. 11. Dependence of the electrical strength and microhardness of pressed gypsum on the dehydration temperature. Breakdown in transformer oil under an impulse voltage \(t_f = 10^{-6}\) sec. \(\ldots\)—\(\mathrm{CaSO_4\cdot 2H_2O}\), \(ooo\)—\(\mathrm{CaSO_4\cdot \tfrac{1}{2}H_2O}\).

Fig. 12. Shape of pressed specimens for determining electrical strength under conditions of a uniform field.

Fig. 13. Gypsum specimens for determining the dependence of the breakdown voltage on structural changes occurring during dehydration. Uniform field. Breakdown in air.

Fig. 14. Breakdown voltage of crystalline gypsum under direct current as a function of the degree of dehydration. Uniform field. Specimen thickness 0.200 mm.
1—breakdown in oil, 2—breakdown in air.

In air, the breakdown voltage of crystalline hydrates whose lattice is substantially deformed during dehydration (of the type \(RSO_4 \cdot nH_2O\)) decreases. The electrical strength of talc under impulse voltage in a uniform-field arrangement remains practically the same both for talc specimens prepared at room temperature and for specimens calcined at a temperature of \(1200^\circ C\) (Fig. 15).

Fig. 15. Dependence of the electrical strength of Onot talc on the dehydration temperature; \(r = 0.250 \pm 10\ \mu\), \(l = 10\) mm, \(\tau_\phi \simeq 10^{-6}\) sec. \(T = 20^\circ C\). Breakdown in transformer oil.

Conclusions

1. Molecules of crystallization water, entering into the structure of a crystalline hydrate of the type \(RSO_4 \cdot nH_2O\), as it were spread the lattice apart, increase the interionic distance, and reduce the electrical and mechanical strength of the compound.

The increase in electrical strength with a decrease in the number of molecules of crystallization water agrees with the ionization theory of breakdown of solid dielectrics. Increases in electrical and mechanical strength due to the scattering influence of dipoles or to densification of the lattice upon the “incorporation” of water molecules into the lattice of the compound were not found experimentally.

1. The electrical strength of crystalline hydrates close in chemical composition, structure, and value of the crystal-lattice energy \((MgSO_4 \cdot 7H_2O,\ ZnSO_4 \cdot 7H_2O,\ FeSO_4 \cdot 7H_2O,\ NiSO_4 \cdot 7H_2O)\) varies within insignificant limits. For the sulfates of nickel, zinc, iron, copper, and magnesium, a connection is observed between electrical strength and the degree of binding of the molecules of crystallization water.

2. The electrical and mechanical strength of lamellar crystalline hydrates (talc, micas) increases with increasing lattice energy of the compound. Thus, for complex crystalline-hydrate compounds, a relationship is observed between the electrical and mechanical properties and the energy of the crystal lattice, analogous to that established earlier [17, 18] for alkali-halide crystals.

3. Qualitative and quantitative X-ray investigations show that removal of crystallization water from the lattice of a crystal of the type \(RSO_4 \cdot nH_2O\) during dehydration causes the lattice of the single crystal to be transformed into the polycrystalline lattice of a crystalline hydrate of lower water content, which is accompanied by loosening, and upon final…

… of final dehydration—destruction of the crystal—and leads to a decrease in the electrical and mechanical strength of crystalline hydrates.

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Errata