The crystallization of potassium nitrate I. Etch pit density and microhardness of potassium nitrate in comparison with some other ionic crystals

,. . . . . . . .

ELSEVIER

CRYSTAl. GROWTH

Journal of Crystal Growth 173 (1997) 481-486

The crystallization of potassium nitrate IO Etch pit density and microhardness of potassium nitrate in comparison with some other ionic crystals A. Herden, R. Lacmann* Institutfiir Physikalische und Theoretische Chemie, Technische Universitiit Braunschweig, Hans-Sommer-Strasse 10, D-38106 Braunschweig, Germany

Received 20 August 1996

Abstract The local etch pit densities (EPD) and microhardness (HV) of KAl(SO4)2' 12H20, KCI and KNO3 were examined. The etch pit density of {1 1 1} faces of KAl(SO4)2" 12H20 was found to be between 104 and 10 6 c m -2, while the microhardness was about 300-1000 MPa. {1 0 0} faces of KC1 showed etch pit densities between 105 and 107 cm- 2 and a microhardness of 90-180 MPa. For {1 l 1} faces of KNO3 etch pit densities in the range of 106 cm -2 and a microhardness of 350-410 MPa have been determined. Any coherence could not be observed between dislocation density and microhardness. For KAl(SO4)2" 12H20 crystals the microhardness depends on the crystal size but is independent of the stirrer velocities during batch crystallization.

1. Introduction Generally, the dislocation density can be used alternatively to X-ray methods to characterize the crystal quality. F r o m the materials science it is known that there should be a coherence between dislocation density and microhardness [1]. Increasing dislocation density should lead to an increase of microhardness. In this paper both properties and their supposed relation were examined for KAl(SO4)2" 12H20, KC1 and KNO3. The aim of

* Corresponding author.

those investigations was to examine the occurrence of growth rate dispersion [2].

2. Etch pit density 2.1. Etch solutions

Several etch solutions are known from literature [3-6] for the {1 1 1} face of KAI(SO4)2" 12H20. Because of poor description of the etching procedures (lack of etching duration, solution temperature,... ) there was a need to find out new etching mixtures and conditions. F o r an etch solution consisting of 4 ml ethanol and 1 ml water, the etching

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A. Herden, R. Lacmann / Journal of Cr)'stal Growth 173 (1997) 481-486

took about 90 s at room temperature. Two discernible kinds of etch pits, trigonal pyramidal and trigonal planar were observed. For KC1 a lot of etching solutions are described [7-12]. Satisfying results were obtained using a solution of 10 mg FeC13 in 25 ml isopropanol and 1 to 2 drops of distilled water [10] at room temperature for about 90 s. A new etching mixture was developed for KC1 crystals. Technical ethanol dried with Na2SO,~ at room temperature and an etching duration of about 5-15 s leads to sharp etch pits. Again two kinds of etch pits are discernible (Fig. 1). The wide flat quadratic ones and those that were small, long and deep. For the {1 1 1} face of K N O 3 no suitable etch solution was known, while for most of the other faces etch solutions are described [13,1. After some efforts, distilled (respectively dried) ethanol at room temperature and an etch duration of about 5-15 s were found out to give sharp etch pits. Blurred etch figures were got with undersaturated KNO3 solu-

tions, technical ethanol and technical acetone at room temperature.

2.2. Etch pit density To determine the etch pit density, the crystals were etched as described below. Photos of the interesting faces were taken. The etch pits were subsequently counted with the aid of enlargements. It is not assured that the etch solution is able to visualize all dislocations. So this paper will only deal with etch pit densities. Several authors [14-16] found a wide scatter in the local etch pit densities. This is in agreement with the results obtained by us. For KAI(SO4)2" 12H/O it could be observed that there is not any etch pit in wide ranges of a face [17-1, whereas other areas of the same face show an accumulation of etch pits, while KC1 and KNO3 faces show nearly no dislocation-free area (mean dislocation densities are higher (Table 1)). Because of this wide scatter in this paper exclusive local densities will be applied to characterize crystal faces. The results of one screen analysis of a {111) face of a KAI(SO4)2" 12H20 (size: about 3 mm) will be summarized in connection with microhardness values later on.

3. Microhardness

Fig. 1. Two kinds of etch pits on a {1 0 0} face of KC1 (wide flat quadratic ones and those that were small long and deep).

Another property that could be used to characterize a crystal is the microhardness. For this paper especially the Vickers hardness [18, 19] was examined. Therefore a microscope (POLYVAR-MET, Fa. Leica) combined with a microhardness tester (MD 4000 E, Reichert-Jung Optische Werke) has been used. The test parameters were chosen in such

Table 1 Etch pit densities

KAI(SO4)2 • 12HzO KAI(SO4)2' 12H20 KCI KNO3

Growth conditions

Etch pit density (cm-2)

Evaporation crystallization Batch crystallization Evaporation crystallization Evaporation crystallization

104-105 l0 6

105 10 v 106

A. Herden, R. Lacmann / Journal of Crystal Growth 173 (1997) 481-486

483

Table 2 Microhardness Substance

Face

Load (10 -3 N)

Slope (10

-3

Time N/S)

KAI(SO,)2" 12H20 KC1

{1 1 1} {1 0 0}

11.8 49.0

4.9 49.0

5 5

KNO3

{1 1 1}

14.7

4.9

5

a way that no cracks could be determined. In Table 2 the used parameters and results are summarized. While for KNO3 the microhardness seems to be homogeneous, for KAl(SO4)2" 12H20 and especially for KC1 a local scatter was obvious. It should be noted that for some KC1 crystals unusual high values had been found. An explanation of those reproducable results has not come yet. 3.1. Growth conditions and microhardness

To vary the crystal quality, KAI(SO4)2" 12H20 crystals were grown from an unstirred solution by evaporation (respectively, by batch crystallization) (100-400 rpm). The results of these investigations are illustrated for two crystal sizes (Fig. 2). Here the mean microhardness (average value calculated from 5 local microhardness values) is drawn versus stirrer velocity. The statistical interpretation characterized by the average value and the range of confidence trc [20] shows that there is no influence of the stirrer velocity on the microhardness, but that the microhardness depends on crystal size. 3.2. Is there a correlation between etch pit density and microhardness ?

The aim of our investigations was to clarify the correlation between etch pit density and microhardness. Therefore, for crystals of KAI(SOa)2" 12H20 and KC1 a screen analysis was carried out. Etch pit densities and mircrohardness had been determined on the same {1 1 1} face of a KAI(SO4)2" 12H20 crystal, so that the obtained

HV (106 Pa)

Literature

(S)

This work

Literature

300-1000 90-180 120-940 350~10

560-760 100-110

[23-26] [23]

130, 470

[23-26]

130

i

i

355

Mm < x

<

500

Mm

o 1000

Mm < x

< 2000

Mm

• F--I

E E

90 70

> T

i

110

\ c) I

t

0

js'|i o o

°

i

t ~

~

o

•

50 30

I

I

I

I

0

100

200

300

400

stirrer velocity [rpm] Fig. 2. The microhardness of KAI(SO4)2"12H20 crystals grown under various conditions; crystals of different sizes belonging to the same growth experiments were examined. A comparison of the average values taking into account the range of confidence shows the size dependence of the microhardness (Iq'q3s5 ,m = 660 + 27 MPa, lqV2ooo tam= 771 _ 13 MPa).

values could be attached to each other directly. After assuring that the microhardness is not influenced by etching the crystal, one 3 mm sized crystal grown by evaporation crystallization was etched as it has already been described. Then a microhardness indentation and its surroundings was photographed to determine the local etch pit density in an area of about two indentation diagonals [21,22]. The results are summarized in Figs. 3 and 4. Fig. 3 shows the distribution of the etch pit density (left) as well as that of the microhardness (right). There is a wide scatter in the local properties on the { 1 1 1} face. The microhardness is drawn versus etch pit density (Fig. 4). It is figured

O!'i!i

A. Herden, R. Lacmann /Journal of Crystal Growth 173 (1997) 481-486

484

Z

T i

~

:

:~

I

i

P

0,3mm

n: 359 E P---D = [ 2.z,-+43).104crn 2

I

iI

I

I

0,3ram

HV / 107 P a L _ _ < 50 5 0 - 59 60-69 7 0 - 79

0 0.1 ° 3.0 3.1 - 6.0 6.1 - 9.0 9.1 - 12.0 12.1 - 15.0 15.1 - 18.0 18.1 - 21.0 21.1 - 24.0 24.1 - 27.0 > 27

80-89

90 - 99

n:238 HV = ( 86t 151.107Po

100- 109

110- 119 120- 129 130- 139 > 140

Fig. 3. D i s t r i b u t i o n of the etch pit density (left) as well as that of the m i c r o h a r d n e s s (right) on a {1 1 1 } face of a KAI(SO4)z" 1 2 H 2 0 crystal; there is a wide scatter in the local properties.

180 160

o 0

140 O..

0

120 100

0

vH U

> -1-

80 60 40 20 0

0 0 m

HV:

85 ~13

90 ±21

86 ~3

~ t9

I

I

I

0

5

10

15

FPD E105/cm2-1

An analogous screening analysis was carried out for a {1 0 0} face of a KC1 crystal grown by evaporation crystallization. The distribution of the etch pit density, respectively, that of microhardness is shown in Fig. 5. Again a wide scatter is observable. In Fig. 6 microhardness is drawn versus etch pit density. It is remarkable that there is also no correlation between etch pit density and the detectable microhardness. For the {1 1 1} face of KNO3 only spot checks were carried out. The result of one face is exemplary shown in Fig. 7. The microhardness is obviously independent of the etch pit density.

Fig. 4. The microhardness on a { 1 1 1 } face of a KAI(SO4)2" 12HzO crystal versus etch pit density; all measurem e n t s were carried out on the same { 1 1 1 } face.

4. Summary

out that there is a wide scatter of the microhardness though the etch pit density is the same. Apparently, the microhardness is not influenced by the observed range of etch pit density (105/cm2).

The perfection of crystals was characterized by etch pit density, respectively, by microhardness. A wide scatter of both properties was observed. For the characterization of a crystal face local values should be used.

A. Herden, R. Lacmann / Journal of Crystal Growth 173 (1997) 481-486

485

I

0,2 mm

I

n:81

n= 7 3

HV-- (14±21.107po

-- ( 3 2 _+ 5.7 ). 106crn-2

Fig. 5. Distribution of the etch pit density (left) as well as that of the microhardness (right) on a { 1 0 0} face of a KCI crystal.

25

2O

I'-'I

,

,

,

O ,-4 i

i

15

O0 O -

0

,

,

,

,

45

o 0 0

[1_

,

I ~

0 0

0

-

O0

0

~ 0 0 oo

i

On

OnO

0

0

CO 0

10

o

40 0

i

0 ~0

0 u

0 0

35

0

> q>

0

0 0

"1-

5

30

0 0

5

10

EPD

'

15

2

o

25

3

0

3

'5

0

i

i

i

i

5

10

15

20

25

40

E106/cm23

Fig. 6. The microhardness on a {1 0 0} face of a KC1 crystal versus etch pit density; all measurements were carried out on the same { 1 0 0} face.

A correlation between etch pit density and microhardness could not be found yet. This may be explained by the fact that each dislocation is surrounded by a strain field. Those strain fields may

n th

measurements

Fig. 7. The microhardness on a {I 1 1} face of a KNO3 crystal; all measurements were carried out on the same {1 1 1} face. (--) average value; (- • .) standard deviation.

interact, leading to an intensification, respectively, to a weakening of strain. So it is impossible to characterize a crystal only by the number of dislocations without taking into account the influence of those dislocation strain fields.

486

A. Herden, R. Lacmann / Journal of Crystal Growth 173 (1997) 481-486

Acknowledgements The authors like to thank the "VolkswagenSiftung" and the "Fonds der chemischen Industrie" for their financial support.

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[12] E.Y. Gutmanas and E.M. Nadgornyi, Soy. Phys. Solid State 11 (1969) 959. [13] J. Rolfs, Thesis, TU Braunschweig (1992). [14] R. Beanland, J. Crystal Growth 130 (1993) 394. [15] U. Roy and D. Glassco, J. Crystal Growth 16 (1972) 227. [16] T. Hasegawa, T. Yakou and S. Karashima, Mater. Sci. Eng. 20 (1975) 267. [17] A. Herden, R. Lacmann and U. Tanneherger, in: Industrial Crystallization 93, 12th Syrup. on Industrial Crystallization, Warschau, Polen, 1993, Ed. Z.H. Rojkowski, Vol. I (Zaklad Poligraficzny, Warschau,1993) pp. 3-39. [18] B.W. Mott, Die Mikroh~irtepriifung (Berliner Union, Stuttgart, 1957). [19] S. Haussfihl, Z. Krist. 116 (1961) 371. [20] R.H. Leaver and T.R. Thomas, Versuchsauswertung, Darstellung & Auswertung Experimenteller Ergebnisse in Naturwissenschaft & Technik, Uni-text (Vieweg, Braunschweig, 1977). [21] Y.S. Boyskaja, R.P. Zitharu, N.A. Palistrant, M.A. Linte and Z.F. Terzi, Cryst. Res. Technol. 23 (1988) 1267. [22] M.P.W. Derks, A.E.D.M. van der Heijden and M. Elwenspoek, J. Crystal Growth 94 (1989) 527. [23] W.v. Engelhardt and S. Haussiihl, Fortschr. Miner. 42 (1965) 5. [24] J. Ulrich and M. Kruse, in: ACS Syrup. Ser. 438, Crystallization as a Separation Process, Eds. A.S. Myerson and K. Toyokura (American Chemical Society, Washington, DC, 1990) p. 43. [25] J. Ulrich and M. Kruse, Cryst. Res. Technol. 24 (1989) 181. [26] J. Ulrich and M. Kruse, Chem. lng. Tech. 61 (1989) 962.