ISSN 10678212, Russian Journal of NonFerrous Metals, 2013, Vol. 54, No. 6, pp. 453–461. © Allerton Press, Inc., 2013. Original Russian Text © S.A. Istomin, A.V. Ivanov, V.V. Ryabov, A.A. Khokhryakov, 2013, published in Izvestiya VUZ. Tsvetnaya Metallurgiya, 2013, No. 5, pp. 35–41.

METALLURGY OF RARE AND NOBLE METALS

Influence of the Mechanical Activation of REE Oxides on the Electrical Conductivity of Borate Melts S. A. Istomin*, A. V. Ivanov**, V. V. Ryabov***, and A. A. Khokhryakov**** Institute of Metallurgy, Ural Branch, Russian Academy of Sciences, ul. Amundsena 101, Yekaterinburg, 620016 Russia *email: [email protected] **email: [email protected] ***email: [email protected] ****email: [email protected] Abstract—The influence of the mechanical activation of oxides M2O3 (La2O3, Ce2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, and Lu2O3) on electrical conductivity (æ) of the B2O3–M2O3 molten systems is investigated. It is assumed that the majority carriers in these melts can be pro tons which enter the melts due to the hydration of B2O3. The variation in the magnitude of æ for borate melts for various M2O3 contents is explained by the corresponding variation in the structure of structural units as a result of the dissociation of the boron–oxygen groups, which include the OH groups. The activation energy of electrical conductivity in the B2O3–M2O3 melts increases due to the decay of superstructural units [B3O4.5] and B3O3O3/2OH as the temperature increases. The dependence of æ on the ordinal number of the lanthanide B2O3–Lu2O3 melts series, this energy follows the intraseries periodicity, is found. In the B2O3–La2O3 which depends on the stabilization energy of fundamental terms of the ions of rareearth elements (REE). Keywords: melt, boron oxide, REE oxides, electrical conductivity, mechanical activation, structure, clusters DOI: 10.3103/S1067821213060084

Borate melts are widely used as fluxes in the crystal growth of semiconductors. The electrical conductivity of molten fluxes, which determines the electrochemi cal doping processes at the melt–crystal interface, is of special interest. The electrical conductivity of borate melts depends on the composition and structure of structural units and interparticle interactions. It is known well [1–3] that molten B2O3 consists of layered fragments, which include nonpolar condensed trian gles [BO3] and boroxole rings [B3O4,5] (3B group). In addition, atmospheric moisture, when depolymeriz ing the B2O3 network, leads to the formation of the O=B–O– end structural fragments [4], the [BO2OH] and [BO3OH] groupings, and the B3O3O3/2OH cyclic groupings, which include the [BO3OH] tetrahedrons. Boron forms strong covalent bonds with oxygen, but it is considered that the interaction between the layers is the van der Waals interaction. The H bonds appearing during the hydration lower the connectivity of the network of molten B2O3. It is known that the mechanical activation (MA) of oxides of rareearth elements (REE) deforms and partially disorders their structure. Chemical reactions can pro ceed on the surface of the MA powders due to the interaction between REE oxides with components of air (H2O, CO2). On the other hand, the MA promotes

the preparation of borate melts more homogeneous in chemical composition, structure, and sizes of cluster zones [5]. It should be noted that the hightemperature inter action of MA oxides of REEs with B2O3 leads to the formation of triborate groups [B3O5] (T group) in the melts [3]. In addition, the concentration of fourcoor dinated BO3OH groupings increases due to a decrease in the concentration of BO2OH groupings and the decomposition of B3O3O3/2OH groupings as the tem perature increases. We note that the OH hydroxyl group enters the composition of these groups instead of one of the oxygen ions. At low temperatures close to the vitrification tem perature, the REE ions in borate melts form the cluster zones (inhomogeneity regions), which contain various sets of polyborate groups. The latter start to destruct as the temperature increases and change the composition and structure of cluster zones. In accordance to this fact, the electrical conductivity of the molten mixtures varies. EXPERIMENTAL To measure the electrical conductivity (æ) of borate melts, we used an ac bridge at a frequency of 5 kHz with

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an error of ±3% [6]. The measurements were per formed in a resistance furnace in air in alundum cruci bles. Electrodes were made of platinum ∅1 mm. The following materials were used for investigations: B2O3 of highpurity grade and La2O3, Ce2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, Tb2O3, Dy2O3, Ho2O3, Er2O3, Yb2O3, and Lu2O3 of reagent grade. All REE oxides were subjected to MA. The MA method consists of the effect of the impact compress ing load on the powder particles. The duration of the effect is determined by the mill rotation rate [7]. MA was performed using an AGO2S installation for 1 and 3 min. Components were mixed with boron oxide for 5 min in a Fritsch centrifugal mill. For the thermal investigation of REE oxides, we used a NETZSCH STA 449C Jupiter device for com bined thermal gravimetry (TG) and thermal analysis (DTA) conjugated with a QMC 403 C Aëolos quadru ple massspectrometer using a capillary heated to 523 K. The experiments were performed upon heating to T = 1173 K with a rate of 20 K/min in air flow (50 mL/min) using the preliminarily calcined Al2O3 crucibles. The investigation was performed in the UralM Collective Use Center (Yekaterinburg). In connection with the low solubility of REE oxides in model boron oxide, we investigated the influ ence of additives of REE oxides on electrical conduc tivity at REE concentrations of 0.5 and 1 wt %. After melting, the mixture and hightemperature holding, æ was measured, lowering the temperature of the borate melt with temperature monitoring using the Pt–PtRh thermocouple. RESULTS AND DISCUSSION It was established that the electrical conductivity of molten boron oxide at temperatures T = 1400–1650 is (1.23–3.22) × 10–6 S/m. These results somewhat differ from the data of authors [6], which is explained mainly by the presence of hydroxyl groups in B2O3 in an uncontrollable concentration. The thermal investiga tion of the M2O3 samples (M–Gd, Ho) subjected to MA and the samples without MA showed (Fig. 1) that the desorption of water molecules occurs upon heating to 553 K, while REE hydroxides decompose in range T = 693–843 K. Similar results were found previously in [8]. Solid B2O3 melts at T > 843 K and, consequently, hydroxyl group OH is introduced into the B2O3–M2O3 molten mixtures mainly with hydrated B2O3. The con centration of hydroxyl groups in the B2O3–M2O3 melts depends on the degree of grinding B2O3 and its residence time in air. It should be noted that, upon heating to 1500 K, hydroxyl groups partially come out of the melt, but their considerable fraction remains.

The dehydration of molten B2O3 can be partially per formed only by prolonged hourslong evacuation. The measured temperature dependence of electri cal conductivity of the boron oxide melt is linear in coordinates lnκ–T –1, which indicates the structural reconstructions upon varying the temperature [9]. As the temperature increases, the concentration of the [BO3] triangles increases due to the decomposition of boroxole rings (3B group), and the concentration of the [BO3OH] fourcoordinated groupings substan tially increases due a decrease in concentrations of the BO2OH groupings and the decomposition of the B3O3O3/2OH groupings. All abovementioned group ings enter the composition of cluster zones forming nonuniformities of a definite size. The sizes of these zones change as the temperature increases. This is indicated by investigations into the structure of vitre ous and molten B2O3 by lowangle Xray scattering [10]. It seems likely that the hydroxyl groups play the main role in the electrical transfer phenomena, since they enter the composition of all types of boron–oxy gen groups. We note that dissolved water in B2O3 is dis tributed chaotically and does not affect the structural nonuniformity [11]. In any case, the boron–oxygen– oxyhydrile network of B2O3 becomes less coherent, which facilitates the ion transfer in the melt. It should be noted that the dissociation energy of cyclic 3B groups with H bonds is considerably lower than that of the products of their dissociation. Therefore, the activa tion energy of electrical conductivity at low tempera tures is lower (60.9 kJ/mol) than at high temperatures (73.6 kJ/mol). The electrical conductivity of the B2O3–M2O3 molten mixtures (M = La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu) in a temperature range of 1400– 1650 K is presented in Tables 1 and 2 for the M2O3 concentrations of 0.5 and 1 wt % without the MA of oxides and with oxides subjected to MA for 1 and 3 min. We can see that the electrical conductivity of all mixtures is lower than that of molten B2O3. As the melt temperature lowers, the electrical con ductivity decreases. Since this installation does not make it possible to measure the high resistance of the melt, magnitudes of æ are given only for temperatures that are higher than the solidification temperature of the melts by 373–523 K. At temperatures close to 1400 K, the electrical conductivity of studied melts is (0.65– 1.61) × 10–6 S/m. It is seen from Tables 1 and 2 that the melts whose components were subjected to MA have both smaller and larger values of æ than samples with out MA. The observed variations in the electrical con ductivity are associated with the corresponding varia tions in the interelectron interactions in the REE ions, the sensor of which is the hydrogen ion (the main par ticipant of the transport process). The proton trans

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INFLUENCE OF THE MECHANICAL ACTIVATION OF REE OXIDES Ion current × 109, A DTA, µV/mg

(a) TG, % 421.2

exo

100.0 TG

0.06

3

1

0.04 1.4

99.0

0.02 1.3 0

298.2

DTA 2

280.4

98.0

1.2

–0.02 1.1

387.0

–0.04 1.0

132.4 97.5 97.0

1.5

Variation in weight of 3.13%

99.5

98.5

455

–0.06 0.9 H2O

–0.08 0.8 200

100

300

400

500

600

700

800

Ion current × 109, A DTA, µV/mg exo

(b) TG, %

900 t, °C

574.1

100.0 474.8

1

TG

0.16 1.0

99.5

0.14

99.0

0.12 541.7

2

98.5

0.10 1.8

3

0.08 Variation in weight of 3.36%

DTA

98.0 97.5

200

0.6

0.02 0.5

H2O 100

0.7

0.06 0.04

97.0

0.9

300

400

500

600

700

800

900 t, °C

Fig. 1. Variation in the weight ((1) TG), the heat flux ((2) DTA), and the moisture content (3) upon the heating of Gd2O3 (a) without MA and (b) with the MA in the AGO2S installation. Charge (a) 61.15 mg and (b) 63.85 mg.

port mechanism for borate glasses was proposed ear lier in [12, 13]. In our case, molecular water is introduced into the melts mainly with boron anhydride. Oxides of the yttrium subgroup absorb water weakly. The presence of hydroxyl in them was not found [7, 14]. MA leads to the interaction of adsorbed water with oxides M2O3 and the formation of hydroxides. It is known [15] that RUSSIAN JOURNAL OF NONFERROUS METALS

the thermal stability of REE hydroxides drops in the series La Lu. Therefore, the fraction of oxyhydrile groups introduced into molten B2O3 with oxides M2O3 will be lowered. It should be noted that the larger duration of MA increases the dispersity of the M2O3 particles, which leads to the enhancement of the interaction of the sur face water with oxides M2O3. Water desorption in such

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Table 1. Electrical conductivity of the B2O3–0.5 wt % M2O3 (La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu) melts in the range T = 1400–1650 K æ × 106, S/m, at T, K M2O3

τMA, min 1400

1450

1500

1550

1600

1650

La2O3

– 1 3

0.85 0.86 –

1.07 0.71 1.08

1.34 0.91 1.36

1.61 1.19 1.80

2.07 1.53 2.33

2.50 2.04 2.96

Ce2O3

– 1 3

– – –

0.65 – –

0.65 0.79 0.63

0.76 1.00 0.78

0.94 1.30 0.94

1.34 1.67 1.25

Nd2O3

– 1 3

0.67 – –

0.84 0.66 –

1.05 0.81 0.72

1.30 0.98 0.89

1.62 1.28 1.20

2.16 1.60 1.67

Sm2O3

– 1 3

0.65 0.70 –

0.82 0.87 0.76

1.00 1.08 0.95

1.24 1.30 1.15

1.65 1.64 1.47

1.92 2.27 2.00

Eu2O3

– 1 3

0.67 – –

0.84 – –

0.97 0.66 0.70

1.26 0.78 0.86

1.59 1.00 1.07

2.09 1.34 1.46

Gd2O3

– 1 3

0.85 0.87 –

0.75 1.07 1.05

0.93 1.34 1.29

1.17 1.61 1.68

1.48 2.14 2.11

1.79 2.72 2.76

Tb2O3

– 1 3

0.69 – –

0.89 0.65 –

1.11 0.85 0.70

1.42 1.06 0.88

1.76 1.19 1.15

2.19 1.52 1.60

Dy2O3

– 1 3

0.74 0.74 –

0.80 0.92 0.91

0.94 1.17 1.12

1.15 1.41 1.35

1.43 1.85 1.76

1.93 2.42 2.19

Ho2O3

– 1 3

0.94 1.16 1.61

1.16 1.55 2.03

1.44 1.89 2.40

1.73 2.38 3.11

2.02 4.24 3.65

2.29 5.36 4.00

Er2O3

– 1 3

0.69 0.65 –

0.87 0.80 0.65

1.09 0.98 0.76

1.37 1.20 0.93

1.64 1.65 1.18

2.03 2.26 1.64

Yb2O3

– 1 3

0.65 – –

0.77 0.70 0.71

0.99 0.91 0.89

1.26 1.20 1.07

1.62 1.56 1.32

2.08 2.32 1.63

Lu2O3

– 1 3

0.89 – –

1.14 – –

1.41 0.69 0.66

1.73 0.82 0.80

2.02 1.08 0.99

2.37 1.50 1.34

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Table 2. Electrical conductivity of the B2O3–1 wt % M2O3 (La, Ce, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu) melts in the range T = 1400–1650 K M2O3

La2O3

Ce2O3

Nd2O3

Sm2O3

Eu2O3

Gd2O3

Tb2O3

Dy2O3

Ho2O3

Er2O3

Yb2O3

Lu2O3

τMA, min

æ × 106, S/m, at T, K 1400

1450

1500

1550

1600

1650

–

–

0.88

1.12

1.35

1.79

2.23

1

–

0.72

0.92

1.18

1.56

2.01

3

–

0.62

0.77

0.96

1.18

1.75

–

–

–

0.72

0.89

1.13

1.66

1

0.64

0.77

0.97

1.20

1.50

1.97

3

–

–

0.67

0.83

1.03

1.38

–

1.00

1.27

1.60

1.96

2.56

3.64

1

–

–

0.65

0.81

0.99

1.31

3

–

0.65

0.82

1.02

1.28

1.76

–

–

0.69

0.89

1.09

1.39

1.78

1

0.75

0.95

1.21

1.47

1.85

2.50

3

0.76

0.93

1.10

1.32

1.57

2.06

–

0.74

0.85

1.09

1.37

1.71

2.19

1

–

–

–

0.71

0.91

1.30

3

–

–

–

0.56

0.71

0.92

–

0.82

0.98

1.21

1.41

1.81

2.08

1

0.78

0.98

1.20

1.44

1.76

2.23

3

–

0.86

1.04

1.29

1.61

2.10

–

1.19

1.52

1.87

2.20

2.74

3.49

1

–

–

0.67

0.84

1.09

1.50

3

–

–

0.69

0.90

1.13

1.27

–

0.73

0.85

1.04

1.27

1.56

1.86

1

0.75

0.92

1.12

1.37

1.71

2.28

3

–

0.73

0.87

1.08

1.35

1.88

–

0.94

1.18

1.43

1.89

2.20

2.59

1

1.18

1.53

1.93

2.40

2.86

3.46

3

0.75

0.94

1.22

1.46

1.85

2.49

–

0.64

0.77

0.93

1.13

1.44

1.60

1

0.63

0.75

0.93

1.15

1.46

2.06

3

–

–

–

0.84

1.04

1.32

–

–

–

0.75

0.91

1.09

1.39

1

0.67

0.81

1.01

1.22

1.59

2.00

3

0.71

0.90

1.16

1.35

1.78

2.30

–

0.80

1.02

1.27

1.56

1.89

2.29

1

–

–

0.69

0.87

1.12

1.53

3

–

–

0.66

0.79

0.99

1.44

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ISTOMIN et al. 4

æ × 106, S/m

3 1 2 2

Lu2O3

Yb2O3

Tm2O3

Er2O3

Ho2O3

Dy2O3

Tb2O3

Gd2O3

Eu2O3

Sm2O3

Pm2O3

Nd3O3

Pr2O3

Ce2O3

0

La2O3

1

Fig. 2. Electrical conductivity of the B2O3 – 1 wt % M2O3 melts at T = 1650 K. The MA of M2O3 for (1) 1 min and (3) 3 min.

systems under heating occurs at higher temperatures and for a shorter time interval. Water passes from the interaction region into the atmosphere; therefore, the fraction of oxyhydrile groups in the B2O3–M2O3 mol ten mixture will be smaller relative to the same solu tion without MA. The activation energy of electrical conductivity (Eæ) in the studied melts is affected by several interac tion mechanisms. On the one hand, this is the lan thanide compression, which determines the increase in ionic potentials of trivalent lanthanide ions and the formation of their stronger bonds with hydroxyl groups in the series from La to Lu. On the other hand, the electrostatic interaction of electrons in the 4f open shells should lead to the tetradic effect, which is indi cated by the zigzag dependence of æ upon an increase in the atomic number of lanthanide. It is seen from Fig. 2 that the regions with minima of æ fall to ions Nd(III), Eu(III), Tb(III), and Er(III) with the highest stabilization energy. The tetradic effect is associated with the corresponding variation in energies of elec tron terms in the lanthanide series, in the Coulomb field of which, the motion of hydrogen ions – is jump like. It is well known [16] that spin pairing energy k1E1, which is symmetric relative to q = 7 (q is the number of electrons) and the energy of coupling the orbital moments of f electrons k3E3 contribute to the energy of the ground level of Ln(III). Here, E1 and E3 are the Rakah parameters, which determine the value of the

interelectron interaction in the states of electron terms. Coefficient k3 possesses symmetry relative to ¼ and ¾ of the f shell. Term k3E3 determines the stability of f shells filled with ¼ and ¾, while k1E1 determines the increased stability of the halffilled shell. We note that, as the MA time increases, the REE oxides form a cluster zone with a higher chemical and structural uniformity, which lowers the number of “defects” in the networks of B2O3–M2O3 melts and enhances their dissipative properties. In our case, the types of H bonds shown in Fig. 3 can be in the network of the B2O3–M2O3 melts. As the temperature increases, the electrical con ductivity of all melts increases, since the concentra tion of fourcoordinated boron groupings containing hydroxyl groups increases. An increase in the activa tion energy of electrical conductivity with temperature is caused by a decrease in the fraction of polyborate groups (T and 3B) and an increase in the fraction of the [BO3] bound triangles, the [BO4OH] tetrahedra, and the O=B=O– end groups, with which the oxyhy drile groups interact. We note that the most mobile protons in molten B2O3 enter the composition of the OH groups forming the strongest H bond. These groups have emission bands at ν = 2980 and 2740 cm–1 in IR spectra, while the lowmobile protons have the weakest H bond. The OH groups associated with these protons have the bands at ν = 3430 and 3125 cm–1 in the emission IR spectra [4]. The schemes of the proton

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B

O

O–

H

O

M

O

H

M

459

1

2

B

B

O

O O

O–

H

M

B 3 B

O

O

Fig. 3. Schemes of the types of H bonds in the network of the B2O3–M2O3 melts. M is the sixcoordinated REE ion and O– is nonbridge oxygen. Possible ways of proton activation jumps are shown by the arrow.

conduction mechanisms in the B2O3–M2O3 molten systems are presented in Fig. 3. Protons are the majority carrier current. The pro ton transfer is performed due to the activation jump under the effect of gradients of the electric field. Acti vation energies of processes 1–3 (Fig. 3) are different. In the experiment, the averaged values of electrical conductivity are measured and, starting from them, the activation energies of the proton transfer are calcu lated (Table 3). An increase in the electrical conductivity with tem perature is caused by an increase in the concentration of the BO3OH fourcoordinated groupings in all borate melts. A decrease in æ with an increase in the M2O3 concentration in the B2O3–M2O3 melts is caused by a decrease in the fraction of hydroxyl groups in the starting melts (Tables 1, 2). The detected depen dence of electrical conductivity in a series of the La2O3–B2O3 Lu2O3–B2O3 melts is associated RUSSIAN JOURNAL OF NONFERROUS METALS

with the tetradic effect. The electrical conductivity correlates with the stabilization energy of ground terms of the Ln(III) ions. CONCLUSIONS The electrical conductivity of the melts B2O3– REE oxides is measured using an ac bridge. It is estab lished that REE oxides in amounts of 0.5 and 1 wt % lower the level of æ of boron oxide. After the MA of REE oxides for 1 and 3 min, the hightemperature and lowtemperature segments with various values of Eæ are established for all the studied melts, which indi cates structural reconstructions upon varying the tem perature. The observed variations in æ are associated with the proton transport mechanisms in borate melts. The experimentally determined electrical conductiv ity in the series of studied borate melts follows the intraseries periodicity (the tetradic effect) upon increasing the atomic number of lanthanide.

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Table 3. Activation energy of electrical conductivity (Eæ) of the B2O3–M2O3 melts Eæ, kJ/mol, at τMA, min System

M2O3, wt % 0

1

3

B2O3–La2O3

0.5 1.0

92.6/78.9 116.0/83.1

119.0/86.6 113.8/85.6

106.8/73.0 188.0/83.1

B2O3–Ge2O3

0.5 1.0

164.7/83.1 180.0/97.7

105.0/78.2 167.0/81.0

166.0/91.0 135.0/90.5

B2O3–Nd2O3

0.5 1.0

132.2/81.3 136.5/80.0

103.0/74.0 136.6/94.4

134.5/80.0 185.0/93.5

B2O3–Sm2O3

0.5 1.0

83.0 103.0/83.0

155.0/76.8 132.0/86.6

152.0/84.7 129.0/79.5

B2O3–Eu2O3

0.5 1.0

107.0/83.1 96.7/56.0

166.0/96.0 124.0

140.0/87.0 103.0

B2O3–Gd2O3

0.5 1.0

120.7/70.0 92.0/74.2

103.0/65.3 102.8/74.2

B2O3–Tb2O3

0.5 1.0

95.3/77.03 100.4/71.0

166.0/96.0 124.0

154.7/126.6 70.5/109.4

B2O3–Dy2O3

0.5 1.0

90.6/64.2 75.0

115.0/55.0 116.0/74.3

108.0/74.3 157.2/81.3

B2O3–Ho2O3

0.5 1.0

71.4 119.3/75.1

125.0/81.2 140.8/79.4

123.4/83.0 126.6/86.5

B2O3–Er2O3

0.5 1.0

83.1 100.0/71.0

127.6/81.6 148.0/79.0

147.0/84.9 111.7/75.5

B2O3–Yb2O3

0.5 1.0

95.0 102.0

136.0/99.7 105.0/72.0

94.4 111.0/83.1

B2O3–Lu2O3

0.5 1.0

74.3 79.2

138.5/79.2 155.0/69.5

146.6/85.0 150.0/76.3

89.5 74.2

The numerator is the value of Eκ in the hightemperature region and the denominator is that in the lowtemperature region.

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Translated by N. Korovin

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