Influence of glass addition and sintering temperature on the structure, mechanical properties and dielectric strength of high-voltage insulators

Materials and Design 31 (2010) 3885–3890

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Short Communication

Influence of glass addition and sintering temperature on the structure, mechanical properties and dielectric strength of high-voltage insulators Muthafar F. Al-Hilli *, Kalid T. Al-Rasoul Al-Jaderiyah, Department of Physics, College of Sciences, University of Baghdad, Jaderiyah Campus, Baghdad, Iraq

a r t i c l e

i n f o

Article history: Received 1 November 2009 Accepted 24 February 2010 Available online 2 March 2010

a b s t r a c t Soda-lime glass as a substituent for the feldspar was used to prepare high-tension electrical porcelain by standard chemical solid reaction technique. The effect of glass substitution and sintering temperature on the physical properties, microstructure, hardness, modulus of rupture, flexural strength and Dielectric breakdown strength were examined. Zero water absorption (WA %) and apparent porosity (AP %) were achieved for the samples with glass content >15 wt.% sintered at 1100 °C. The apparent density was found to increase with sintering temperature. The Vicker’s micro-hardness increased with both glass addition and sintering temperature. Both of the modulus of rupture (MOR) and flexural strength (rf) had maxima values at 15 wt.% glass addition. The structure and morphology were characterized by X-ray diffraction and scanning electron microscope (SEM). It showed the formation of mullite needles at sintering temperature of 1100 °C, which enhanced the mechanical and electrical properties of the porcelain. The dielectric breakdown strength increased with sintering temperature and glass addition. The highest dielectric strength was found at 10 wt.% of glass addition depending on the Na2O and Fe2O3 content. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Porcelain insulators and porcelain shells are important equipments in the operation of power plants and transformer substations insulation and supporting wire [1]. The electrical porcelain are usually prepared using pressing. The firing of electrical porcelain is usually performed at 1300 °C. To decrease the sintering temperature of porcelain, different flux-forming additives (natural materials and production wastes) are used. The fluxes based on natural alkali alumino-silicates (feldspar, pegmatite, perlite, nepheline syenite, etc.) or carbonates of alkali-earth metals are usually utilized for this aim. However, in many cases, the use of glassforming components to ceramic bodies result in decrease in firing temperature, in the amount of crystalline phases due to their dissolution in the glass phase during firing and in mechanical and thermal properties of porcelain. The manufacture of porcelain of high properties at with low sintering temperatures is possible up to certain limits. The properties of the porcelain are determined mainly by the presence of sufficient quantity of well-crystallized mullite. The sintering temperature of porcelain is therefore connected to with the development of the processes of mullite formation in the clay–nonplastics-flux system, i.e. the firing temperature of the porcelain must not be lower than the temperature at which the mullite forming processes can be completed (not below

* Corresponding author. Tel.: +964 7704509125. E-mail addresses: [email protected], (M.F. Al-Hilli).

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0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.02.048

1150–1200 °C) [2]. The present work aims to recycle glass waste in the development of high-voltage porcelain insulators sintered at low temperatures. 2. Experimental The chemical compositions of the kaolin and soda-lime glass used in the porcelain preparation are shown in Tables 1 and 2, respectively. The glass obtained from high transparency soda-lime glass bottles free of coloring oxides. Both kaolin and glass were milled using pestle and mortar then sieved through (63 lm) screen (using British sieve). The glass powder was added to kaolin in proportions of (5, 10, 15, 20 and 25 wt.%), then dry mixed using a ball mill to homogenize the mixture. Samples in the shape of pellets were prepared by uniaxial semi-dry pressing using a pressure of 374.3 MPa. The samples were finally sintered at different temperatures (800, 900, 1000 and 1100 °C). The sintering process carried out in air with heating rate of 3 °C/min and soaking period 30 min using Carbolite muffle furnace. The linear shrinkage (LS %) was determined using Eq. (1) by measuring the sample dimensions before and after sintering. The water absorption was determined from Eq. (2) using the vacuum/boiling technique [3,4]. The samples were weighed immediately after sintering to find dry weight (WD). The samples were soaked in distilled water and contained in suitably protected desiccators for 2.5 h at pressure of 25 mbar. Afterwards, the samples were left to boil gently in the water for 1 h, and soaked for an additional 24 h at ambient temperature. The samples were removed and weighed after being wiped with

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Table 1 Chemical analysis of kaolin.

%

Oxide SiO2 47.34

Al2O3 36.37

Fe2O3 0.63

TiO2 2.20

CaO 0.02

MgO 0.08

Na2O 0.31

K2O 0.53

L.O.I. 12.42

using INSTRON instrument. The specimen was fixed between the device platens to start compressing at a crosshead speed = 0.5 mm/min until fracture occurred. The flexural strength (diametrical compression strength) was determined by applying Eq. (7) [7].

rf ¼ Table 2 Chemical analysis of soda-lime glass.

%

Oxide SiO2 72.36

Na2O 15.13

CaO 6.67

MgO 3.78

Al2O3 1.60

SO3 0.36

Fe2O3 0.08

K2O 0.02

moistened tissue to find the soaked weight (WS). The sample suspended from the arm of a balance pan and immersed in distilled water to find the immersed weight (WI). The apparent porosity (AP %) and apparent density (AD) were determined using Eqs. (3) and (4), respectively. The measurements were carried out according to Archimedes principle [ASTM test method (20-83, 493-70)].

SL % ¼

dry length  fried length  100% dry length

WA % ¼

AP % ¼

AD ¼

WS  WD  100% WD

WS  WD  100% WS  WI

WD WD  WI

ð1Þ

ð2Þ

ð3Þ

Fa 2

d

ð4Þ

ð5Þ

where Fa is the applied indenter load (N) and d is the average diagonal length for the indent (mm) [5,6]. INSTRON instrument was used for modulus of rupture measurement. The test was performed using three-bearing cylinders mechanism mounted between the platens of the testing device. The crosshead speed was (0.5 mm/ min) with maximum load of (2000 N). Rectangular shaped samples (5  5  22 mm) sintered at 1100 °C were broken over (17 mm) span in this test. Only the results of the specimens broken at mid span were taken into account. The maximum load of fracture was recorded and Eq. (6) was applied to obtain the modulus of rupture (MOR)

MOR ¼

3FS 2bt

2

ð7Þ

where Ffract. is the fracture load (N), D is the sample diameter and h is the sample thickness. X-ray diffraction was performed on selected samples. The samples were milled and tested using Philips PW1050/70 Goniometer with Cu Ka radiation (k = 1.540598 Å). The surface morphology was characterized by scanning electron microscope instrument JEOL JSM-6400. The samples were grinded and polished then etched in 40% HF for (30 s) and washed with distilled water. The samples were coated with gold thin film of thickness 230 Å using JEOL JFC-1100E Ion Sputter at working pressure (101 Pa) and coating duration 3 min. The dielectric strength was performed on ‘‘High Voltage Potronics Device” at 50 Hz with maximum negative voltage of 80 kV. The testing ambient medium was pure transformer oil with breakdown strength about 40 kV/mm, to prevent the surface breakdown or flash-over voltage. The voltage was raised at a rate of 1 kV/s to ensure eliminations of any thermal effects until breakdown occurs at the maximum breakdown voltage (Vbr). The dielectric strength (Ebr) was determined using the following relation:

Ebr ¼

V br h

ð8Þ

where h is the sample thickness in mm.

where LS % is the linear shrinkage percent, WA % is the water absorption percent, AP % is the apparent porosity percent and AD is the apparent density (g/cm3). Vicker’s micro-hardness instrument HPO250 equipped with microscopic screen and diagonal measuring device was used to measure the hardness. The samples were grinded and polished to mirror-like surface and coated with an aluminum thin film (100 Å) to ensure a clear evident indent on the sample surface. The optimum indentation load was (49 N) for loading time of (15 s). Three indents were made on each sample surface and the average of diagonals dimensions was taken. The Vicker’s hardness (HV) was determined using

HV ¼ 1:8544

2F fract: pDh

3. Results and discussion The XRD patterns shown in Figs. 1 and 2 are for samples with 5% and 25% glass additions sintered at 1100 °C, respectively. The XRD patterns showed the presence of the secondary mullite phase crystallized at 1100 °C. The crystallization of the needle-like mullite crystals are shown in the SEM micrograph in Fig. 3, where the needle-shaped crystals termed secondary mullite (since they form later in the firing process) [8–10]. The results obtained for linear shrinkage (LS) %, water absorption (WA) %, apparent porosity (AP) % and apparent density (AD) are shown in Table 3. The linear shrinkage increased with sintering temperature increase, since the body becomes richer with molten glass. The molten glass helps to fill up the inter-particles spacing as a result of which volume is reduced. Among the important factors for the electrical porcelain are the water absorption and apparent porosity, which were correlated to each other. However, knowing that the water absorption must not exceeds (0.5%) for the electrical porcelain, highlights the importance of the zero and AP which was attained with 25 wt.%

ð6Þ

where F is the load at failure (N), S is the distance between supports (span), b is the sample breadth and t is the sample height [3]. The flexural strength (rf) was measured by the diametrical compression of a solid disk referred to as the Brazilian disk fracture test. The test performed on a disk sample (height = 5 mm  diameter = 13 mm)

Fig. 1. XRD patterns of kaolin with 5 wt.% soda-lime glass additive sintered at 1100 °C, where M = mullite [ASTM 15-776], C = cristoballite [ASTM 11-695] and B = (Ca, Na2, K2)–Si Al.

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Fig. 2. XRD patterns of kaolin with 25 wt.% soda-lime glass additive sintered at 1100 °C, where M = mullite, C = cristoballite and B = (Ca, Na2, K2)–Si Al.

effect of glass density. Except the case when apparent porosity drops to zero at the sintering temperature 1100 °C, closed pores may increase owing to the bloating, and then density decreases. The source of bloating is the expansion of O2 gases released from the reaction of Fe2O3 to Fe2O4, or the expansion of the gases enclosed within the pores [13,14]. Fig. 4 shows the Vicker’s micro-hardness of samples with different glass content sintered at different temperatures. Hardness was found to increase with glass addition and sintering temperature as shown in Figs. 4 and 5. This behavior may be discussed based on the surface vitrification and self-glazing by the molten glassy phase. The decrease of apparent porosity and enrichment in the glassy phase content is shown in Table 3 and Fig. 6. Moreover, the hardness increased by the stimulated formation of mullite at sintering temperature 1100 °C (mullite hardness  15 GPa). This is in agreement with the results of Sidjanin et al., Ramaswamy et al. and García et al. [15–17]. The bending test is the most reliable one for mechanical properties assessment for ceramics. However, the modulus of rupture (MOR) versus the glass addition is plotted in Fig. 7. The MOR reaches maximum at 15 wt.% glass addition corresponding to almost complete vitrification, since the MOR varies exponentially

Fig. 3. SEM photograph of kaolin with 25 wt.% soda-lime glass sintered at 1100 °C.

of glass addition to kaolin sintered at 1100 °C [11,12]. The apparent density reached maximum due to the vitrification, then decreased due to the lower density of soda-lime glass (2.47 g/cm3). At higher additions, the relatively lower viscosity of glass suppresses the

Fig. 4. Vicker’s hardness (GPa) versus glass additive wt.%, sintered at different temperatures.

Table 3 Effect of glass addition wt.% on linear shrinkage %, water absorption %, apparent porosity % and apparent density at different sintering temperatures. Water absorption (%)

Apparent porosity (%)

Apparent density (g/cm3)

0 5 10 15 20 25

800

2.9 3 3.1 2.8 2.8 2.9

15.1 14.3 14 12.9 13.9 12

26.1 27.2 25.1 24.2 24.8 23.8

2.28 2.57 2.48 2.47 2.39 2.61

0 5 10 15 20 25

900

4.9 5.5 6 6.1 6.2 7

13.8 12.8 9.9 8.9 8.4 6

25.4 24.9 19.9 18.9 18.2 14.5

2.48 2.67 2.51 2.52 2.58 2.67

0 5 10 15 20 25

1000

6.5 8.2 10.5 11.9 12 12.5

11.8 11.2 8.4 3.9 4.8 2.6

23 23.9 16.3 9.7 11 8

2.58 2.8 2.61 2.56 2.64 2.72

0 5 10 15 20 25

1100

12.6 14.4 14.5 14.6 14 15.5

5.5 1.5 0.3 0.1 0 0

13 4.7 1.8 0.5 0 0

2.68 2.58 2.72 2.57 2.56 2.52

Glass addition (wt.%)

Sintering temperature (°C)

Linear shrinkage (%)

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240 220

MOR ( MPa)

200 180 160 140 120 100 80 60 40 0 Fig. 5. Dependence of Vicker’s hardness (GPa) on sintering temperature for the samples, (d) kaolin and (j) kaolin + 15 wt.% glass addition.

5

10

15

20

25

30

Glass additive % Fig. 7. Modulus of rupture (MOR) (MPa) versus glass additive wt.%, sintered at 1100 °C.

Fig. 8 shows the relationship of the diametrical compression strength (rf) with the of glass addition. The behavior of rf agrees with that of the MOR, but with lower values. This may be due to the difference in stresses distribution. This shows that the relationship between the two tests is not straightforward in agreement with the postulate of Vardar and Finnie [19]. However, in the bending test the shear stress criterion is the dominant mechanism of fracture. Fig. 9 shows the relationship between the flexural strength and sintering temperature for samples of kaolin and kaolin + 15 wt.% glass. The behavior of rf with sintering temperature agrees with that of the hardness. The high flexural strength for sample sintered at 1100 °C may be attributed to the formation of needle-like mullite crystals as shown in Fig. 3. Image indicates

Fig. 8. Flexural strength (rf) (MPa) versus glass additive wt.%, sintered at 1100 °C.

Fig. 6. SEM photograph of kaolin with (a) 5 wt.% soda-lime glass sintered at 1100 °C, (b) 25 wt.% soda-lime glass sintered at 1100 °C and (c) 25 wt.% soda-lime glass sintered at 800 °C.

with the porosity, in agreement with the observations of Karamanov et al. [18]. The decrease of MOR with glass addition >15 wt.% may be due to the decrease in apparent density, since MOR varies proportionally with the density.

Fig. 9. Dependence of flexural strength (rf) (MPa) on sintering temperature for the samples, (d) kaolin and (j) kaolin + 15 wt.% glass addition.

M.F. Al-Hilli, K.T. Al-Rasoul / Materials and Design 31 (2010) 3885–3890

Fig. 10. Undissolved quartz grain shown by SEM photograph of kaolin with (a) 5 wt.% soda-lime glass sintered at 1100 °C.

mullite’s preferential orientation on the surface of kaolinite matrix. Probably the mullite in the kaolinite matrix is the seed for the crystallisation of the mullite needles. The morphology of the mullite needle is like acicular crystal. The mullite needles seem to be interlocked, which actually act as the strength increaser. On the other hand, the undissolved quartz grains shown in Fig. 10 acts to increase the flexural strength. Where the difference between thermal expansion coefficient of quartz and the surrounding glassy phase causes stress around the crystal during cooling. This stress in the glassy phase creates tensile forces perpendicular to the compressive forces parallel to the grain boundaries. This might causes a favorable effect on the porcelain mechanical strength [20–23]. The results of dielectric strength listed in Table 4, increases with glass addition up to 10% mainly due to the porosity decreases. At higher addition of soda-lime glass, the dielectric strength drops down may be because of the increase in Na2O content. The high content of Na2O in the glass suppresses the advantage of porosity decrease. The dominant mechanism of breakdown here is, a small number of carriers in the conduction band are accelerated in the

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Fig. 12. Relation between dielectric breakdown strength and sample thickness for kaolin + 15 wt.% soda-lime glass addition sintered at 1100 °C.

electric field and these collide with atoms, ionizing them. This releases more carriers for further impact ionization and the current rapidly builds. However, the resultant Joule heating causes insulators to become better conductors that can pass more current. The process feeds on itself until a thermal runaway result in local failure. Shaping insulators in special ways can effectively reduce their tendency to break down or conduct at high voltages. Fig. 11 shows variation of breakdown strength with sintering temperature. For sintering temperatures (800–900) °C the breakdown strength does not raise significantly may be because the meta-kaolin does not show any structural transformation up to 980 °C. The breakdown strength increased rapidly above sintering temperature of 1000 °C. This is due to the crystallization and growth of secondary mullite from the molten amorphous aluminosilicate phase [24], as shown in Figs. 1 and 2. Moreover, the breakdown strength increased with porosity decrease above 1000 °C, in agreement with the results of Jongprateep et al. [25]. The breakdown strength was found to change inversely linearly with the sample thickness, as shown in Fig. 12. This behavior of the breakdown strength with sample thickness coincides with Eq. (8). 4. Conclusion

Table 4 Dependence of dielectric breakdown strength upon glass addition to kaolin sintered at 1100 °C. Glass addition (wt.%)

Dielectric strength, Ebr (kV/mm)

0 5 10 15 20 25

20.39 21.17 54.44 36.14 29.25 28.51

Fig. 11. Dielectric breakdown strength versus sintering temperature for kaolin + 15 wt.% soda-lime glass addition.

It is feasible to substitute feldspar by soda-lime glass in the preparation of high-voltage porcelain insulators. The glass waste powder shows to be an efficient fluxing agent when used as an additive to ceramic mixture. The prepared samples showed selfglazing depending on the glass content and sintering temperature. The obtained mechanical properties were quite applicable for the high-voltage insulators. Vicker’s micro-hardness increased as a function of porosity decrease. Both of the modulus of rupture (MOR) and flexural strength (rf) showed maximum values at 15 wt.% of glass addition. This may suggest that the material exhibit anisotropic mechanical properties. The needle-like mullite showed a positive impact on the mechanical properties. The mullite distribution is very critical for the mechanical properties. However, mullite formation sacrificing corundum content should not be allowed. The strength can be greatly increased by undissolved quartz. If there is a large amount of undissolved quartz then it creates an obstacle in the way of conductive path that is glassy phase in which mobile ions can easily move. If it were possible to retain the undissolved quartz with higher amount of mullite then the strength would be greatly increased. The dielectric strength was remarkably influenced by the sintering temperature, porosity and glass content. For Glass addition >10 wt.%, the glassy phase gives a free path to the mobile ions like Na+, K+, Fe3+, Al3+ to move and hence reduce the dielectric breakdown. It is valid for the glass additive to contain up to 15 wt.% of Na2O. For better electrical insulation, the Fe2O3 content should be <1.0%.

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