phase, thermal and impedance studies of nanosize via mechanical milling and sintering

Superlattices and Microstructures 49 (2011) 17–31

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Superlattices and Microstructures

journal homepage: www.elsevier.com/locate/superlattices

Phase, thermal and impedance studies of nanosize Li2WO4via mechanical milling and sinteringTan Kim Han ∗, Roslina Ahmad, Mohd Rafie JohanAdvanced Materials Research Laboratory, Department of Mechanical Engineering, University of Malaya, Lembah Pantai,50603 Kuala Lumpur, Malaysia

a r t i c l e i n f o

Article history:Received 29 December 2009Received in revised form1 October 2010Accepted 20 October 2010Available online 3 November 2010

Keywords:Nanosize Li2WO4Structural propertiesThermal propertiesConductivityMechanical milling

a b s t r a c t

Nanosize Li2WO4 is successfully synthesized using a mechanicalmilling method. The mean particle size of milled Li2WO4 is 37 nmand 31 nm as measured by Transmission Electron Microscopy(TEM) and X-ray diffraction (XRD) analysis, respectively. Thecalculated lattice parameter of as-received and milled powderare 0.8326 and 0.8321 nm, respectively. Both powders have beenhydrated at room temperature and are highly crystalline withthe presence of different types of phases at varying sinteringtemperatures. There is no Li2W2O7 (JCPDS 28-0598) phasepresent in the milled powder during the whole sintering process.Meanwhile, the thermogravimetric analysis (TGA) of both powdersis in accordance with their phase changes as presented in theXRD spectra. The impedance measurements show that the milledpowder has a lower conductivity than that of the as-receivedpowder in overall. The conductivity of the as-received powderincreases with sintering temperature but this phenomenon is viceversa for the milled powder. All these trends can be correlatedwith the Field Emission Scanning Electron Microscopy (FE-SEM)micrographs on both powders.

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

The studies of different phases of ceramic materials have already gained much attention due totheir potential electrical, mechanical and chemical properties [1,2]. These properties could be furtherenhanced if their structures were turned into the nanoscale. Since nanostructured materials have a

∗ Corresponding author. Tel.: +60 3 79676873; fax: +60 3 79675317.E-mail address: [email protected] (T.K. Han).

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18 T.K. Han et al. / Superlattices and Microstructures 49 (2011) 17–31

larger surface to volume ratio, their surfaces are highly energetic and reactive. Therefore, the sizereduction affects the role of the surfaces or interfaces during chemical adsorption.

Lithium Tungstate (Li2WO4) is among the ceramic materials with a polycrystalline structure andhygroscopic behavior in nature as it is strongly susceptible to atmospheric moisture. As it getshydrated, Li2WO4 possesses a completely different chemical behavior [3]. Anhydrous Li2WO4 is whiteand has a stable thermal property, with its melting point as high as 742 °C [4]. Although this materialis a poor ionic conductor at ambient temperature, it still has a potential application as a lithium ionconducting solid electrolyte due to its increasing electrical conductivity, which is contributed by ionicconduction at elevated temperature [5–7].

Bulk crystalline Li2WO4 has received much attention in recent years and its properties havebeen well studied [5–11]. However, this is not the case for nanocrystalline Li2WO4. In thiswork, the nanosize Li2WO4 is synthesized using a mechanical milling method. Phase, thermaland electrical conductivity studies on the nanosize Li2WO4 are conducted at a certain range ofsintering temperatures. The results were compared with their bulk crystalline Li2WO4 powder. Toour knowledge, this is the first study on nanosize Li2WO4 powder and this paper could give a clearpicture on some properties of nanosize Li2WO4.

2. Experimental setup

The as-received Li2WO4 powder (purity >99.9%, particle size ≈0.1 µm) was procured from AlfaAesar. Mechanical milling was performed by a planetary ball mill (Retsch-PM 400, Germany) for 10 hat 200 revolutions per minute (rpm) using zirconia balls as grinding media in a zirconia milling jar. Aball to powder ratio of 3:1 was applied in the milling process to produce nano-scale powder. The as-received andmilled powderswere thenmade into the cylindrical pellets 20mmand 3mm in diameterand height respectively. The powders were pelletized in a hydraulic press at 8 tons. The pelletizedsamples with a diameter of 2 cm were heated in the air at successively higher temperatures (400,450, 500, 550, 600, 650 and 700 °C) for 6 h.

A Transmission Electron Microscope (TEM) (LIBRA r⃝ 120, Germany) with 120 kV was used tomeasure the particle size of milled Li2WO4. Selected area electron diffraction (SAED) was alsoconducted to characterize the structure of milled Li2WO4. Surface morphology of crystalline solidswas observed by using a Field Emission Scanning Electron Microscope (FE-SEM), (AURIGA, ZEISS,Germany) with the electron beam generated by 3 kV. Phase analysis was also studied by using aPhillips X’pert MRD PW3040 X-ray Diffractometer with Cu Kα radiation of wavelength 1.54056 Å. Inorder to investigate any changes in crystallite size and internal strainswithin the structures of powder,the Scherrer and William–Hall methods were applied. The crystallite size of powder was determinedby using the Scherrer formula [12–14]:

S =Kλ

β cos θ(1)

where S is the crystallite size, K is a constant whose value is 0.9, λ is the X-ray wavelength andthe width peak β (in rad) was determined as full width at half-maximum (FWHM). Meanwhile, theresulted internal strain was calculated using the Williamson–Hall equation [13]:

β cos θ =Kλ

S+ 2ε sin θ (2)

where ε represents the internal strain. A graph of β cos θ against sin θ is plotted from Eq. (2) and theinternal strain is then obtained from the slope of the graph. Besides, the lattice parameter (ao) of crystalstructure that belonged to a phase was estimated using extrapolation technique by plotting latticeparameter, (a) against the Nelson–Riley function, (N–R) [15,16]. For a cubic system, the magnitudeof a could be calculated by using the value of interplanar spacing (dhkl) and Miller indices (h, k and l)obtained from the XRD data as reflected in Eq. (3) as shown below:

a2 = d2hkl(h2+ k2 + l2). (3)

T.K. Han et al. / Superlattices and Microstructures 49 (2011) 17–31 19

Rs (Ω) / Z cos θ

X (Ω

) / Z

sin

θ

Fig. 1. A typical Nyquist plot which is generated by IS.

The magnitude of N–R due to a crystal structure is calculated using Eq. (4) as shown below:

NR =

cos2 θ

sin θ

+

cos2 θ

ϑ

. (4)

Thermal gravimetric analysis (TGA/DTG) and simultaneous differential thermal analysis (SDTA) wereconducted using TGA/SDTA 851 Mettler Toledo under the flow of 50 ml/min of nitrogen at a heatingrate of 5 °C/min which was heated up to 750 °C. Impedance spectroscopy (IS) measurements wereperformed by using a complex impedance analyzer (HIOKI LCR Hi-Tester Model 3532-50, Japan)within the frequency range of 50Hz–1MHz’s. The impedance of a sample is the product of the complexresistivity and the length over area ratio. The sample should be preparedwith parallel faces and awell-defined cross section. Therefore, samples in pellet form are necessary in order to obtain consistent andreliable conductivity. The bulk resistance of a sample, Rb (�) can be then measured in such a way asshown in Fig. 1. By substituting Rb, thickness and cross section of a pelletized sample, t (cm) and A(cm2) respectively into Eq. (5), electrical conductivity, σ (S cm−1) of the sample is estimated.

σ =t

RbA. (5)

3. Results and discussion

3.1. Phase and crystal structure

Fig. 2 shows TEM image of the milled Li2WO4. The particles are agglomerated and vary in shapeand size. Fig. 3 indicates a narrow size distribution of the milled Li2WO4 and the mean particle size is37 nm.Mechanical ballmilling has successfully reduced the size of particles of as-received Li2WO4intothe nanoscale. The selected area electron diffraction (SAED) pattern with concentric rings is shown inFig. 2. The pattern confirmed the crystalline nature ofmilled Li2WO4. The SAED pattern can be indexedbased on the major phase of this powder, the face centered cubic phase (fcc) of 7Li2WO4·4H2O (JCPDS35-0826). The calculated cell lattice parameter is 0.8401 nm with three observable rings for (111),(211) and (300) diffractions.

Fig. 4 shows the XRD patterns of both as-received and milled Li2WO4 with well-desired peaksat room temperature. This clearly indicates that both samples are highly crystalline. The diffraction


Fig. 2. TEM image of the milled Li2WO4 at room temperature.

patterns of both samples are similar except for the peak broadening. The FWHM of the broadenedpeaks at the plane (300) where the peaks have the highest density are used to calculate the averagecrystallite size, according to Eq. (1). The average crystallite sizes for as-received andmilled samples are61 nm and 31 nm, respectively. Both Figs. 5 and 6 indicated theWilliamson–Hall plots are obtained byusing Eq. (2). The internal strain of the as-received sample is calculated as 0.0944%,which is lower thanthe calculated internal strain of the milled sample, 0.1862%. The milled Li2WO4 has a broader peak inits XRD pattern which indicates a smaller crystallite size with an increase in its internal strain [13,14].Due to the introduction of defects throughmechanical milling, the crystallite size decreases, while theinternal strain increases.

Fig. 7 indicates both samples have major peaks which are identified as face centered cubic phases(fcc) of 7Li2WO4·4H2O (JCPDS 35-0826). All these predominant peaks at respective 2θ angles withtheir corresponding planes are tabulated in Table 1. By applying the relevant data as tabulated inrespective Tables 2 and 3 into both Eqs. (3) and (4), a plot presented in Fig. 8 is formed and themagnitude of lattice parameter (ao) of 7Li2WO4·4H2O for the as-received and milled powder arecalculated as 0.8321 nm and 0.8326 nm, respectively. They are close to the lattice parameter of7Li2WO4·4H2O (JCPDS 35-0826), i.e. 0.8323 nm. During milling, the impact energy is subjected uponthe powder. Thus, a distortion of the lattice parameters of the milled powder is promoted by theintroduction of defects or lattice strain. This explains the peak broadeningwhich caused the reduction


Fig. 3. Particle size distribution of the milled Li2WO4 at room temperature.

Fig. 4. Comparison of XRD spectra of Li2WO4 at room temperature.

of crystallite size [13,14,17]. The lattice parameter of milled powder is thus slightly higher than theas-received one.

Some minor peaks are also observed at 21° and 30° for both powders in Fig. 7. These minor peaksare matched to the rhombohedral/phenacite phase of Li2WO4 (JCPDS 12-0760) as listed in Table 4.Interestingly, an additional peak is also observed at 25° but is limited to the milled powder. Bothpowders are supposedly anhydrous but hydrated to a certain extent due to the presence of a major7Li2WO4·4H2O phase. The anhydrous Li2WO4 powder is strongly susceptible to the atmosphericmoisture and easily hydrates with an inconsistent content of water [3,4,10].

Figs. 9 and 10 show the diffraction pattern for both powders at various sintering temperatures.Initially, peaks due to 7Li2WO4·4H2O are the major phase. With the increase of temperature, peaksattributable to Li2WO4 have increased and become the major phase while peaks attributable to7Li2WO4·4H2O have diminished gradually. Only peaks attributable to the Li2WO4 phase are observedat the highest sintering temperature of 700 °C. No peak appears for the 7Li2WO4·4H2O phasebeginning at 500 °C. All hydrated water content is removed at this temperature [7,9,10].


Table 1Major peaks in XRD spectra of Li2WO4 powder at room temperature and JCPDS No. 35-0826.

Miller indices (hkl) Angle 2θ (°)As-received powder Milled powder JCPDS No. 35-0826

100 10.72 10.79 10.60111 18.55 18.57 18.44210 24.00 24.07 23.88211 26.31 26.37 26.20300 32.34 32.39 32.24310 34.14 34.19 34.03222 – 37.57 37.40400 – 43.69 43.45322 44.96 45.05 44.87330 – 46.34 46.24331 47.70 47.71 47.58420 49.00 49.07 48.90511 – 57.65 57.47

Fig. 5. Williamson–Hall plot for the as-received Li2WO4 .

Fig. 6. Williamson–Hall plot for the milled Li2WO4 .

There is no peak attributable to the Li2W2O7 (JCPDS 28-0598) phase in the complete spectraof milled powder, which is different from previous studies [9,10]. As the temperature increases,7Li2WO4·4H2O is gradually decomposed to Li2WO4 and water according to the following equation:

7Li2WO4·4H2O → 7Li2WO4 + 4H2O. (6)

The water is then dehydrated with increasing heat. Since the crystallite size of the milled sampleis reduced into nanometer region due to milling effect, their surface to volume ratio is increasedtremendously. They have high surface energy and their surface is highly reactive. Since the nature


Fig. 7. Detailed XRD spectra of Li2WO4 at room temperature.

Fig. 8. Lattice parameter of major phase for the as-received and milled Li2WO4 .

of reaction is chemical surface adsorption, the milled powder is completely decomposed into Li2WO4and water. As a result, no Li2W2O7 peak is present.

For as-received powder, there are some relatively small new peaks attributed to Li2W2O7. Thisphase starts to appear at 400 °C and disappears above 650 °C. According to Eq. (7), 7Li2WO4·4H2O


Fig. 9. XRD spectra of the as-received Li2WO4 at various sintering temperatures.

Table 2XRD data for 7Li2WO4·4H2O phase in the as-received powder at room temperature.

Miller indices (hkl) Angle 2θ (°) Angle θ (°) d-spacing (Å) NR Lattice parameter, a (Å)

1 0 0 10.72392 5.36196 8.24293 21.1914 8.24291 1 1 18.55132 9.27566 4.77887 12.0543 8.27722 1 0 23.99788 11.99894 3.70517 9.1670 8.28502 1 1 26.30746 13.15373 3.38489 8.2934 8.29133 0 0 32.33687 16.16844 2.76619 6.5785 8.29863 1 0 34.14372 17.07186 2.62383 6.1768 8.29733 2 2 44.96230 22.48115 2.01444 4.4065 8.30573 3 1 47.70194 23.85097 1.90494 4.0760 8.30344 2 0 49.00143 24.50072 1.85742 3.9309 8.3066

is gradually decomposed to Li2W2O7, Li2WO4 and LiOH·H2O as the temperature increases [10]. BothLi2W2O7 and LiOH·H2O are subsequently decomposed to Li2WO4 and water. However, no LiOH·H2Ophase is detected in XRD spectra due to its insignificant intensity (Fig. 9).

7Li2WO4·4H2O → 5Li2WO4 + Li2W2O7 + 2LiOH.H2O + H2O. (7)

Water is still present in the lattice at high temperature due to the strong affinity of Li2WO4 towardswater. The nature of the interaction is surface adsorption and the action of heat can regenerate theanhydrous form. The moisture content is considerable and heating leads to the formation of newcomponents such as Li2W2O7 and LiOH·H2O [10].


Fig. 10. XRD spectra of the milled Li2WO4 at various sintering temperatures.

Table 3XRD data for 7Li2WO4·4H2O phase in the milled powder at room temperature.

Miller indices (hkl) Angle 2θ (°) Angle θ (°) d-spacing (Å) NR Lattice parameter, a (Å)

1 0 0 10.79 5.3950 8.19118 21.0595 8.19121 1 1 18.57 9.2850 4.76471 12.0416 8.25272 1 0 24.07 12.0350 3.69452 9.1372 8.26122 1 1 26.37 13.1850 3.37937 8.2718 8.27773 0 0 32.39 16.1950 2.76008 6.5661 8.28023 1 0 34.19 17.0950 2.62080 6.1670 8.28772 2 2 37.57 18.7850 2.39357 5.5145 8.29164 0 0 43.69 21.8450 2.07189 4.5727 8.28763 2 2 45.05 22.5250 2.01444 4.3953 8.30573 3 0 46.34 23.1700 1.95363 4.2359 8.28863 3 1 47.71 23.8550 1.90494 4.0750 8.30344 2 0 49.07 24.5350 1.85742 3.9234 8.30665 1 1 57.65 28.8250 1.59773 3.1157 8.3020

Table 4Minor peaks in XRD spectra of Li2WO4 powder at room temperature and JCPDS No. 12-0760.

Miller indices (hkl) Angle 2θ (°)As-received powder Milled powder JCPDS No. 12-0760

211 21.08 21.17 20.98300 21.51 24.93 21.39113 30.46 30.69 30.52

Fig. 11 shows the FE-SEM micrographs of the as-received Li2WO4 pellets sintered at varioustemperatures for 6 h. A porous and homogenous microstructure with narrow grain size distributionwas observed in the as-received samples (Fig. 11(a)). The increase of sintering temperature


Fig. 11. FE-SEM micrographs of the as-received Li2WO4 pellets sintered at: (a) room temperature (b) 500 °C (c) 550 °C (d)600 °C (e) 650 °C (f) 700 °C.

significantly promoted the grain growth and microstructural densification (Fig. 11(b)–(c)). Thenumber of pores was reduced at a higher sintering temperature as a result of which the individualgrains got closer to each other and the effective area of grain contact increased. This, in turn, resultedin greater densification or less porosity. The grain boundaries of the sintered samples became clearalong which the grains grew perfectly straight. Prominent features observed in this case are presenceofwell-defined grains and less porosity giving impression of grain growth. Comparedwith the gradualincrease of grain size with sintering temperature in the range of 500–550 °C, there is a facilitatedgrain growth in the samples sintered at 600–700 °C, respectively, implying a considerable increaseLi2W2O7 along with Li2WO4 (Fig. 11(d)–(f)). The presence of this secondary phase will lead to thereduction of porosity after 550 °C. As mentioned before, the presence of liquid (water) due to thedecomposition of Li2W2O7 during sintering can enhance the mass transport and microstructuraldensification. Therefore, the liquid phase sintering causes the reduction of porosity. This enhancedthe electrical conductivity in the as-received samples [18].

Fig. 12 shows the FE-SEM micrographs of the milled Li2WO4 pellets sintered at various temper-atures for 6 h. Overall, the morphology of the milled samples at all sintering temperatures is al-most similar. As compared to Fig. 11, the milled samples are much finer. This is consequence of the


Fig. 12. FE-SEM micrographs of the milled Li2WO4 pellets sintered at: (a) room temperature (b) 500 °C (c) 550 °C (d) 600 °C(e) 650 °C (f) 700 °C.

fragmentation of the large composite particles due to themilling process. A porous and heterogeneousmicrostructure was observed in the milled samples (Fig. 12(a)–(d)). The increase of sintering temper-ature also promoted the microstructural densification. However, the grain growth phenomenon wasnot noticeable at higher sintering temperatures (Fig. 12(e)–(f)). Compared visually with the increaseof grain size with sintering temperature in the as-received samples, there is a reduced grain size inthe milled samples, implying a decrease in the grain structure of milled samples.

3.2. Thermal analysis

Figs. 13 and 14 show that the TGA and DTG curves of both samples indicate that dehydration ofwater and decomposition process has occurred [7,10]. Two mass losses for as-received powder are


30 40

200 250 300 350 400 450 500 550 600 650 700

50 60 70 80 90 100 110 120 130 min

98.2

98.4

98.6

98.8

99.0

99.2

99.4

99.699.8

100.00%

Fig. 13. TGA, DTG and SDTA curves of the as-received Li2WO4 .

30 40

200 250 300 350 400 450 500 550 600 650 700

50 60 70 80 90 100 110 120 130 min

98.2

98.4

98.6

98.8

99.0

99.2

99.4

99.6

99.8

100.00%

Fig. 14. TGA, DTG and SDTA curves of the milled Li2WO4 .

observed in Fig. 13, inwhich themajor onehas occurredbetween500 and550 °Cdue towater removal.This is supported by the XRD spectra in Fig. 9 which indicate all peaks due to the 7Li2WO4·4H2O phasehave disappeared within the temperature range of 450–500 °C. Another minor mass loss in both TGAand DTG curves at 400 °C as shown in Fig. 13 is due to the formation of Li2W2O7 and LiOH·H2O phases,according to Eq. (7) mentioned in Section 3.1.

For the milled powder, the dehydration of water occurs at the beginning of heating, as shown bythe TGA curve in Fig. 14, whereby a continuous mass loss is observed. However, the distinct mass lossoccurs at 430–480 °C, which is presented by the DTG curve. This is in accordance with the phases thatare present in Fig. 10 which indicates the gradual decomposition of 7Li2WO4·4H2O into the Li2WO4and water. The water is dehydrated with the increase of heat [7].

Meanwhile, for both types of powder, a strong endothermic peak is also present in the respectiveSDTA curve at 740 °C, suggesting a melting point (Tm) for Li2WO4, which is close to the theoretical Tm,742 °C [9,19].

3.3. Complex impedance measurements

The complex impedance spectra (Nyquist plots) of both Li2WO4 samples at the various sinteringtemperatures are presented in Figs. 15 and 16. The plots show the presence of spikes and asymmetricsemicircles at respective lower and higher frequencies. The spike can be attributed to the grain


Rs (Ω) / Z cos θ

X (Ω

) / Z

sin

θ

0.00E+000.00E+00

1.00E+05

1.00E+05

2.00E+05

2.00E+05

3.00E+05

3.00E+05

4.00E+05

4.00E+05

5.00E+05

5.00E+05

6.00E+05

6.00E+05

7.00E+05

7.00E+05

8.00E+05

Fig. 15. Impedance spectra of the as-received Li2WO4 pellets at various sintering temperatures.

Rs (Ω) / Z cos θ

X (Ω

) / Z

sin

θ

0.0E+000.00E+00

5.0E+05

1.0E+06

1.00E+06

1.5E+06

2.0E+06

2.00E+06

2.5E+06

3.0E+06

3.00E+06 4.00E+06 5.00E+06 6.00E+06

Fig. 16. Impedance spectra of the milled Li2WO4 pellets at various sintering temperatures.

boundary effect, while the semicircles may be associated by the resistance of bulk (grain) of thestudied material [20,21]. The bulk resistance and electrical conductivity of both types of samples arecalculated by using Eq. (5) presented in Table 5. In comparison, the as-received samples have a lowerbulk resistance and thus have a higher electrical conductivity than that of the milled samples. This isdue to the grain boundary effectwithin themicrostructures. Since themilled sample is a nanostructurematerial, it has high densities of grain boundaries. The grain boundaries act as barriers towardsmobileions. The higher the amount of grain boundary present, the more difficult it is for the mobile ions tocross over them [20].

Besides, the reduced grain structure in the milled samples increases the volume fraction of grainboundaries, which have a highly disordered structure. This structural disorder leads to an increase oftemperature independent contribution to the resistivity [22]. The resistivity increaseswith decreasinggrain size. This excess resistivity is directly proportional to the volume fraction of atoms in grainboundary and is associated with grain boundary scattering. The increased grain boundaries in themilled samples have the net effect of increasing impedance.

Sintering treatment is able to cause grain growth and microstructural densification to both as-received and milled samples. However, the secondary phase of Li2W2O7 which is only present in the


Table 5Bulk resistance and conductivity of Li2WO4 .

As-received Li2WO4 powder

Sintering temperature, T (°C) T (K) Bulk resistance, Rb (�) Conductivity, σ (S cm−1)

550 823 1.73 × 106 5.94 × 10−8

600 873 7.54 × 105 1.39 × 10−7

650 923 4.22 × 105 2.60 × 10−7

Milled Li2WO4 powder

Sintering temperature, T (°C) T (K) Bulk resistance, Rb (�) Conductivity, σ (S cm−1)

550 823 2.47 × 106 3.94 × 10−8

600 873 2.86 × 106 3.37 × 10−8

650 923 6.20 × 106 1.64 × 10−8

sintered pellets for the as-received samples gives a larger reduction of porosity due to liquid phasesintering. This gives the as-received samples a higher electrical conductivity [18].

The diameter of the semi-circle for the as-received samples diminishes with the temperature(Fig. 15). The bulk resistance for the as-received samples decreases with temperature, and thusimproves its conductivity [23]. Sintering drives the grains to grow larger and reduces porosity withinthe microstructures. Reduction of porosity is also assisted by the presence of secondary phase thatcauses liquid phase sintering. Therefore, the increase in conductivity with increasing temperature isthe result of the increase in ionic mobility due to the grain growth and the reduction of porosity.

In contrast, the conductivity of milled samples reduces with temperature since its bulk resistanceincreases (Table 5), due to the gradual increase in diameter of semi-circle as shown in Fig. 16. In themilled samples, the sintering process may lead to formation of narrower conducting pathways in thisnanostructure, causing a decrease in ion mobility. The greater grain boundary resistance within thisnanosize material also contributes to this phenomenon.

4. Conclusion

Nanosize Li2WO4 (milled powder) has been successfully synthesized via mechanical ball milling.The average particle size is 37 nm, as measured by TEM. The crystallite size of milled powder is 31 nmand is much smaller than that of the as-received powder, which is 61 nm. The introduction of defectsduring the milling process causes the lattice parameter of milled powder (0.8326 nm) to be slightlyhigher than that of as-received powder (0.8321 nm). Both powders are highly crystalline and havebeen hydrated at room temperature by exhibiting dominant peaks attributable to the 7Li2WO4·4H2O(JCPDS 35-0826) phase in XRD spectra. The Li2WO4 (JCPDS 12-0760) phase is the minor phase. Withincreasing temperature, Li2WO4 (JCPDS 12-0760) becomes the major phase. No 7Li2WO4·4H2O phaseis present at a temperature beginning at 500 °C due to decomposition process and complete waterdehydration. It is further confirmed by the observation of a mass loss in the TGA and DTG curvesof both powders. However, no Li2W2O7 (JCPDS 28-0598) phase appeared in the milled powder butis present in the as-received powder throughout the whole sintering treatment. The milled powder(nanosize) has an overall lower conductivity than its bulk powder due to the grain boundary effect,disorder structure of grain boundary, and absence of a secondary phase that contributes to liquidphase sintering. The incidents of grain growth and greater reduction of porosity due to liquid phasesintering make the conductivity of the as-received powder increase with the sintering temperature.The conductivity of the milled powder decreases with sintering temperature. This is probably dueto the formation of narrower conducting pathways in this nanostructure, causing a decrease in ionmobility.

Acknowledgements

The financial support received from Science fund (13-02-03-3068), Ministry of Science and Inno-vation Malaysia is gratefully acknowledged.


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phase, thermal and impedance studies of nanosize via mechanical milling and sintering

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