LZS/Al2O3 Glass-Ceramic Composites Sintered by Fast Firing
LZS/Al2O3 Glass-Ceramic Composites Sintered by Fast Firing
Sabrina Arcaroa b *
Maria Isabel Nietoc
Maria Dolores Salvadord
Antonio Pedro Novaes de Oliveiraa b
aLaboratory of Glass-Ceramic Materials - VITROCER, Federal University of Santa Catarina - UFSC, 88040-900, Florianópolis, SC, Brazil
bGraduate Program in Materials Science and Engineering - PGMAT, Federal University of Santa Catarina - UFSC, 88040-900, Florianópolis, SC, Brazil
c Institute of Ceramics & Glass - CSIC, Madrid, Spain.
d Institute of Materials Technology, Polytechnic University of Valencia, Spain
In this work, nanometric Al2O3 (1-5 vol.%) particles (13 nm, 100 m2/g) were added to a 19.58Li2O•11.10ZrO2•69.32SiO2 (mol%) (3.5 µm, 2.5 m2/g) parent glass-ceramic matrix to prepare composites with the purpose of studying the influence of Al2O3 on their structure, microstructure, mechanical, thermal and electrical properties when sintered by fast firing. The parent glass-ceramic was prepared by melting and fast cooling (in water) to obtain a glass frit. The resulting glass frit was milled according to a two-step procedure consisting on a dry milling stage followed by a long wet milling step down. Each composition was wet homogenized and then dried at 110 ºC for 48 h for disaggregation. The obtained powders were uniaxially pressed (100 MPa) and compacts sintered by fast firing (175 ºC/min) between 800 and 900 ºC for 30 min. The composites, with relative densities ranging from 89% to 93%, showed zircon and β-spodumene as main crystalline phases. The hardness and Young's modulus varied from 4.5 to 6.5 GPa, and from 65 to 102 GPa, respectively. The formation of β-spodumene in the obtained composites leads to reduce the CTEs, whose values ranged from 13 to 7 x 10-6 ºC-1.
Keywords: Alumina nanoparticles; Fast firing sintering; Crystallization; Glass-ceramics
The constant advances in the industry require the permanent development of new and efficient solutions for many different applications. In fact, in the last 20 years, the emergence of highly complex structures derived from the junction of several classes of materials. Many efforts have been devoted to improve the mechanical, thermal, electrical and chemical behavior of ceramic materials, exploiting different strategies, such as the study of new compositions, use of reinforcements with particles, fibers, and the use of nanometric secondary phases for ceramic matrix composites and nanocomposites1-5.
The LZS (Li2O-ZrO2-SiO2) glass-ceramic system has been investigated since 19966 due to its interesting properties, particularly from the point of view of its mechanical strength and its hardness and relatively high resistance to abrasion and chemical attack. In fact, relatively recent studies7,8 have demonstrated that LZS glass-ceramics containing lithium and zirconium silicates (Li2Si2O5 and ZrSiO4) as main crystalline phases achieved hardness of 8 ± 0.5 GPa, bending strength of 190 ± 13 MPa and fracture toughness of 3.65 ± 0.20 MPa.m1/2. However, most of the LZS glass-ceramic compositions have a relatively high coefficient of thermal expansion, CTE (8.8 - 10 x 10-6 ºC-1) which constitutes a limitation in some applications. Thus, the production of such LZS glass-ceramics with their inherent properties but with controlled CTE is of practical interest. Some studies9-14 have been conducted in order to determine the influence of alumina additions in a LZS glass-ceramic matrix, particularly with the purpose of reducing the CTE.
Materials that have low CTE are being developed for applications requiring rapid temperature changes such as heat exchangers, ceramics for use in household cookers, high performance cutting tools and precision optical instruments as well as burner nozzles. Furthermore, in applications involving the joining of materials, the thermal expansion requires a fairly narrow compatibility to match the shrinkage, as in glass and glass-ceramic/metal systems for hermetic seals, laser tubes, electronic devices for measuring and monitoring, sealants for solid oxide fuel cells (SOFCs), and substrates used in microelectronic packaging on LTCC technology (low temperature co-fired ceramics)15-19.
In previous works20,21 it was demonstrated that the addition of nanosized alumina in a LZS glass-ceramic matrix, produced by conventional sintering, was able to reduce the CTE significantly. This happens because of the alumina affinity with respect to lithium silicates to form β-spodumene (LiAlSi2O6), a crystalline phase having a CTE nearly zero (0.9 x 10-6 oC-1). In this case, the CTE changed from 9.5 x 10-6 ºC-1 for the LZS glass-ceramic to 4.4 x 10-6 ºC-1 for a 5 vol.% nanoparticulate alumina LZS glass-ceramic matrix composite.
In this context, this study aims to evaluate the possibility of designing and obtaining of LZS glass-ceramic composites with different nanometric alumina contents sintered and crystallized by fast firing with the aim of generating thermal energy in the furnace at high speed and transmit it to the pieces surfaces to obtain a product of acceptable quality and economically viable. Thus, the influence of the nanometric alumina additions on the structure, microstructure, mechanical and electrical properties and coefficient of thermal expansion, will be evaluated.
2. Experimental Procedure
In this work the following raw materials were used: a LZS glass (parent glass-ceramic powder) with an average particle size dv.50=3.5 µm, composition 19.58Li2O·11.10ZrO2·69.32SiO2 (mol%), and a commercially available Al2O3 nanopowder (Aeroxide® AluC, Evonik-Degussa, Germany) with an average particle size dv.50=13 nm, a specific surface area of 100 m2/g and made up of a mixture of δ/γ-alumina phase.
Batches to produce the parent glass were prepared from well-mixed powders containing appropriate amounts of Li2CO3 (Synth, purity 99%), ZrSiO4 (Colorminas, purity 99%) and SiO2 (Colorminas, purity 99%) as raw materials. Subsequently, each batch was placed in a Pt crucible (100 mL) and melted at 1550 ºC for 2 h in a high temperature bottom loading furnace (Jung, CPM45, Brazil). The melts were cast into deionized water to provide frits for milling. The resulting glass frit was milled according to a two-step procedure consisting on a dry milling stage followed by a long wet milling step down to an average particle size of dv.50=3.5 µm and surface area of 2.5 m2/g. More details of the milling procedure are available elsewhere21.
The chemical composition of the milled powder (LZS parent glass) was determined by X-ray fluorescence (Philips, PW 2400, The Netherlands) and atomic absorption spectroscopy (Unican, 969, United Kingdom). Further details on the preparation and processing of the LZS frit are reported in a previous work20,21.
The differently prepared compositions were labeled as LZS, 1An, 2.5An, and 5An, for contents of nanosized alumina of 0, 1, 2.5, and 5 vol.%. Each composition was wet homogenized (using a water to powder weight ratio of 60/40) on a propeller mechanical agitator (IKA RW Digital 200, Germany). Subsequently, the suspensions of the formulated compositions were dried at 110 ºC for 48 h and then disaggregated into a fast laboratory ball mill (SERVITECH, CT-242, Brazil) for 15 min. Thus, samples of these compositions were uniaxially pressed in a cylindrical steel die by means of a hydraulic press (ST Bovenau P10, Brazil) at 100 MPa. The obtained samples (10 x 6mm) were fired (for sintering and crystallization) in a Bottom Loading Furnace (Energon S.L., Spain) with a heating rate of 175 ºC/min (named fast firing), at 800 and 900 ºC for 30 min. The samples were then immediately taken off and cooled to room temperature with cold air. The firing temperatures were selected based on previous work20,21. These works use traditional burning processes. In this new work, the objective is to verify how these composites behave when burned by a rapid burning process. The true densities (ρt) of powdered samples were determined by using a helium pycnometer (AccuPyc 1340, Micromeritics, USA). The apparent densities (ρa) of fired samples were determined by relating their geometrical measurements, obtained using a caliper (Mitutoyo, Japan, accuracy ± 0.01 mm), and their masses (Shimadzu AX200, Japan, at 0.001 g). The relative densities (ρr) were determined relating the apparent densities and the true densities of the samples according to Equation 1.
To determine the evolution of crystalline phases in studied samples, X ray diffraction was performed using a powder XRD (Philips, model X'Pert, The Netherlands) diffractometer using Ni-filtered Cu-Kα radiation (1.5418 Å) at 40 kV and 30 mA. Samples of the composites were rotated to minimize the effect of preferential orientation and analyzed in powder form with particle size smaller than 45 µm, using a step size of 0.02, dwell time of 2 s per step and 2θ between 5 and 80º 2θ angle range. JCPDS data banks were used for identification of the resulting crystalline phases. The quantitative analysis of the crystalline phases has been performed by the Rietveld method22,23. The refinement of the X-ray patterns as well as the simulation and quantification of the crystalline phases were performed by the X'Pert HighScore Plus® software (Philips, The Netherlands).
The microstructure of the fired samples was observed on fracture surfaces using a field emission gun scanning electron microscope (FE-SEM-S-4700 type I, Hitachi, Japan). Pore size of the composites were determined from counting corresponding to the specified pore diameters range, based on the linear intercept method24, where the ratio between the average length string (t) and average sphere diameter (D) is given by Equation 1 to better represent the measurement of a 3D unit (pore) by an 2D image.
In this case, five images of the fracture surfaces of obtained were used and 200 measurements in each image (in average) were made, with aid of a software (ImageJ®).
Mechanical properties were evaluated by micro and nanoindentation techniques. Hardness (H) and Young's modulus (E) were measured with a nanoindenter G-200 of Agilent Technol. (Inc., Santa Clara, CA) under a 2000 nm constant indentation depth program. A Berkovich tip was used after calibration of the function area in fused silica sample. Stiffness was recorded in depth by Continuous Stiffness Measurement (CSM). The oscillation amplitude was programmed to 2 nm with a frequency of 45 Hz. Sixteen indentations with 100 µm spacing between them were arranged in a matrix (4 x 4) for all samples analyzed.
Two-point probe electrical conductivity measurements have been performed in the thermal treated samples (disks with 6.5 mm in diameter). The electrical contact was made by sandwiching the disk shaped samples with Pt foils using coils to ensure good contacts. The sandwiched sample was inserted into the furnace and the electrical resistance was measured in the range of 300-900 ºC in air atmosphere by impedance measurements in the range of 1 MHz - 10 Hz (Agilent 4294A) with an amplitude of 50 mV. It is noteworthy that for temperatures below 300 ºC it has not been possible to obtain the impedance curve with precision, because the electrical conductivity is very low in relation to the effects of the electrode.
The acquired impedance spectra were analyzed using the ZVIEW® fitting software. The activation energy and electrical conductivity at room temperature were obtained from an Arrhenius plot25,26. According to the Arrhenius equation the relation between ionic conductivity in solids and the temperature T is given by Equation 2:
where EA is the activation energy for the electrical conduction processes26.
The coefficient of thermal expansion (CTE) of the composites was measured linearly from the thermal expansion curve, and was determined using a contact dilatometer (Netzsch Gerätebau model 402 EP, Germany) at a heating rate of 5 ºC/min in the temperature range between 25 and 500 ºC.
3. Results and Discussion
Table 1 shows the theoretical chemical composition and experimental values obtained by chemical analysis of the LZS parent glass powder.
Table 1 Chemical composition of the LZS parent glass.
Constituent oxides Composition (wt%) Theoretical6 Analyzed Al2O3 --- 0.95 CaO --- 0.05 Fe2O3 --- 0.05 Li2O 9.6 8.63 Na2O --- 0.05 SiO2 68.1 68.02 TiO2 --- 0.05 ZrO2 22.4 22.20
As can be seen, there is a small difference between the theoretical chemical composition and experimental values obtained by chemical analysis of the LZS parent glass powder. In fact, there is a small decrease in the percentage related to the lithium oxide, probably due to some evaporation of Li during melting. The detection of aluminum oxide can be associated to the contamination produced by the wear of the alumina balls used for milling. In spite of this alumina contamination it, apparently, did not affect the expected performance of the obtained LZS glass-ceramics.
Table 2 shows the relative density for LZS glass-ceramic samples (without Al2O3) and for the composites containing 1, 2.5 and 5 vol.% of nanosized Al2O3 fired by fast firing at 800 ºC and 900 ºC for 30 min. It can be seen that the relative densities of the samples are between 89 and ~ 93 %. From the data collected one can realizes that for the two studied fast firing temperatures, the relative density of the sintered samples slight decreases with increasing amount of added alumina, but less than it could be expected. This behavior can be explained due to the presence of nanosized alumina which is more reactive than the LZS glass matrix, reducing the viscous flow during sintering step, and retarding densification. This retard is related to the crystallisation of spodumene crystalline phase, which greatly reduces the sintering rate. Viscous flow reduction may in this case be by mechanical impediment (thermal stability of the alumina) and/or by dissolution of the latter in the glass forming crystalline phases (such as lithium aluminum silicates) more viscous. In fact, some works in the literature27,28 support these possibilities since there is a decrease in the relative density of glass samples containing different amounts of alumina particles. It is also verified that the effect intensifies as the particle size decreases27. In this context, according to28 a strong coupling between alkali ions in borosilicate glass and Al+3 in alumina, forms an Al+3 and alkali ion-rich reaction layer around alumina particles. Since the above reaction continuously depletes the concentration of alkali ions in the glass during sintering, it causes a rise in viscosity of the glass and thus slows down the densification kinetics and increases the activation energy of densification.
Table 2 Relative densities of samples sintered at 800 ºC and 900 ºC for 30 min.
Compositions Sintered density (%TD) 800 ºC 900 ºC LZS GLASS-CERAMIC 93 ± 0.2 94 ± 0.5 LZS-1An 90 ± 0.5 92 ± 0.5 LZS-2.5An 90 ± 0.5 93 ± 0.5 LZS-5An 89 ± 0.5 90 ± 0.5
Table 2 shows that the relative densities of samples fired (sintered) at 900 ºC are slightly higher than those of samples fired at 800 ºC for 30 min. This behavior can be related to the differences in the resulting microstructures for composites fired by fast firing at 800 ºC (a) and 900 ºC (b) as shown in Figure 1.
Figure 1 SEM micrographs of fracture surfaces of fast fired LZS. (a) 800 ºC, and (b) 900 ºC for 30 min.
It can be seen by analysis of Figure 1 that samples fired at 800 ºC did not readily sinter to high density. At 900 ºC, the particles form a single homogeneous body, although some porosity is still remaining, which was expected for such glass-ceramics produced from powders in accordance with the obtained density measurements. The microstructural difference enables the selection of composites with enhanced performance for further evaluation of the behavioral properties. Thus, selected composites were sintered by fast firing at 900 ºC. Figure 2 shows SEM micrographs of fracture surfaces of the LZS glass-ceramic (a) without and with different alumina additions after fast firing at 900 ºC for 30 min (b, c, d).
Figure 2 SEM micrographs of fracture surfaces of the LZS glass-ceramic without (a), with 1, 2.5, and 5 vol.% (b, c, and d, respectively) alumina additions after fast firing at 900 ºC for 30 min and LZS glass-ceramic (e) and LZS with 5 vol% alumina additions (f) for a high magnification.
The micrographs confirm the information obtained from the relative density versus firing temperature curves, i.e., the porosity is very similar in all cases, although there is a small increase as the alumina content increases from 1% to 5%. However, it must be noted that the pore size decreases with increasing amount of alumina added to the LZS glass-ceramic. Figures (e) and (f) are at higher magnification to better demonstrate pore sizes. It was possible to calculate the mean pore diameter size from the measurements performed by the intercept method. For the LZS sample an average value of 3.50 ± 0.5 µm was obtained. For the 5An sample, a value of 2.75 ± 0.6 µm was obtained.
Figure 3 shows the obtained X-ray diffraction patterns of the LZS parent glass (a), LZS glass-ceramic sintered by fast fired at 800ºC (b), LZS glass-ceramic sintered by fast fired at 900 ºC (c) and LZS with different alumina additions (d, e, and f) fast fired at 900 ºC for 30 min. The X-ray diffractogram shown in Figure 3 (a), exhibits a band at about 23º which is characteristic of amorphous phase, i.e., the LZS parent glass-ceramic. In addition, it is possible to observe that the crystallization phenomenon practically did not happen for the LZS glass ceramic sintered at 800 ºC by fast firing, i.e. it is possible to observe the band at about 23º. This fact, as well the density and microstructure, supported in the selection of the samples at 900 º C for further work. It was verified from the data collected from the XRD patterns related to the LZS glass-ceramic (c) that zirconium silicate, ZrSiO4, lithium disilicate, Li2Si2O5 and β-quartz are the crystalline phases formed. It is observed also that the addition of Al2O3 (d, e, and f) promotes the formation of β-spodumene, LiAlSi2O6 and lithium metasilicate, Li2SiO3 phases.
Figure 3 X-ray diffraction patterns of the LZS parent glass (a), LZS glass-ceramic sintered by fast fired at 800 ºC (b), LZS glass-ceramic sintered by fast fired at 900 ºC (c), and LZS with different alumina additions (c, d, and e) fast fired at 900 ºC/30 min. (Z) zirconium silicate, ZrSiO4; (D) lithium disilicate, Li2Si2O5; (Q) β-quartz; (S) β-spodumene, LiAlSi2O6 and (L) lithium metasilicate, Li2SiO3.
Table 3 shows the relative amounts of crystalline phases determined by Rietveld refinement of the LZS glass-ceramic samples (without Al2O3) and of the LZS/nano Al2O3 composites containing 1, 2.5 and 5 vol.% of nanosized Al2O3 fast fired at 900 ºC for 30 min. It can be noticed that the LZS glass-ceramic has 35.8% of zirconium silicate, 44.7% of lithium disilicate and 19.5% of β-quartz. Note that when adding alumina in the LZS glass-ceramic composition, there is a gradual decrease of lithium disilicate and the formation of β-spodumene phase, so that lithium metasilicate formation is promoted. This is due to the presence of alumina, which has high affinity for lithium and they easily react to form β-spodumene. Since the molar ratio between alumina and lithium is not stoichiometric the β-spodumene phase is accomplished by the formation of lithium disilicate and lithium metasilicate. For the composition containing only 1 vol.% of Al2O3, the formation of β-spodumene determinated by Rietveld method to occur to an extent of 16.1%. For compositions containing 2.5 and 5 vol.% nano alumina, the determined β-spodumene contents are 19.3 and 28.4%. This crystalline phase is very interesting from the point of view of properties that can enhance the glass-ceramic behavior, mainly due to its low coefficient of thermal expansion12-14,29. No crystalline phases based on alumina could be detected by XRD.
Table 3 Relative amounts of crystalline phases(Rietveld refinement)for samples of the LZS glass-ceramic without and with different alumina additions fired by fast fired at 900 ºC/30 min.
Crystalline phase Chemical formula Relative amounts of crystalline phases (wt%) LZS glass-ceramic LZS -1An LZS - 2.5An LZS - 5An Zirconium silicate (ICSD 100248) ZrSiO4 35.8 31.5 30.8 29.5 Lithium disilicate (ICSD 15414) Li2Si2O5 44.7 31.5 20.4 15 β-Quartz (ICSD 64980) SiO2 19.5 16.6 19.7 13.4 β-spodumene (ICSD 14235) LiAlSi2O6 - 16.1 19.3 28.4 Lithium metasilicate (ICSD 28192) Li2SiO3 - 4.3 9.9 13.7 GOF: Godnness of fit 1.84 1.73 1.48 1.87
Table 4 shows the values of Vickers microhardness and Young's modulus, for LZS glass-ceramic samples (without Al2O3) and for the composites containing 1, 2.5 and 5 vol.% of nanosized Al2O3 fired by fast firing at 800 ºC and 900 ºC for 30 min. It can be seen that both values of microhardness and Young's modulus for composites sintered at 900 ºC are higher than those for samples sintered at 800 ºC, as expected. According to that seen in Figure 1, the later has a high porosity, open microstructure and heterogeneous particles that still retain their identity, thus explaining the lower values of hardness and Young modulus. However, good results of Vickers microhardness were obtained for samples fired at 900 ºC for 30 min with values ranging from 4.9 to 6.5 GPa. These good results can be associated with the presence of zirconium silicate crystals which have hardness between 9 and 10 GPa24. It is possible to observe that there was a slight decrease in Vickers microhardness values with the addition of Al2O3. This fact can be explained for although the amount of the β-spodumene crystalline phase which has a hardness value lower than that of zircon29-31. Furthermore, considering the increase of porosity with an increase of the addition Al2O3, is possible residual porosity exerts a negative influence on the mechanical properties of the sintered ceramics32-34.
Table 4 Mechanical properties of samples fast fired at 800 and 900 ºC for 30 min.
Compositions Mechanicalproperties Vickers Young’s Microhardness, HV (GPa) Modulus, E (GPa) 800 ºC 900ºC 800ºC 900ºC LZS GLASS-CERAMIC 6 ± 0.3 6.5 ± 0.4 92 ± 0.5 102 ± 0.5 LZS-1AN 5.7 ± 0.4 6.4 ± 0.4 90 ± 0.5 98 ± 0.5 LZS-2.5AN 5.7 ± 0.4 5.0 ± 0.2 90 ± 0.4 79 ± 0.3 LZS-5AN 4.3 ± 0.2 4.9 ± 0.1 65 ± 0.3 72 ± 0.2
The Young's modulus (E), have a fluctuation with a slight decrease with increasing amount of added alumina ranging from 102 GPa to 72 GPa for the LZS composition without and with 5% Al2O3, respectively. This fact could be associated mainly with the increased porosity with the addition of alumina since it is well known that the Young's modulus decreases exponentially with the porosity32-34.
The electrical conductivity as a function of the temperature was investigated using two-point probe measurement, getting the resistance (Z '= R when Z' = 0 in the impedance diagram, the side of low frequencies), with the aid of Zview® software so that an Arrhenius plot (Figure 4) can be drawn from which the activation energy for the conduction process can be calculated.
Figure 4 Arrhenius plot (Log conductivity versus the inverse of temperature) for samples of the LZS glass-ceramic (without Al2O3) and for the composites containing 1, 2.5 and 5 vol.% of nanosized Al2O3 sintered by fast firing at 900 ºC for 30 min.
In all composites the electrical conductivity increases with temperature showing a semiconductor behaviour, from values around 10-6 S.cm-1 at 300 ºC to values around 10-2 S.cm-1 at 900 ºC, as it could be expected. In general, the Arrhenius plots indicate that for the whole temperature range tested the electrical conductivity remains higher with the addition of alumina. This fact is in opposition to the expected behavior, since alumina is an insulating material, as for example the values measured at low temperatures, in this case 300 ºC, the conductivity obtained is about 8 orders of magnitude higher that expected for alumina and silica, which exhibits conductivities around 10-14 S.cm-1 at this temperature. But the conduction process of these materials is heavily influenced by the concentration of Li ions that have high mobility, exhibiting ionic conductivities as reported in the literature26,29,31,35.
The activation energy for all cases decreased with increasing addition of alumina. This fact is probably related to the presence of increasing contents of β-spodumene phase with larger additions of alumina. Other authors have reported that glass-ceramics containing this phase have electrical conductivity values in the same range26,29.
It can be observed as the conductivity as the CTE are directly related to the phases crystalized when Al2O3 is added, LiAlSi2O6 and Li2SiO3. The conductivity and thermal expansion coefficient are influenced by the materials partners of the matrix. Conductivity is higher when Al2O3 because three Li-materials exhibit high lithium mobility, even Li2SiO3 does not appear to be a particularly good ionic conductor, it is still likely that the local mechanism of Li+ site exchange is similar to that in better-studied silicates36. Also, the CTE of a glass-ceramic material depends on the single CTEs of its crystalline and amorphous phases and their proportions37-39 and at this respect, the lower CTE of LiAlSi2O6 than Li2Si2O5 is doing to decrease the CTE of LZS/Al2O3 sintered in this work.
To complete the characterization of LZS-Al2O3 composites obtained by fast firing the coefficients of thermal expansion were determined. According to the results shown in Figure 5, the coefficients of thermal expansion (CTE) decrease gradually with the amount of alumina added from 11.3 x 10-6 ºC-1 for the LZS glass-ceramic (fast fired at 900 ºC for 30 min) to 6.0 x 10-6 ºC-1 for the composition LZS-5An. This significant CTE decrease with increasing amount of added alumina is related to a fraction of transformed zirconium silicate (which has a relatively low CTE, i.e., 4 x 10-6 oC-1) and in particular to the formation of the β-spodumene phase (which has a much lower CTE, i.e., 0.4-2 x 10-6 oC-1) as reported in the XRD patterns of Figure 3.
Figure 5 Coefficients of thermal expansion for the LZS glass-ceramic (without Al2O3) and for the LZS + Al2O3 composites containing 1, 2.5 and 5 vol.% of nanosized Al2O3 fast fired at 900 ºC for 30 min.
LZS-Al2O3 composites with glass-ceramic matrix 19.58Li2O.11.10ZrO2.69.32SiO2 (3.5 µm) and nanoparticles (13 nm) of Al2O3 (1-5 vol.%) were prepared by melting of a LZS parent glass and further mixing and reaction in the solid state with Al2O3 nanoparticles. The obtained composites were sintered by fast firing at 800 and 900 ºC for 30 min, leading to relative densities between 89 and 94%, containing zirconium silicate and β-spodumene as major crystalline phases. Composites sintered by fast firing at 900 ºC, had hardness between 4.5 and 6.5 GPa and Young's modulus between 65 and 102 GPa, the highest value being achieved for the LZS glass-ceramic. The electrical conductivity was maintained within ± 10-6 S.cm-1. The formation of β-spodumene in the obtained composites leads to reduce the CTE, whose values ranged from 11.3 to 6 x 10-6 ºC-1.
This work has been supported by CAPES in the frame of the International Cooperation Program Science without Borders for Special Visiting Researcher PVE (MEC/MCTI/CAPES/ CNPq/FAPESC/Nº71/2013), Project Nº A011/2013 (Brazil) and CNPq (National Council for Scientific and Technological Development, Brazil). Authors greatly acknowledge the financial Support of the Spanish Ministry of Economy and Competitiveness (MINECO, grant MAT2015-67586-C3-R, ENE2013-49111-C2-1-R, and IJCI-2014-19839).
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Received: October 31, 2016; Revised: March 25, 2017; Accepted: April 09, 2017
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