Effect of the addition of glasses as sintering aids on microstructure and properties of nanoalumina

Alumina-Glass compositions were prepared to evaluate the effect of glass as sintering aids. The composites showed densification below 1100°C. The highest density values (~95%) were obtained for compositions based respectively on borosilicate (G2 and G3) and soda lime glasses (G4), all containing Na in the precursor powder. Samples G1 (K-based) and G5 (no K or Na in the precursor glass powder) presented irregular morphology with the presence of intergranular porosity. The composition G2, G3, and G4 presented uniform morphology corresponding to higher densification.


INTRODUCTION
In recent years, many efforts have been devoted to improving the thermal, chemical and mechanical behavior ceramic materials and composites, exploring different strategies, such as new compositions including sintering aids that make up the ceramic matrix [1], design of tailored microstructures for the production of ceramic laminates [2], the use of nanosized secondary phases [3,4].
Alumina-based ceramics are widely used materials in many industrial applications. These materials have exceptional combined properties and are often used in advanced structural ceramics. Alumina compounds with secondary oxides phases are often manufactured to improve some properties, such as corrosion and oxidation resistance, hardness and mechanical stability. Particularly, the formation of a glassy phase during sintering at high temperatures may aid the densification of alumina ceramics [5].
Several studies have been carried out using a mixture of glass and alumina in different proportions for enhancing processing conditions and final properties of a single matrix or composites based on alumina [6][7][8][9]. The association of aluminate a glass forming system generally contributes to the reduction of sintering temperatures [10]. If a suitable glass composition is selected, the composite may have controlled closed porosity and properties, such as dielectric constant, thermal expansion and tightness required for highperformance microelectronic substrates [11].
Sintering aids are intentional chemical modifiers that act to promote greater particle contact and allow reactions in the pores of the constituent oxides, thus reducing the energy required for the diffusive and viscous flow processes to occur improving densification of the material [12]. These aids might be combinations of one or more oxides which are added as well to control grain growth, in addition to improving densification by forming a liquid phase [13,14].
In this context, the present study aims to obtain alumina ceramics using glass compositions as sintering aids to improve physical and microstructural features.

MATERIALS AND METTHODS
The compositions chosen in this work were based on the research done by [6,7,8,9] as presented in Table 1. ing, the powder compositions were dried at 100°C for 24 h in an oven. The resulting mixes were granulated through a 200 mesh continuous vibration screen for 50 min. Then, prismatic blocks (4 mm x 4 mm x 4.7 mm) were compacted via dry uniaxial pressing at 30 MPa followed by cold isotactic pressing at 200 MPa.
Thermal expansion behavior of the samples was investigated trough dilatometry (Netzsch DIL 402 PC), with heating rate of 1°C/min up to 1200°C, hold time of 5 h, and cooling rate of 1°C/min down to room temperature. The green bodies were then sintered in an air oven (HT Nabertherm D 2804) with a constant heating/cooling rate 5°C/min with hold at 1200°C for 60 min.
The morphology of the samples was obtained by scanning electron microscopy (Zeiss Supra 55VP FEG-SEM) of the alumina with additives samples after sintering.
Apparent density measurements of the sintered samples were performed using the Archimedes principle [13] using distilled water as the immersion liquid. The respective specific theoretical densities of the oxides used in this work were those provided by the manufactures. The hardness of the samples was determined using the Vickers technique, with load of 1 kgf for 30 s in each indentation [14], in a dedicated equipment (Zwick 3212).
The powders (G1P, G2P, G3P, G4P and G5P) and sintered samples (G1S, G2S, G3S, G4S and G5S) were analyzed by X-ray diffraction (XRD, Bruker AXS D8 Discover), using CuK radiation, 40 kV and 40 mA, in the range of 2 between 20° and 80°.  It was observed that compositions G2, G3 and G4 presented lower sintering temperatures, 1075°C, 1075°C and 1050°C, respectively. This was related to a similar composition of G2 and G3, both containing 3.5 -3.9% Na 2 O in the precursor glass, which is a well-known fluxing agent. In the case of composition G4, which is based on soda lime glass, the content of Na 2 O in the soda lime glass is much higher (22%), so that it forms a liquid phase at the lowest temperature.

Sintering Behavior, densification and mechanical performance
For composition G1, which contains K 2 O, sintering started at about 1100°C. In this case, K 2 O present in the precursor glass (G1) is slightly less active as a flux when compared to Na 2 O (used in G2 and G3 for example). Alkaline oxide act as modifiers connecting through ionic bounds to anions ne twork and such ions act breaking covalent bonds, as in the case of Si-O-Si, thereby causing the onset of oxygen atoms unbound in silica-based materials [16][17].
Thus, the addition of fluxing aids, in special Na-based, break the links between bridges of polyhedrons (SiO 4 ) forming a terminal anion which neutralizes the charge of cation, positioned in the interstices of the three-dimensional structure, causing a strong flux that is slightly stronger than K -based aids [18].
The Figure 2 illustrates the values of relative density and Vickers hardness in function of the compositions studied. For G4 and G2, the density values were around 95%, and for the G3 curves, the density presented a value above 90%. Similar results were found by Montedo et al. 2017 [19], who studied the effects of a glass ceramic LZSA (11.6Li 2 O 16.8ZrO 2 68.2SiO 2 3.4Al 2 O 3 ) in the sintering behavior of the alumina obtained by phase sintering.
The authors showed that the glass ceramic increased the densification of the alumina studied, but one of the samples reached a relative density of 95% in the sintered samples at 1600°C/40 min compared to 85% for the sintered at 1600°C/4h, being a potential candidate to improve the densification of alumina in applications where wear resistance is the primary requirement.
The additives used in G1 and G5 have a strong influence on the hardness and the density of the samples presenting lower values in relation to the other compositions. The alpha alumina exhibits a hexagonal corundum structure. In this structure, the Al 3+ cations occupy only two-thirds of the available sites and an idle interstitial site arises between alternating Al 3+ pairs. The capture of the alumina filler may occur around defects resulting from the dissolution of impurities (i.e. doping cations and their charge compensation defects). In sintered materials, it is also necessary to consider the effect of grain contours, segregation of impurities and defects in the interfaces [20]. These dopant atoms may have contributed to the formation of punctual defects, thus increasing the concentration of native defects in the crystal, which may hinder the sintering kinetics, generating a high porosity for samples G1 and G5.
The porosity produces adverse effects on the mechanical properties, since the existence of pores can result in the reduction of hardness. When there is a decrease in the pores, the penetrator is more difficult to retreat into fully dense regions.

Structural and microstructural analysis
Alumina () is in JCPDS (COD 90077496) and quartz in JCPDS (COD 9006307) were identified in all samples. In general, the appearance of the andalusite (Al 2 SiO 5 ) phase in JCPDS (COD 9000919). It is possible to verify that the peaks referring to all the phases in the powder samples intensify with the sintering. The Figure  3 shows the XRD results of the powdered samples and for the sintered specimens.

SEM
The morphological aspects of the sintered samples were observed in Figure 4. It is possible to observe that samples G1 (a) and G5 (d) exhibited an irregular morphology with the presence of intergranular porosity. For G5, it was observed the formation of necks between the particles and a decrease of the voids, which was related to the beginning of sintering. In contrast, the G2 (b) and (G4) composition had higher densification, as can be seen in the dilatometric curves and in the density plot. The same morphological aspect was observed for the G3 composition, with similar densification to the G2 composition.

CONCLUSION
Alumina/glass systems were formulated, processed and thermally treated in order to reduce the sintering temperature as well as to improve microstructural characteristics. According to the SEM images, it was possible to observe an irregular morphology with the presence of intergranular porosity for samples G1 and G5. Samples G2, G3, and G4 were denser and more uniform. Glass used as sintering aids were effective in lowering the alumina sintering temperature. However, for the compositions G1 and G5 an increase of porosity was observed, which influenced the mechanical properties.

ACKNOWLEDGEMENTS
The Cooperation Program PROBRAL/CAPES/DAAD is acknowledged for the financial support.