The Effect of Temperature on Synthetic Nano β-Eucryptite and Alumina Ceramic Materials: Thermal Expansion, Mechanical, and Physical Properties

The capability to fabricate ultra-low and customizable CTE materials with good mechanical properties in a simple method was demonstrated in this work. For this purpose, nano beta-eucryptite and alumina powders were synthesized and used in the composite's fabrication. Four composites of alumina and a second-phase beta-eucryptite were prepared, containing 10, 20, 30, and 40 wt.% beta-eucryptite. The temperature effect on prepared composites is investigated. The results of XRD analysis and the microstructures of prepared composites are discussed with results of mechanical strength and thermal expansion at temperature ranges of 1400, 1500, and 1550 °C. The CTE of alumina-beta-eucryptite composites decrease as the beta-eucryptite content increases, reaching -1.036 × 10–6 °C−1. The findings show that a composite with a very low thermal expansion coefficient and good mechanical properties can be designed and used in different applications.


Introduction
Low coefficient of thermal expansion (CTE) materials is getting a lot of attention due to their wide variety of applications, which range from cookware to aerospace [1].
The first glass ceramics fabricated were ceramic composites based on Li 2 O.Al 2 O 3 .SiO 2 (LAS), which have grown in economic and industrial significance. The predominant crystalline phases in these systems are metastable solid solutions with beta-quartz or keatite structures. The lithium ions fill the structure's cavities, providing charge neutrality. The three most important crystalline phases in this system are beta-petalite (Li 2 O.Al 2 O 3 .8SiO 2 ), beta-eucryptite (Li 2 O. Al 2 O 3 .2SiO 2 ), and beta-spodumene (Li 2 O.Al 2 O 3 .4SiO 2 ). The low negative coefficients of thermal expansion of these phases (ranging from 86 × 10 -7 °C −1 for beta-eucryptite to 9 × 10 -7 °C −1 for beta-spodumene) facilitate the production of glass ceramics with dimensional stability and high thermal shock resistance [2][3][4][5][6].
Alumina is one of the most appealing ceramic alternatives due to its high melting point and strong mechanical properties. For example, it has high compressive strength, high hardness, high elastic modulus, and good chemical and thermal stability. However, alumina's low fracture toughness is one of its disadvantages, which may limit its employment in high-mechanical applications. This is attributable to the easiness with which cracks propagate in alumina ceramics. Incorporating a second phase that interacts with alumina boundaries is one method of increasing fracture toughness. Materials with two crystalline phases, for example, could have higher fracture toughness, which improves wear resistance. With alumina ceramics, eucryptite has been employed as a second phase [7][8][9][10][11][12][13].
For decades, eucryptite (Li 2 O.Al 2 O 3 .2SiO 2 ) has been widely employed as a Lithia carrying flux and low expansion filler in whiter ware bodies in the glass and ceramics industries. Because of its very-low thermal expansion, high thermal stability, superior thermal shock resistance, and high chemical stability, eucryptite is utilized as a structural material. They are commonly employed in industrial furnaces as refractory materials and as heat exchangers in gas turbines [13][14][15][16][17].
In this study, we will focus on the production of an alumina-eucryptite ceramic composite using the Pechini method. The Pechini method is extensively used for the synthesis of ceramics and composites. This procedure is successful in the synthesis of ceramic powders because it works at low temperatures and achieves good uniformity in the solution phase and gels. The process allows the preparation of ceramic powders with good sinterability, stoichiometry control, and good management of the morphology of particles and agglomerates. The synthesis of lithium aluminosilicate gels utilizing various metal alkoxide starting materials has been extensively studied. Metal alkoxides, which are used as sol-gel synthesis precursors, are extremely costly. The Pechini method now uses inorganic salts due to the time-consuming laboratory synthesis of metal alkoxides, their high cost, and commercial unavailability. Therefore, the second goal of this work is to prepare these composites using chemically pure materials and characterize their phase composition, microstructure, mechanical, and physical properties.

Preparation of Beta-Eucryptite
The method devised by Pechini involves the complexing of cations in an aqueous organic medium and the utilization of low cost chemical precursors. A homogenous ion distribution is also obtained at the molecular level. It works on the principle that certain-hydroxy carboxylic organic acids can form stable chelates with a variety of cations. After adding a poly hydroxylic alcohol to this mixture, the chelate is transformed into a polymer with a uniform distribution of cations. At temperatures as low as 300 °C, the organic component is removed, leaving reactive oxides.
The Pechini method was employed to prepare betaeucryptite, which has previously been used to synthesize poly cationic powders. The process is based on metallic citrate polymerization with ethylene glycol. A hydro carboxylic acid (citric acid) was utilized in an aqueous medium to chelate cations. When a poly alcohol, such as ethylene glycol, is added, an organic ester is formed, and polymerization is facilitated by heating, resulting in a homogenous form. Metal ions are dispersed equally throughout the organic matrix [18].
At 60-70 °C, 30 gm of citric acid was added with continuous agitation, followed by the addition of 38.6 gm of aluminum chloride hexahydrate (AlCl 3 .6H 2 O) as the polymeric former, (11.032 gm) lithium nitrate (LiNO 3 ), and (33.30 gm) tetraethyl orthosilicate (TEOS) were mixed in a stoichiometric ratio with continuous stirring to get beta-eucryptite. After completely dissolving the salts, 20 gm of ethylene glycol was added to the solution. To speed up the reaction, the citric acid-ethylene glycol mass ratio was set to 60:40%, the temperature was raised to 110-210 °C, and a polymeric gel was generated at the end of the reaction.

Preparation of Nano Alumina
For nano alumina preparation, the same procedure is carried out with utilizing AlCl 3 .6H 2 O as alumina source.
The resulting gel was heat treated for 1 h at 600 °C to partially decompose the polymeric gel. The obtained materials were deagglomerated in porcelain and passed through a 100 mesh sieve. The powders were fired at temperatures ranging from 600 °C to 1200 °C at a heating rate of 10 °C/min for 1 h to follow the phase compositions.
We used the prepared alumina and beta-eucryptite after calcinations at 1200 °C in this study. Four composites of alumina and a second-phase beta-eucryptite were prepared, containing 10, 20, 30, and 40 wt.% beta-eucryptite and designated as AE1, AE2, AE3, and AE4 respectively. The samples were pressed at 100 MPa and fired at a temperature range of 1400, 1500, and 1550 °C for 2 h.

Characterization
X-ray diffraction analysis (XRD) was done on the prepared powders fired at 600, 800, 1000, and 1200 °C using Bruker D8 equipment with Cu Kα radiation at a scanning rate of 1 degree per minute. The particle size and morphology of the prepared powder calcined at 1200 °C were characterized using a transmission electron microscopy (TEM, JEOL JEM-1230). The microstructure of the sintered samples was examined using a scanning electron microscope (SEM-Philips XL 30). The Archimedes method was used to measure the apparent porosity and bulk density of the different samples. The coefficient of thermal expansion was measured from 25 to 700 °C at a heating rate of 10 °C min −1 . An automatic hydraulic testing machine (Shimadzu Co., Kyoto, Japan) with a maximum capacity of 1,000 KN was used to test the compressive strength of sintered bodies.

X-ray Diffraction of Calcined Powders
The XRD patterns of beta-eucryptite and alumina powders at different calcined temperatures are shown in Figs. 1 and 2.
When the gel powders of all prepared samples were calcined at 600 °C for 2 h, the amorphous structure was preserved. In Fig. 1, XRD peaks of the beta-eucryptite phase were detected when the samples were calcined at 800 °C to 1200 °C. With increasing calcination temperatures, the XRD peaks' intensity increased. This shows how heat treatment causes crystallinity to grow in powders. The XRD patterns for the prepared alumina powders demonstrate the development of boehmite (AlOOH) (Fig. 2), which transforms into gamma alumina (γ-Al 2 O 3 ) after calcination at 600, 800, and 1000 °C. Other previous studies have shown that increasing the calcination temperature causes a series of transformations: [19]. At high temperatures (1200 °C), some of the gamma alumina phases that formed at low temperatures changed to α-Al 2 O 3 . Low activation energies are required for transitions from γ phase ended to θ phase, while α transformation proceeds occur via nucleation, growth, and higher temperature [20]. At low temperatures, alumina appeared in an amorphous state, which changed to a crystalline state over 1000 °C [21]. Figure 3 shows TEM images of samples that had been annealed at 1200 °C. The particles are uniform in size and shape, with an average size of 5-50 nm for beta-eucryptite. For beta-eucryptite, the particle size was observed to be on the nanometer scale. On the contrary, alumina appears in hexagonal form with large sizes between 100 and 500 nm. The EDS analysis indicates that the well crystallite for both powders is at 1200 °C.

SEM Images
SEM of fired powders at 800 °C and 1200 °C is demonstrated in Figs. 4 and 5. Beta-eucryptite particles of small size and irregular forms have been observed. It appeared as dark grey particles that fused together to form a tridimensional structure. Beta-eucryptite is uniformly dispersed in the matrix, with an average particle size varying from 17-25 nm (Fig. 4). At high temperatures, the final structure of eucryptite particles can be related to the big microscopic air gas that exists between the particles, resulting in a reduced diffusion rate that inhibits particle growth (Fig. 5).
On the other hand, scanning electron microscope for alumina powders fired at various temperatures; 800 °C and 1200 °C are shown in Figs. 4 and 5.
The change in morphology and the size of the powders after calcinations are observed with increasing the firing temperature. For powders fired at low temperatures (800 °C), the aggregation of nano particles into micro particles was observed to be referred to δ-Al 2 O 3 . For the alumina powder fired at 1200 °C, plate α-alumina was found in the matrix. The α-alumina particles have a size greater than one micron. Plate α-alumina is the main phase for this fired temperature (1200 °C).

Physical Properties of the Prepared Samples
The bulk density and apparent porosity of beta-eucryptitealumina composites sintered at 1400, 1500, and 1550 °C with different beta-eucryptite contents (10, 20, 30, and 40 wt.%) are shown in Figs. 6 and 7.
It is widely known that obtaining pore-free sintered betaeucryptite (Li 2 O.Al 2 O 3 .2SiO 2 ) by the conventional firing process or without the addition of sintering aids during the preparation of beta-eucryptite from micro-sized starting materials is difficult because of the high grain boundary energy of lithium-alumina-silicates that causes the prevailing of grain coarsening [22,23].
As a result, preparing beta-eucryptite materials at the nano scale may facilitate sintering by reducing pores. Furthermore, in lithium-alumina-silicates (LAS), the Si-rich zone has higher grain boundary mobility than the Al-rich zone. Likewise, magnesium aluminate spinel doped with lithium may help in sintering and grain boundary mobility. According to the figures, increasing the sintering temperature causes an increase in bulk density and a decrease in apparent porosity with increasing beta-eucryptite content up to 1500 °C. This is because when the sintering temperature rises, the solid state reactions between the particles increase. The densification was improved at higher temperatures by the formation of a liquid-phase obtained from the LAS materials. Grain rearrangement, viscous flow, and glass redistribution are thought to be the causes of that behavior up to 1500 °C. Also, it is clear from Figs. 6 and 7 that the bodies always show a decrease in the apparent porosity with increasing sintering temperature and beta-eucryptite content up to 30 wt.% at 1550 °C. Then an increase in apparent porosity and %. This result is due to the formation of microcracks and grain growth of beta-eucryptite [6]. In general, the existence of the ternary oxide as the nucleus to obtain the beta-eucryptite grain and the very big alumina grain indicates that the particle size distribution is different (PSD). Such a likely scenario could lead to good densification and a high final relative density [24].

XRD Analysis
XRD patterns of alumina-beta-eucryptite composites sintered at different temperatures; 1400, 1500, and 1550 °C are shown in Figs. 8, 9, and 10. It is observed that the beta-eucryptite intensities increase with increasing temperature up to 1500 °C and beta-eucryptite contents. However, increasing the temperatures to 1550 °C increases the eucryptite contents by up to 30%. In the case of beta-eucryptite 40 wt.% at 1550 °C, the intensity of the peak decreases due to the formation of a high quantity of glassy-phase and the dissociation of some beta-eucryptite into spodumene.
According to the previous work [24,25], as the heat treatment proceeds, the crystal transforms into a different phase. For example, at high temperatures, crystal-line phases of a eucryptite solid solution convert to the more stable phase of spodumene. The observation of spodumene was observed for samples containing 30 wt.% of beta-eucryptite fired at 1550 °C.

SEM Analysis of the Sintered Samples
Microstructure images of alumina containing different amounts of beta-eucryptite and sintered at 1550 °C are shown in Fig. 11 at different magnifications. As seen from these figures, the porosity in most of the samples are appeared for 10, and 20 wt.% beta-eucryptite. The increase in beta-eucryptite content reduces the porosity up to 30 wt.% of beta-eucryptite due to the increase in the glassy phase.
As shown, by increasing the eucryptite content, the alumina plate shape (gray particles) are embedded in a more liquid phase. The highest content of beta-eucryptite (40 wt.%) shows the formation of microcracks (Fig. 11(d)).The presence of microcracking is due to the material's very anisotropic thermal expansion behavior, which produces thermal residual stresses and spontaneous microcracking. Due to the significant anisotropy in the beta-eucryptite crystal structure, which causes the a and b axes to expand while the c axis contracts, microcrack formation is a hallmark feature of beta-eucryptite at high temperatures. Internal stresses are produced when grains break up spontaneously and microcracks form [26,27]

Thermal Expansion Coefficient
The thermal expansion and the coefficient of thermal expansion for different alumina-beta-eucryptite proportions sintered at 1500 °C are shown in Fig. 12 and Table 1, respectively.
In general, since there is no reaction between alumina and the LAS, it is possible to predict the final phase composition and thus design various properties in the material, including its CTE. The CTE value depends on the initial composite composition, resultant body phases, and microstructure. As seen from Table 1, the CTE of alumina-beta-eucryptite composites decreases with increasing the beta-eucryptite content, which was considered a characteristic of beta-eucryptite behavior . As it is known, the thermal expansion coefficient of alumina is 10.3 × 10 -6 °C −1 [32], while betaeucryptite has a very low thermal expansion coefficient of -6.2 × 10 -6 °C. −1 [33]

Mechanical Strength
The mechanical strength of alumina-beta eucryptite composites is shown in Fig. 13. It was observed that samples showed high strength up to 40 wt.% beta-eucryptite content at 1400 °C and 1500 °C, whereas samples with higher beta-eucryptite content (40 wt.%) showed lower strength at 1550 °C. The increase in mechanical strength for samples sintered at 1400 °C and 1500 °C with increasing beta-eucryptite content and at 1550 °C up to 30 wt.% betaeucryptite is due to the porosity effect [6]. The decrease in mechanical properties of samples containing 40 wt.% beta-eucryptite and sintered at 1550 °C is due to a decrease in density and an increase in porosity as a result of betaeucryptite grain size enlargement (grain growth) and crack formation.

Declarations
Ethics Approval Not applicable: the research did not involve human participants and/or animals.

Consent to Participate
All authors have agreed to participate in this research.

Consent for Publication
The article was written by the named authors, who are all aware of its content and have given their permission for it to be published.

Competing Interests
The authors have no relevant financial or nonfinancial interests to disclose.
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