Alumina, assignement di gruppo ceramic materials

Properties and Applications of Alumina in the Advanced Ceramic Industry

Despite over 90% of alumina being used to produce metal aluminium, the remainder has found an increasing use in the advanced ceramic industry. Its properties along with its low cost, have made alumina the ideal material for a number of applications such as refractories, cutting tools, catalyst and catalyst supports, spark plugs, light bulbs, prostheses, bulletproof panels and in composites (both MMC and CMC) [1].

This work intends to be a summary of its properties, production process along with a selection of its application as an advanced ceramic by itself or with other materials.

Properties of Al O based materials

Alumina materials are usually made by pressed and sintered powder, with purity from 80% to 99.9% (the rest is either glassy impurities or added components). Pure alumina is white, but impurities make it pink or green. The maximum operating temperature increases with increasing alumina content. It has a low cost and a useful and broad set of properties: electrical insulation, high

mechanical strength, good abrasion and temperature resistance up to 1650°C, excellent chemical stability and moderately high thermal conductivity, but it has limited thermal shock and impact resistance. Competing materials are magnesia, silica and borosilicate glass [2].

Table 1: Values from GRANTA EduPack

Alumina is available in a range of standard shapes: rods, tubes, plates. The lower-density grades with up to 10% porosity are easily cut and ground. Fully dense alumina is hard and abrasion resistant, so it requires more specialized shaping methods. Subjecting alumina to different techniques we can produce different technical ceramics. All technical ceramics start as powders which are mixed with a binder and often a glass-forming ingredient, then are moulded, extruded, or pressed to the desired shape. Most grades of alumina (included those used for electrical insulators) contain a good deal of silica (up to 10%). This content of silica forms a glassy phase when heated, allowing firing at

A relatively low temperature. Pure alumina, required for substrates of microcircuits, requires firing at a much higher temperature, making it expensive [2]. Gamma phase mesoporous aluminas (γ-Al O ) are of tremendous interest for technological applications, 2 3 particularly when they possess large surface areas, tuneable pore sizes, narrow pore size distributions, and high thermal stability. Generally, the alumina pore structure collapses at high temperatures due to phase transformations from metastable phases to the stable alpha phase. To improve the thermal stability of mesoporous γ-alumina, dopants such as lanthanum, magnesium, zirconia, and silica are often introduced [3]. Above 900 °C, the pore structure changed significantly as metastable phases began to transition into the more thermodynamically stable phases [4]. Aluminas prepared with several dopants, including Ba, Sr, La, Sn, SiO , and PO , exhibited larger pore size and 2 4 higher surface area stability under steam.

Undesirable structure changes during thermal and hydrothermal treatments can be retarded by inclusion of dopants, though the mechanism for structure stabilization remains unclear. The pore structure and particle size of these aluminas can be easily controlled by changing synthetic parameters, such as the starting material, water to aluminium molar ratio, and rinsing agent ^[3]. As we said, the main problem of alumina is its brittleness and in order to solve it toughening dopants are considered. An example is zirconia toughened alumina (ZTA) which is used in structural, biomedical and cutting tool sectors due to its high strength and hardness. The coarse grain formation of alumina leads to poor densification ^[5]. Therefore, nano-sized particles of yttria-stabilized zirconia (Y-TZP) are added as toughening agent in the alumina matrix ^[5][6]. They lead to grain boundary refinement and transformation toughening effect, which means crack propagation is hindered by zirconia particles phase.

transformation. The grain boundary refinement leads to a finer grain and uniform morphology, enhancing mechanical properties. During sintering, some degree of grain coalescence and solid-state diffusion takes place. However, zirconia is almost insoluble to pure alumina. Sintering time and temperature are the predominant factors influencing the sintering process and the former significantly dominates the diffusion kinetics of the material. Higher times reduce the porosity and develop the mechanical properties. However, excessive sintering can lead to abnormal grain growth of the matrix and reduces the strength consequently [5]. Cr O can also be added to counterbalance the loss of hardness caused by zirconia and by adding strontium2 3oxide we can obtain strontium aluminate during the sintering process that forms platelets able to dissipate crack energy and limit their advancement [6].

Moving to Alumina matrix composites, SiC is one of the most used reinforcement. The addition of SiC increases both

Young's modulus and fracture toughness, thanks to several induced toughening mechanisms as crack bridging, wake toughening, crack deflection, and pull-out. Furthermore, SiC induces a decrease in the overall coefficient of thermal expansion (CTE) whereas an increase in thermal conductivity, a better thermal shock and erosion resistance [7]. TiC can also be used as reinforcement, leading to overall similar changes (below 30% wt.%) [b]. Carbon nanotubes (CNT) and graphene nanoplatelets (GNP) fillers have also been studied for their unique properties. A recent work theorized industrial feasibility of CNT+GNP/Al2O3 nanocomposites production wet techniques [8]. Production Since advanced ceramics are used typically for high-end applications in harsh environmental conditions, their properties must be very precise and reliable, which means highly controlled composition and so using powders that are no more minerals (as bauxite) but proper chemicals (typically over 90% pure Al2O3 powders) [2].

than 1 mm in diameter). Nowadays several production routes for alumina powders exist. The first is the well-known Bayer process, which however produces lower grades of purity along with environmental-hazardous red muds. Another established route is calcination of gel-based Al(OH) , decomposition of Al-3 containing salts or CVD. These processes can all produce ultra-high purity powders but involve high temperatures (above 1100 °C) leading to powders aggregations needing further milling and making the control of the particles morphology troublesome [9]. Mechanical milling is frequently used in order to mix in additives and reduce particles dimensions but is unable to obtain particles smaller than 100 nm and the medium may contaminate the powder and the process usually takes several hours. However, the possibility to have reactions among the powders can be exploited to have the in-situ formation of compounds for composites use, as graphene (though with poor control on morphology) [10].

the powders can be pressed into a desired shape using a hydraulic press. This process is called compaction and it involves applying pressure to the powder in a mold to form a solid object. Compaction is commonly used for producing ceramic filters, where the desired shape is a porous structure that allows for the passage of fluids while retaining solid particles. Another method for shaping the powders is through injection molding. In this process, the powders are mixed with a binder material to form a paste-like mixture. The mixture is then injected into a mold under high pressure, where it solidifies and takes the shape of the mold. Injection molding is often used for producing complex shapes with high precision, such as ceramic components for electronic devices. In addition to compaction and injection molding, other shaping methods such as slip casting, tape casting, and extrusion can also be used depending on the specific requirements of the application. Once the powders are shaped, they need to be sintered to achieve the desired properties. Sintering is a heat treatment process that involves heating the shaped object to a high temperature, but below its melting point. During sintering, the powders bond together and densify, resulting in a solid ceramic material with improved mechanical strength and other properties. Overall, the production of zirconia-toughened alumina involves a combination of powder synthesis, shaping, and sintering processes. Each step plays a crucial role in determining the final properties of the material and its suitability for different applications.catalyst carriers or fibres-composite a good solution is the extrusion of an alumina paste along with pore forming agents to be burned afterwards (for example carbon fibres) [12]. Another well-developed method used mostly in the production of substrates for the Figure 2: Mechanical properties of ZTA as a function of sintering time [13] electronic industry is tape casting in which alumina powder is mixed with a solvent, binder and other additives in order to form a slurry to be continuously deposited on a tape and dried in the process. Organic solvents have been typically used but the trend nowadays is to use water to avoid health and environmental concerns. Kristoffersson et al. [14] tested common binders as latexes, PVA and cellulose ethers highlighting the remarkable good behaviour of latexes, though the final impurity content needs to be addressed. As for metals and polymers, 3D printing has been developed to produce complex ceramics parts. In the inkjet printing a binder (typically

cellulose or PVA) is dropped following the CAD file information and will be then dried and debounded. To increase the final density and so mechanical properties, infiltration should be performed as in the work of Maleksaeedi et al. [15] in which a vacuum process was used. Another possible 3D printing technique is stereolithography, which uses photoactive resins that are cured after each layer. In the work of Wu et al. [16] is suggested that a bimodal distribution of micro and nanoparticles, together with a vacuum debinding are a good strategy to obtain better products through stereolithography. Independently from the forming process chosen, sintering at high temperature (about 2/3 of melting temperature) must be performed at the end in order to decrease porosity and form strong chemical bonds between particles due to solid-state diffusion. It can be performed through hot isostatic pressing in order to combine it with forming and achieve a near full density and a net shape but, being a very

expensive technique, pressure-less sinteringis also common. It is the last step in the production route and surely one of the most critical since dependingon the setting of its parameters (pressure, time and temperature) the final properties can largely change.The results of Dhar et al. [13] are represented in the figure, depicting mechanical properties changes of ZTAas a function of sintering time.

Biomedical implantsThanks to Alumina's chemical inertia this ceramic material is highly biocompatible which is a property thatmakes it a good candidate for biomedical applications [18]. Biocompatibility is "The ability of a material toperform with an appropriate host response in a specific application" [17].One of the main applications of Alumina in the medical field is in ball heads and liners of hip replacementprostheses. A scheme of the different component of a hip prosthesis is presented in Figure 1, while in Table1 the possible material couplings of ball head and

nce, Germany, Switzerland), the use of Co-Cr alloy liners is preferred due to their excellent wear resistance and biocompatibility. In other countries, such as the United States, UHMWPE (Ultra-High Molecular Weight Polyethylene) liners are more commonly used. Alumina liners are also an option, especially for patients with metal allergies or sensitivities. Alumina is a ceramic material that is highly biocompatible and has low wear rates. However, it is more brittle than Co-Cr alloy or UHMWPE, so it may not be suitable for all patients. The choice of liner material depends on various factors, including the patient's age, activity level, and any existing allergies or sensitivities. The surgeon will consider these factors and make a recommendation based on the individual patient's needs. It is important to note that the choice of liner material does not affect the type of ball head material used. The ball head is typically made of Co-Cr alloy or alumina, regardless of the liner material chosen. Overall, the selection of liner and ball head materials is a decision that is made by the surgeon in consultation with the patient, taking into account the patient's specific needs and preferences.