Development of Magnesium Aluminate (MgAl2O4) Nanoparticles for refractory crucible application

Ceramics are the oxides of metals and nonmetals with excellent compressive strength. Ceramics usually exhibit inert behavior at high temperatures. Magnesium aluminate (MgAl2O4), a member of the ceramic family, possesses a high working temperature up to 2000°C, low thermal conductivity, high strength even at elevated temperatures, and good corrosion resistance. Moreover, Magnesium Aluminate Nanoparticles (MANPs) can be used in the making of refractory crucible applications. This study focuses on the thermal behavior of Magnesium Aluminate Nanoparticles (MANPs) and their application in the making of refractory crucibles. The molten salt method is used to obtain MANPs. The presence of MANPs is seen by XRD peaks ranging from 66° to 67°. The determination of the smallest crystallite size of the sample is achieved by utilizing the Scherrer formula and is found to be 15.3 nm. The SEM micrographs provided further information, indicating an average particle size of 91.2 nm. At 600°C, DSC curves show that only 0.05 W/g heat flows into the material, and the TGA curve shows only 3% weight loss, which is prominent for thermal insulation applications. To investigate the thermal properties, crucibles of pure MANPs and the different compositions of MANPs and pure alumina are prepared. During the sintering, cracks appear on the crucible of pure magnesium aluminate. To explore the reason for crack development, tablets of MgAl2O4 are made and sintered at 1150°C. Ceramography shows the crack-free surfaces of all the tablets. Results confirm the thermal stability of MANPs at high temperatures and their suitability for melting crucible applications.


Introduction
Improvement and the development of the materials regarding their properties and specially the industrial applications have always been the great interest of scientists, engineers, and researchers.Search of the most efficient and economical materials is still being done even after the development of the advanced materials.Ceramics is one of the classes which is nonmetallic and inorganic materials and is indispensable in our daily life.Furthermore, engineered ceramics have received substantial focus because of their applications [1][2][3].
Spinel structure materials are represented by the general formula AB 2 X 4 where A and B are the divalent and trivalent cations while X is typically chalcogens that have extensive applications nowadays due to their properties.Magnesium aluminate is a spinel structure ceramic material that has several advantages, including thermal stability at higher temperatures, lack of sensitivity to chemicals, insensitivity to heat, and an adsorptive surface [4,5].
Magnesium aluminate spinel (MgAl 2 O 4 ) is a well-established refractory material that has received significant attention over the years due to its unique properties.These properties include a good thermal conductivity, low thermal expansion coefficient at elevated temperatures, high melting point (2105˚C), chemical inertness, and good chemical and mechanical strength.As a result of these advantageous characteristics, MgAl 2 O 4 is suitable for application in various refractory applications [6,7].The manufacturing of high-quality MgAl 2 O 4 powder is an important step in the application of MgAl 2 O 4 spinel.The conventional approach for creating MgAl 2 O 4 is a solid-state reaction of Al 2 O 3 and MgO at around 1500˚C.This technique is simple to implement and ideal for large-scale manufacturing [8], but the reaction temperature is quite high, and the end product lacks the requisite purity and particle size.In addition to the conventional method of synthesis of spinel MgAl 2 O 4 , molten salt synthesis is also used.In recent years, the molten salt synthesis (MSS) approach has received a lot of interest.In this process, the reaction medium is low melting salts, which allow reactants to combine in an atomic scale liquid fraction [9].The MSS liquid/solid system enables more homogeneous mixing and diffusion of input materials than the conventional solid-state synthesis technique, resulting in a significant reduction of temperature and reaction time [10].Safaei-Naeini et al. [11] employed the molten-salt approach to successfully produce MgAl 2 O 4 nanoparticles at 850˚C by heating various MgO-and Al 2 O 3 -containing precursors in KCl in stoichiometric proportions, nanosized spinel powders were created.Fazli et al. [12] used a molten-salt method to create nanocrystalline MgAl 2 O 4 spinel.Nano alumina, magnesia, and lithium chloride were used as starting materials.The ideal sintering procedure for best outcomes was discovered as 850˚C sintering temperature with a 3h soaking period.The optimal salt-to-oxide ratio has been stated to be 5:1.Zhang et al. [13] have described a molten synthesis process for Mg-Al spinel.The initial precursors were high-purity alumina and magnesia, and the solvent was KCl, which was utilized to investigate how varied salt concentrations influenced the shape.Zhang et al. [14] used molten-salt synthesis to create a MgAl 2 O 4 (MA) spinel layer on a Ti 3 AlC 2 substrate in a different study.
Magnesium aluminate spinel's have been observed to possess a range of favorable features, making them highly utilized as refractory materials and structural materials in metallurgical applications [15].Magnesium aluminate spinel, characterized by its diverse stoichiometric compositions, finds extensive utilization across a range of applications depending upon the specific environmental conditions of the intended application area.The primary utilization domains of magnesium aluminate spinel as refractory materials include cement rotary kilns, steel-teeming ladles [16][17][18], and the regenerators checker work element of glass tank furnaces [19,20].Spinel magnesium aluminate has been produced by numerous researchers, who have also suggested its potential application in the metallurgical industry [21].Despite the existence of several refractory applications, there is a noticeable absence of literature regarding the crucible development of MgAl 2 O 4 as it possesses highly desirable features that are essential for an efficient crucible, such as a high melting point, chemical inertness, high thermal stability, good chemical, and mechanical strength.Therefore, some efforts have been carried out in this paper to demonstrate the preparation of crucibles from Magnesium aluminate nanoparticles (MgAl 2 O 4 ).Firstly, the synthesis of MgAl 2 O 4 was conducted by using the molten salt method in different salt-to-oxide ratios.The resulting samples were characterized using SEM, XRD and Ceramography was also performed to investigate the surface of ceramic samples.Finally, a variety of crucibles were fabricated utilizing both pure MgAl 2 O 4 and a combination of Al 2 O 3 .

Supporting chemicals and equipment used in synthesis
Following are the chemicals and equipment used in the synthesis of Magnesium aluminate nanoparticles. 2.1.

Different methodologies used for preparing MgAl 2 O 4 nanoparticles
As illustrated in Fig 1, two different samples were employed in the case of MgAl 2 O 4 .The process approach of Sample A was derived from the author's own trial-and-error method.Sample A is a mixture of equimass quantities.In this method, precursors were prepared using a ratio of 15:1 (i.e., 15 g of KCl with 1g of MgO and an Al 2 O 3 mixture).MgO and Al 2 O 3 were then mixed together in equal amounts using ethanol as the solvent.In sample B, an equimolar mixture of MgO and Al 2 O 3 is prepared in ethanol [13].All methods remained consistent with Sample A, with the exception of an additional step involving the mixing of MgO and Al 2 O 3 using a mortar and pestle for Sample B. The primary distinction between Sample A and Sample B lies in the composition of the mixtures, specifically the utilization of equimass and equimolar quantities, respectively.
The Experimentation Section provides a comprehensive discussion of the steps involved in each sample preparation.

Experimentation
A brief experimentation plan of the complete setup is shown in Fig 2. 2.3.1.Preparation of Sample A. Magnetic stirrer was calibrated after placing the petri dish on the weight machine.6g of fine Al 2 O 3 powder was weighted and placed in a beaker.6 g of MgO powder was weighted and placed with Al 2 O 3 powder in the same beaker.15ml of ethanol was added to the mixture with the help of a pipet.Then the beaker was placed on a magnetic stirrer for half an hour to mix the sample completely.After one hour, the powder settled at the bottom and excess ethanol drained out.The wet sample was placed in a ball milling machine for 3 hours.The sample was collected in the beaker and placed in a drying oven at 80˚C for 4 hours after ball milling.The dried sample was then mixed with KCl powder.30 g of KCl powder was mixed with 2g of dried sample (i.e.15:1) and then placed in the ball milling machine for 1 hour.
After preheating the graphite crucible with its lid at 1150˚C for 4 hours along with furnace cooling, the resulting sample was then placed in the preheated graphite crucible and tightly packed with its lid.The crucible was placed in a furnace for calcination at 1150˚C for 4 hours.The sample was furnace cooled and ready for examination after calcination.

Preparation of Sample B.
1 M solutions of both MgO and Al 2 O 3 were prepared by mixing 1 g of MgO powder in 25 ml of ethanol and 2.5g of Al 2 O 3 powder in 25 ml of ethanol.Table 1 shows the details of Sample B solution preparation.
The two solutions are then put in a beaker.The beaker was placed on a magnetic stirrer for half an hour in order to mix the solution completely.The beaker was covered with aluminum foil and placed in a fume hood for 22 hours.The powder settled down at the bottom and excess ethanol drained out.The wet sample was placed in a ball milling machine for 3 hours.After ball milling, the sample was collected into the beaker and placed in a drying oven at 80˚C for 4 hours.120 g of KCl powder is mixed with 4g of dried sample (i.e., 30:1) and placed in a ball milling machine for 1 hour.Then again, milled in a mortar pestle for 15 min.The resulting sample was then placed in a preheated graphite crucible and tightly packed with its lid.The crucible was placed in a furnace for calcination at 1150˚C for 4 hours.The sample was furnace cooled and ready for examination after calcination.

Characterization
Characterization is an organized examination or formal evaluation exercise.It involves the measurements, tests, and gauges applied to certain characteristics regarding an object or activity.Table 2 shows the equipment used to characterize different characteristics of Magnesium aluminate nanoparticles.

Result of Sample A
Laser Particle Size Analyzer was used to obtain the average particle size.  in the nano range (<100 nm).This is the case of mixing the quantities of magnesium oxide and aluminum oxide in equimass quantities rather than equimolar quantities, as stated in [13].

Results of Sample B
The sample obtained after performing the experiment as shown in Fig 5 was then characterized using different techniques to find out different characteristic properties.

XRD. Powder diffraction (XRD
) is a method employed to determine the preferred orientation, crystallite size (grain size), and crystallographic structure of solid samples that are either polycrystalline or powdered.The sample needs to be homogenized, finely ground, and its average bulk composition is necessary for XRD.Magnesium aluminate spinel was    3 shows the particle size calculation of Sample B.

D ¼ Kl b cosy
Where, β is the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians.
λ is the X-ray wavelength, K is a dimensionless shape factor, with a value close to unity.The shape factor has a typical value of about 0.9, D the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size, θ is the Bragg angle (in degrees).

Thermal analysis.
The differential scanning calorimeter (DSC) is a powerful apparatus used in the area of thermal analysis.The thermal analysis approach involves the quantification of the difference in heat energy required to increase the temperature of a specimen in comparison to a reference material.The difference is assessed as a function of temperature.The smallest and average crystallite size of the prepared annealed sample was determined from X-ray line broadening using the Scherrer formula and it is found to be 15.3 nm and 46.7 nm respectively. https://doi.org/10.1371/journal.pone.0296793.t003

Scanning electron microscopy analysis.
The scanning electron microscope (SEM) is a variant of the electron microscope that utilizes a high-energy electron beam to scan and generate images of a sample in a raster scan pattern.JEOL Ltd.Scanning electron microscopy (SEM) is used to obtain micrographs, which confirm the presence of nanoparticles.The SEM micrograph shown in Fig 8(A) is 20,000X (particle size is in the range of 120 nm to 143 nm), but the SEM micrograph in Fig 8(B) is 30,000X (particle size is in the range of 86.7 nm to 99.1 nm), which is under nanometric scale, demonstrating the presence of irregularly shaped and sized particles.In some regions, the cohesiveness of the particles has increased, resulting in the formation of agglomerated places within the sample.Thus, it is believed that some particles within the sample are used as condensation nucleation sites around which smaller particles condense, resulting in agglomeration.Sintering of MgAl 2 O 4 materials is challenging due to their distinct chemical stability and high melting point.The sintering properties of MgAl 2 O 4 are influenced by the uniformity, size, and agglomeration of the particles.The irregularity and non-uniformity of the particles are attributed to the elevated calcination temperature [22][23][24].Particles were agglomerated in Fig 8(A)-8(C) of SEM which was due to the high calcination temperature (i.e., 1150˚C).Therefore, particles diffused into one another and connected through a physical bond.This problem can be controlled by minimizing the calcination temperature to 800˚C, which is also the melting temperature of KCl.The ratio of mixed powder to solvent was also high enough that it also produced residues in sufficient quantity, and it resulted in the agglomeration of particles.If that ratio was minimized to 1:3 (i.e., mixed powder to KCl) instead of 1:30, the problem was eliminated completely.
Table 4 provides a concise overview of the synthesis and characterization of magnesium aluminate nanoparticles.

Making of crucibles
In the making of the crucible, the binder was the major component which allows the particle to bind compactly.PVA is mostly used as a binder for making the crucibles of ceramics.Table 5 shows the steps of preparing crucibles.PVA solution was made by taking PVA as a solute and water as a solvent.5 gm of PVA powder was mixed in 100 ml of water and the mixture was stirred until all the solute particles were dissolved.

Sample Brief overview of the obtained results
Sample A Synthesis 6 g of each Al 2 O 3 and MgO were mixed with 15 ml ethanol.The mixture was then placed in a ball milling and after that in a drying oven at 80˚C. 2 g of the sample was mixed with 30 g of KCl.The same mixture was finally placed on a preheated graphite crucible at 1150˚C.This crucible with the sample was placed in a furnace at the same temperature for 4 hours.

Characterization
The Laser Size Analyzer revealed the average size of the particle 6 μm which was beyond the nano range.Upon reviewing the experimental procedure, the main reason of the large particle size was equal mass proportion of magnesium oxide and aluminum oxide, instead of equal molar ratios.

Sample B Synthesis
1M solutions of each MgO and Al 2 O 3 in ethanol were prepared separately and then mixed completely.This mixture was then placed in a fume hood to get the powder of the mixture after draining out the excess ethanol.Ball milling was done of the wet mixture and dried in the oven at 80˚C. 4 g of the sample was mixed with 120 g of KCl.This mixture is again milled in a mortar pestle.The same mixture was finally placed on a preheated graphite crucible at 1150˚C.This crucible with the sample was placed in a furnace at the same temperature for 4 hours.Characterization XRD detected the presence of MgAl 2 O 4 in a sample.The smallest and average crystallite size of the prepared annealed sample was determined from X-ray line broadening using the Scherrer formula and it is found to be 15.3 nm and 46.7 nm respectively.DSC curve indicated 0.05 W/g heat flowed into the material at 600˚C and TGA curve showed only 3.5% weight loss at the same temperature.This made MgAl 2 O 4 good candidate for its thermal insulation application.SEM images confirmed the presence of nanoparticles with an average particle size of 91.2 nm.There was agglomeration of the particles seen due to high calcination temperature of 1150˚C and high ratio of the sample i.e., 1:30.This agglomeration of the particles can be controlled by reducing the temperature up to 800˚C and the sample ratio up to 1:3.
https://doi.org/10.1371/journal.pone.0296793.t004composition.After cleaning the die and punch, oil was applied in order to avoid friction.Then the sample was placed in a die.Die and punch was placed on a hydraulic press of 10 tons.A pressure of 6 tons was applied for 5 min.The pressure was then released; the die and punch were removed from the pressing machine.The crucible was then removed from the stillattached die to the punch.The crucible was removed from the punch very carefully.
Result: The powder was not compressed, and it was shattered because of the low content of PVA.
4.1.2Second attempt.1.5g of PVA is added to 20 g of MgAl 2 O 4 powder.The powder was then put in a mortar and pestle and mixed vigorously in order to achieve the homogenous composition.After cleaning the die and punch, oil was applied in order to avoid friction.Then the sample was placed in a die.Die and punch were placed on a hydraulic press of 10 tons.Pressure of 6 tons was applied for 5 min.Pressure was then released; die and punch were removed from the pressing machine.The crucible was then removed from the still-attached die to the punch.The crucible was removed from the punch very carefully.
Result: The compressed powder did not get the required shape when it was detached from the punch because of the low content of PVA.
4.1.3Third attempt. 2 g of PVA is added to 20 g of MgAl 2 O 4 powder.The powder was then put in a mortar and pestle and mixed vigorously to achieve the homogenous composition.After cleaning the die and punch, oil was applied to avoid friction.Then the sample was placed in a die.Die and punch were placed on a hydraulic press of 10 tons.Pressure of 6 tons was applied for 5 min.Pressure was then released; die and punch were removed from the pressing machine.The crucible was then removed from the still-attached die to the punch.The crucible as demonstrated in Fig 9(A)-9(C) was removed from the punch very carefully.

Making of crucibles from 100% Al 2 O 3
2 g of PVA is added to 20 g of Al 2 O 3 powder.The powder was then put in a mortar and pestle and mixed vigorously in order to achieve the homogenous composition.After cleaning the die and punch, oil was applied in order to avoid friction.Then the sample was placed in a die.Die and punch were placed on a hydraulic press of 10 tons.Pressure of 6 tons was applied for 5 min.The pressing machine's die and punch were removed.The crucible was then removed from the still-attached die to the punch.The crucible as illustrated in  A summary of different compositions used in Experiment A and Experiment B is shown in Table 6.
All four unsintered crucibles are depicted in Fig 12.

Sintering of crucibles
Sintering of all four crucibles (i.

Making of tablets to carry out the investigation of sintering of 100% MgAl 2 O 4
Tablets of 100% MgAl 2 O 4 were made to study the effects of sintering and densification of particles as the crucibles made in the experiment did not seem to be sintered at the described conditions.4.5.1 Preparation of tablets.1g of PVA was added to 10g of the sample and mixed vigorously till complete homogenization.Then the powder was divided into three equal quantities.Three tablets were made for the confirmation of tests.The tablets were made with a 1cm diameter die and punch.A pressure of 5000 psi was applied to compact the powder.The densities of tablets were calculated after compaction by measuring the height and diameter of the tablets with a vernier caliper.The green tablets were sintered at 1150˚C for 2 hours, followed by 30 minutes at 300˚C and 30 minutes at 700˚C.The densities of tablets were calculated after sintering by using the same procedure as mentioned above.Stereographic images were also taken before and after the sintering of tablets.For Ceramography, samples were hot mounted.Grinding of samples was done using 400 grit size of silicon carbide paper for 2 min, followed by grinding on 600 grit size paper for 2 min and finally on 1000 grit size paper for 2 min.The samples were washed and polished using coarse alumina powder (i.e., 1μm) for 2 min, followed by polishing using fine alumina powder (i.e., 0.5 μm).The samples were washed once more before being etched for 10 minutes with concentrated Aqua regia and hot blowing for 3 minutes.The samples were then taken to the Olympus Multicolor Microscope and images were taken at 400 X.

Density of tablets.
The density of any material can be affected by two parameters (i.e., mass and volume).During sintering, both parameters may be affected.Loss of PVA and moisture content results in weight loss, and shrinkage of geometry results in volume decreases.However, a marginal change in density was observed due to both of the above-mentioned reasons.Table 7 shows the density of tables before and after sintering.Before sintering, the blackish area on the surface of the sample indicated the oil content which was applied during compression of the tablet.Moreover, the metal particle inclusions were embedded on the sample from the die which can be seen as tiny sharp black spots.In addition, the tablets' edges were damaged during their removal from the die.After sintering, the surface of the sample appeared to be clearer and brighter due to the removal of oil content.The particles were well densified and compacted and other inclusions like black spots were also minimized due to the sintering as shown in Fig 15 .4.5.4Ceramography.Images of the surfaces of sintered tablets were taken by using a metallurgical microscope at 400X as demonstrated in Fig 16.
The surfaces were found to be dull and clear.It has a compact and dense structure and was unaffected by the etchant used.White spots on the surface were due to the alumina which was embedded on the surface during polishing.Sintering was achieved at 1150˚C, which could be seen from the micrographs.Compactness can be enhanced by increasing the sintering temperature and the holding time.
4.5.5 Hardness of sintered sample.Hardness was taken by Rockwell hardness tester.Steel ball indenter applied 100 kg of force for a dual time of 10 seconds.These indents were taken close to each other.Stereographic images showed that very less damage occurred on the surface.During testing, a crack developed in the mounting area, but the sample remained intact, and the hardness of the sample was estimated to be above the measuring scale.Ball penetrated less than 1mm into the surface which can be seen in the images as illustrated in Fig 17.

Conclusion
The molten salt technique was employed in order to obtain nanoparticles of MgAl 2 O 4 .Potassium chloride (KCl) was employed as the solvent in the experiment.The laser particle analyzer was utilized to characterize Sample A, revealing that the average particle size measured 6 μm, exceeding the nano range.It is evident that the primary cause for the significant particle size

MgAl 2 O 4 4. 1 . 1
First attempt.1g of PVA was added to 20 g of MgAl 2 O 4 powder.The powder was then put in a mortar and pestle and mixed vigorously in order to achieve a homogenous Fig 10 was removed from the punch very carefully.

4 . 5 . 3
Stereo micrographs.Stereographic images were taken at 15X of tablets as shown in Fig 14.