The effect of calcination on hydroxyapatite composition, atomic structure, and microstructure

Hydroxyapatite in an important bioceramic used for orthopedic and dental implants.  Its structure and chemistry are similar to the natural materials in human bone which encourages bonding between the body and the implant.  The wet chemical precipitation method is the most common manufacturing method for hydroxyapatite, and a key stage in this process is calcination.  In this process step the hydroxyapatite is converted from a powder with low crystallinity, and high chemical disorder, to stoichiometric hydroxyapatite, with larger crystal domains and higher overall crystallinity.  The result is a powder with higher strength and high biocompatibility.  In this paper the effect of calcination on the material is discussed, the impact of process parameters described, and the choice of crucible is explained.

Hydroxyapatite Production

Hydroxyapatite is the primary natural mineral in bone and teeth.  It can be manufactured synthetically and is a key bioceramic material for dental and orthopaedic implants.  Its chemical composition and microstructural similarity to bone means that bone cells can successfully grow into the structure, enhancing the bonding between implant and the body.   Key to the success of this augmentation between the bone and the implant is the chemical composition and the microstructure of the hydroxyapatite.  This paper outlines the most common manufacturing route for this important bioceramic and discusses the effect of calcination on its chemical composition and degree of crystal ordering, which affect both strength and biocompatibility.

Wet Chemical Precipitation

The most common and scalable production route for hydroxyapatite is the wet chemical precipitation route.  This route can be used to produce grammes as well as tonnes of material.  In this process, a calcium source such as calcium nitrate, calcium chloride or calcium hydroxide is mixed with a phosphate source, such as ammonium phosphate or phosphoric acid.  The reagents react and form hydroxyapatite.  The composition of hydroxyapatite is stoichiometric and therefore additions need to be made carefully with the solution constantly stirred.  To achieve the correct composition the pH must be maintained between pH 9 and 11.

Once reacted, the slurry can then be aged to increase crystallinity as much as possible before being filtered, washed, and dried. 

Calcination is then carried out at around 600°C to 1100°C.  This has the effect of removing volatile species from the powder, reducing unit cell distortion, and improving overall crystallinity.  These changes improve the mechanical strength and biocompatibility of the material, making it more suitable for orthopaedic & dental implants.

Hydroxyapatite Structure

Prior to calcination the atomic structure, microstructure and overall composition is produced by the precipitation process.  At this stage, there are many small crystallites that are typically around 5-20 nm.  These stem from the rapid nucleation and the relatively low temperature of the solution.  As a result, many crystallites form, but the low atomic mobility results in them being small, with vacancies and positional disorder being common.  The high amount of nucleation, but relatively low growth mean that these crystallites are often growing in different directions and are misaligned with each other.  As the atomic mobility is low, not all atoms can move to the ideal unit cell location; such that the unit cells do not achieve their lowest energy state.  Vacancies and substitutions are common and create lattice strain.   Thus, the crystal domains are small and poorly defined prior to calcination.

As well as a crystalline phase, the powder also contains an amorphous phase between the crystalline areas.  This is less ordered and contains regions rich is ions and hydrated hydroxyapatite.  These ions are typically ammonia, nitrate and chloride ions from the pH control and starting chemistry. 

Prior to calcination the material is not suitable for orthopaedic or dental implants, as the lack of composition and structural order results in poor thermal stability, poor mechanical strength, and poor bioactivity. 

Calcination improves crystallinity and lattice order by providing sufficient energy that volatile components such as residual water, carbonates and undesirable HPO₄^2- decompose and evaporate.

At elevated temperatures atomic migration can occur much more easily and vacancies become filled.  The calcium and phosphate sites become fully occupied and achieve stoichiometric hydroxyapatite (Ca/P ratio of 1.67) which reduces lattice strain. 

The improved atomic migration means that crystals can restructure; crystals become more aligned; domains become better defined and grow.  Prior to calcination domains are relatively small at around 5 to 20 nm, and after calcination they are 50 to 200 nm or more.  The amount of amorphous material is reduced as these crystallite domains grow and volatile species decompose and evaporate.  The increased crystallinity and reduced structural strain result in an increased strength.  

The Affect of Calcination Process Parameters on Hydroxyapatite.

During calcination not all processes happen at once.  Table 1 describes what happens at each temperature stage as the powder is heated.  It can be seen that restructuring begins at around 400°C, but carbonate losses do not occur until 600°C and dehydroxylation does not occur until 800°C.

Table 1 the effect of temperature on precipitated hydroxyapatite powder

The key parameters for calcination are peak temperature, hold time, atmosphere, and the condition of the initial uncalcined powder.

Higher temperatures increase diffusion as a result larger domains and high crystallinity occurs as the material seeks to reduce its energy.  The parameter is limited by the decomposition of the hydroxyapatite.  This can occur at temperatures around 1050°C, decomposing to the more soluble tricalcium phosphate (TCP) and lime. 

The hold time is another way of improving crystallinity.  Simply allowing more time for diffusion to occur allows the composition to change and the crystal domains to grow.  However, excessive hold time results in higher energy costs, reduced cost effectiveness, and reduced capacity of the manufacturing process. As with all diffusion-based processes it has a much lower effect than temperature on the final powder properties.

The atmosphere can also influence the calcination process.  A key stage of the process is when chemically bound water and other volatiles evaporate.  If the humidity is too high this reduces the dehydroxylation of the hydroxyapatite, and the subsequent rearrangement and crystal domain growth.  The desired degree of dehydroxylation will depend on the final application.

The final consideration is the condition of the starting powder.  If the disorder is high, more work must be done by the calcination to get the powder into to a satisfactory condition.

Calcination Crucibles

During calcination, the powder is placed in a crucible for ease of handling and to maximise furnace capacity.  The choice of crucible shape, size and material is important for a number of reasons.  First the design of the crucible must:

• Allow them to pack well in the furnace.  This maximises capacity but also allows an even heat distribution between them.
• They must also have a low thermal mass to minimise their effect on heating rate and cooling rate.

In both cases the hydroxyapatite powder must reach the peak temperature.  A poor heat distribution will result in cool regions within the furnace such that not all the hydroxyapatite achieves the required properties.  Similarly, if the thermal mass of the crucible is high it will slow the heating rate of the powder, such that all the hydroxyapatite in the furnace will fail to achieve the target temperature and the required properties.  A high thermal mass also means the cooling rate will be too slow, the capacity will be reduced, and the overall cost effectiveness of the calcination process will also be reduced.

The choice of crucible material is also a key consideration for calcination.  The material must:

• Be capable of reaching the required temperature.
• Show chemical compatibility and not react or influence the material.
• Not drop particles from the crucible walls or cause contamination.
• Have a high thermal shock resistance to enable longer life during cycling.
• Have an acceptable cost and availability.

When considering the above factors, high purity alumina crucibles are often chosen.  The best way to explain this is to consider the alternatives.  In most cases chemical compatibility and material cost limit them.

• Porcelain – this would have sufficient temperature resistance, but is limited by the fluxes used (Na₂O, K₂O and CaO), which can react with the hydroxyapatite.  They also tend to show lower strength and lower thermal shock resistance compared to alumina.
• Quartz – this would be just capable of the temperature, but it is not resistant to strong alkalis or high calcium phosphate reactions that occur at elevated temperatures.
• Zirconia – is chemically inert, has the required temperature resistance and thermal shock resistance but is more expensive than alumina.
• Silicon Carbide – is likely to contaminate at elevated temperatures.
• Platinum alloys – these would work from a material property point of view but are cost prohibitive.

A high purity alumina crucible (>99.5%), have the required chemical inertness, temperature resistance, and cost effectiveness.  The crucibles are typically available in two forms based on porosity and thermal shock resistance.

Fully dense, alumina crucibles can be made with a thin wall thickness, and hence low thermal mass.  They have an excellent surface finish, and the low surface area means all powder can be easily removed from the crucible quickly.

The alternative form is a more thermal shock resistant material.  This material has a similar composition (>99.5% alumina) but has a very different microstructure.  Based on a range of fused alumina sizes, not all the porosity is eliminated during manufacture.  As a result, this porosity, acts to retard and deflect cracks that can grow during aggressive thermal cycling.  The wall thickness is generally double that of fully dense alumina, and such the thermal mass increases, but the furnace cycle can be much more aggressive.  A longer life is generally found with these crucibles.

The choice of alumina is often down to the hydroxyapatite manufacturer, and their choice of production method and calcination parameters.   Almath crucibles supplies a number of leading hydroxyapatite manufacturers around the world.  Almath crucibles have supported these businesses since their establishment and are proud to have improved so many lives by supplying into the orthopaedic and dental implant market.

Almath Crucibles

Almath crucibles manufactures and supplies crucibles and technical ceramic components for the medical device industry.  Products include high purity alumina, in fully dense and thermal shock resistant forms.  It is also one of the few companies in the world to produce fully dense MgO and won a prestigious King’s Award for slip casting MgO in 2024 following its Queens Award in 2021 for International Trade.  If you wish to find out more get in touch at:

Almath Crucibles

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