Powder mixture for the production of microporous ceramics based on hydroxyapatite

Powder mixture with given molar ratio Ca/P = 1.67 consisting of brushite (calcium hydrophosphate dihydrate) CaHPO4·2H2O, calcium oxalate monohydrate CaC2O4·H2O in form of whewellite and weddellite and some quantity of quasi-amorphous phase was obtained as a result of the interaction of hydroxyapatite powder Ca10(PO4)6(OH)2 with an aqueous solution of oxalic acid H2C2O4 at a molar ratio of Ca10(PO4)6(OH)2/H2C2O4 = 1:4 under mechanical activation conditions. This powder mixture was used to produce microporous monophase ceramics based on hydroxyapatite Ca10(PO4)6(OH)2 with aperient density of 1.25 g/cm3 after firing at 1200 oC. Microporosity of sintered ceramics was formed due to presence of particles with plate-like morphology, restraining shrinkage during sintering. Microporous ceramics based on hydroxyapatite Ca10(PO4)6(OH)2 with roughness of the surface as a consequence of the created microporosity can be recommended as a biocompatible material for the bone defects treatment and as a substrate for bone cell cultivation.


Introduction
The creation of ceramic materials based on calcium phosphates is one of the intensively developing areas of modern materials science for medicine [ 1 ]. These materials are biocompatible and can be used in medicine as a porous matrices for replacing lost or damaged bone tissue or as substrates for cell cultivation. Ceramics based on hydroxyapatite Ca10(PO4)6(OH)2 (HA) are widely used as material for bone implants creation due to its stability and the similarity of chemical and phase compositions of inorganic part of natural bone [ 2 , 3 ].
Ceramics for bone implants have to be porous with at least two levels of porosity to mimicking the natural bone. Macro pores have to be not less than 100 m and the dimension of micro pores should be about 10 m [ 4 , 5 ].
Microporosity of ceramics giving roughness to the surface can improve biointegration and osteoconductivity of material and ensure effective fixation and reproduction of bone tissue cells, as well as fusion of the implant with the body [ 6 ].
Ceramics based on hydroxyapatite are very often prepared from HA powder previously synthesized by various methods. The simplest and most common way to obtain hydroxyapatite powder is its precipitation from an aqueous solution of the corresponding substances, which are phosphoric acid or soluble phosphates of ammonium, sodium, potassium as phosphate ion sources and, soluble calcium salts (acetate, nitrate, chloride) as calcium ion sources [ 7 , 8 ]. Another way to prepare HA powders or ceramics consisted in heat treatment of preliminarily homogenized powder mixtures of different salts with the preset molar ratio of Ca/P = 1.67 [ 9 ].
To create microporosity one can use special thermal treatment scheduler to get undersintered ceramic material [ 10 ] or specially via sol-gel synthesis prepared powder system [ 11 ]. Another method of microporosity creation consists in using organic [ 12 , 13 ] or inorganic [ 14 , 15 ] additives in form of particles with a small dimensions as a sacrificed porogens. Special additives which have ability to decompose with the release of a sufficiently large volumes of gases at different stages of ceramics production can be used to generate microporosity. CO2, NH3 from NH4CO3 can be used for example at the stage of slurry preparation [ 16 ] or CO2 from sodium of potassium carbonates in presence of melt can be used at the stage of heat treatment [ 17 ] for creation of microporosity of material.
In the present work for preparing of microporous HA-ceramics we used intentionally prepared powder mixture including particles with the plate-like morphology expecting that particles with this form would restrain sintering.
The purpose of this work consisted in preparing and investigation of powder mixture with preset molar ratio Ca/P=1,67 including plate-like particles which able to restrain shrinkage during sintering of HA-ceramics for microporosity creation. To prepare powder mixture with preset molar ratio Ca/P=1,67 powder of HA was treated in water solution of oxalic acid H2C2O4 under mechanical activation conditions. We expected that interaction of basic calcium phosphate salt (Ca10(PO4)6(OH)2, HA) with water solution of oxalic acid H2C2O4 give us opportunity to prepare powder mixture including brushite (calcium hydrophosphate dihydrate) CaHPO4·2H2O and calcium oxalate monohydrate CaC2O4·H2O.
Pre-ceramic powder compacts in the form of discs with a diameter of 12 mm and a height of 2-3 mm were made from prepared powder mixture using a manual press (Carver Laboratory Press model C, USA) at 100 MPa using a steel mold. Then the samples were fired in a furnace at 500 °C, 1000 °C, 1100 °C and 1200 °C with exposure at the specified temperatures for 2 hours (the heating rate of the furnace was 5 °C/min). The mass, linear dimensions of the samples were measured before and after firing. Then linear shrinkage and density of samples were calculated. Prepared powder mixture was additionally heat treated at 200 °C with exposure at this temperature for 30 minutes (the heating rate of the furnace was 5 °C /min) for better understanding of processes of transformation of phase composition of powder system under heating.
The phase composition of the powder mixture after treatment in a planetary mill and after heat treatment at 200 o C as well as ceramic samples after firing was determined by X-ray powder diffraction (XRD) analysis on a Rigaku D/Max-2500 diffractometer (Japan) with a rotating anode, using Cu-Ka radiation (average wavelength λ = 1.54183 Å), accelerating voltage 50 kV, tube current 250 mA, angle interval 2Ѳ: from 2° to 70°, step 2Ѳ -0.02°, speed 4°/min). Phase analysis was performed using ICDD PDF2 database [ 18 ] and Match!3 software (https://www.crystalimpact.com/). Prepared powder mixture after drying was examined by synchronous thermal analysis (TA), which was performed on a NETZSCH STA 449 F3 Jupiter thermal analyzer (NETZSCH, Germany) at a heating rate of 10 °C/min. The mass of the sample was at least 10 mg. The composition of the gas phase formed during the heating of powder mixture was studied using a quadrupole mass spectrometer QMS 403 Quadro (NETZSCH, Germany) combined with a thermal analyzer NETZSCH STA 449 F3 Jupiter. Mass spectra (MS) were recorded for the mass numbers 18 (H2O); 44 (CO2).
Powder mixture after treatment in a planetary mill and ceramics after firing were examined by scanning electron microscopy (SEM) on LEO SUPRA 50VP electron microscope (Carl Zeiss, Germany; auto-emission source). This investigation was carried out at an accelerating voltage of 3-20 kV in secondary electrons (SE2 detector). The surface of the samples was coated with a layer of chromium (up to 10 nm).

Results and Discussion
According to the XRD data ( Figure 1), the phase composition of powder mixture obtained as a result of the interaction of HA Ca10(PO4)6(OH)2 powder with a 1M water solution of oxalic acid H2C2O4 under mechanical activation condition and drying in air for a week consisted of brushite (calcium hydrophosphate dihydrate) CaHPO4·2H2O and calcium oxalate monohydrate CaC2O4·H2O in form of whewellite (PDF card 20-231) and weddellite (PDF card 17-541).  The XRD data confirmed that reaction (1) [ 20 ]. Additionally, the Match! Phase Analysis Report marked 30% as unidentified peak area. Phase composition of prepared powder mixture determined by using different programs are in good correlation. It should be noted that some quasi-crystalline phases not detected via XRD could form during interaction of HA Ca10(PO4)6(OH)2 powder with water solution of oxalic acid H2C2O4. Figure 2 shows a micrographs of a powder mixture obtained as a result of the interaction of HA Ca10(PO4)6(OH)2 powder with an 1M water solution of oxalic acid H2C2O4 under conditions of mechanical activation and drying in air during 1 week. On the micrograph, one can see two kinds of particles: particles of a plate-like morphology with dimensions about 10-20 mm (Figure 2, a) and particles of isometric morphology and dimensions up to 100 nm. The plate-like morphology is inherent to brushite CaHPO4·2H2O according to scientific literature data and our experience [ 21 ]. So, we could assume that particles with isometric morphology and dimensions up to 100 nm were weddellite or whewellite (calcium oxalate monohydrate) CaC2O4·H2O. One can see that these isomeric particles presented in powder as aggregates with dimensions of 500-1000 nm and as individual particles on the surface of plate-like brushite CaHPO4·2H2O particles.   Figure 3 shows the data of synchronous thermal analysis: thermogravimetry (TG) and differential scanning calorimetry (DSC) curves for the studied powder mixture when heated from 40 °C to 1000 °C. Figure 4 shows the mass spectra of evolving gases with m/Z = 18 (H2O) and m/Z = 44 (CO2) resulting from the thermal decomposition of components of the powder mixture. The total mass loss of the powder mixture when heated up to 1000 o C was 33%. It should be noted that if powder mixture consisted only from product formed according reaction (1) the total mass loss according calculation would be 42%. This fact can additionally points on possible presence of non-detected by means of XRD less hydrated quasi-amorphous products formed during treatment of HA Ca10(PO4)6(OH)2 powder in water solution of oxalic acid H2C2O4 in the condition of mechanical activation. There are 3 noticeable steps on the curve of mass loss. The mass loss at the first step is estimated as 14% (90 -300 o C), at the second step -10 % (300-550 o C) and at the third step -9 % (550-750 o C). . It should be noted that simulations presence of these two hydrated salts can influence on thermal decomposition processes of each of them. Probably the mass loss at 130 o C can be explained with possible interaction of these two hydrated calcium salts. Also, this peak may reflect the process of thermal transformation of any undetected by XRD quasi-amorphous phase presence of which estimated as possible.
Phase composition of ceramics after firing at 1100 o C included HA (Ca10(PO4)6(OH)2) and small quantity of -tricalcium phosphate -Ca3(PO4)2. And finally phase composition of ceramics after firing at 1200 o C included the only phase -HA (Ca10(PO4)6(OH)2). *small quantity XRD data of samples after heat treatment at different temperatures shows that formation of single-phase HA-ceramics from multi-components homogenized powder mixture took place as complicated sequence of different heterogeneous reactions, including thermal decomposition reactions and solid-state reactions. As it is knowing from the scientific literature heterophase reactions can take place during firing and accompany sintering process of HA-ceramics [ 26 , 27 ]. Investigation presented in this article emphasizes the importance of the preset Ca/P=1,67 molar ratio in starting powder mixture when preparing ceramics based on HA. Presetting of Ca/P=1,67 in starting powder mixture in this work was guaranteed both by high quality of HA powder used and by preparation of powder mixture in via acid-base reaction in mechanical activation condition. This method excludes changes in preset Ca/P molar ratio due to difference in solubility of starting components or influence of pH on preferability of formation of one or other phases as it would possible in case of precipitation of calcium phosphate powders from solutions.
Micrograph of the surface of the ceramic sample based on powder mixture including brushite (calcium hydrophosphate dihydrate) CaHPO4·2H2O and calcium oxalate monohydrate CaC2O4·H2O in form of whewellite and weddellite after firing at 1200 °C is presented at Figure 6. Microstructure of ceramic sample fired at 1200 o C consisted of polycrystalline plate-like particles with dimensions 5-15 m, arched groups of particles 0,5-2 m, and two kinds of pores with dimensions about 10 and 1-2 m. Obviously microstructure of HA-ceramics inherits microstructure of starting powder mixture. It can be assumed that formation of phase of HA was realized both on the surface of nano sized particles of calcium oxalate/calcium carbonate and on the surface of micro sized plate-like particles of brushite/monetite/calcium pyrophosphate.  Apparent density (g/cm 3 ) and relative diameter (D/D0, %) of ceramic samples after firing at different temperatures are presented at Figure 7. Linear shrinkage increased very slowly from 1% at 500 oC up to 2,7% at 1100 o C and reach the maximum 7 % at 1200 o C. Liner shrinkage of HA-ceramics based on uniform synthetic HA powders consisted of isometric particles could reach about 20% [ 28 ] or even more than 20% [ 29 ,]. Ceramic samples prepared from powder mixture (CaHPO4·2H2O, CaC2O4·H2O, non-identified quasi-amorphous phase) had the maximum density 1.25 g/cm 3 (~ 40 % relatively theoretical density of HA) after firing at 1200 o C. So, microstructure of HA-ceramics ( Figure 6), data of apparent density and relative diameter after firing (Figure 7) confirm possibility to created microporosity via using of powder mixture with plate-like particle restraining shrinkage during sintering.

Conclusions
Powder mixture with given molar ratio Ca/P = 1.67 consisting of plate-like particles of brushite (calcium hydrophosphate dihydrate) CaHPO4·2H2O; nanosized isometric particles of calcium oxalate monohydrate CaC2O4·H2O in form of whewellite and weddellite; and some quantity of non-identified quasi-amorphous phase was obtained as a result of the interaction of HA powder Ca10(PO4)6(OH)2 with an aqueous solution of oxalic acid H2C2O4 at a molar ratio of Ca10(PO4)6(OH)2/H2C2O4 = 1:4 under mechanical activation conditions. This powder was used for creation of microporous monophase HA Ca10(PO4)6(OH)2 ceramics. Components of prepared powder mixture with preset Ca/P=1.67 molar ratio take part both in sequences of thermal transformations such as dehydration and decomposition and then in sequences of hetero phase reactions leading to final and target phase composition of ceramics presented by HA Ca10(PO4)6(OH)2. It was shown that plate-like particles presented in the powder used for ceramic creation can restrain sintering process and provide the formation of microporosity of HA Ca10(PO4)6(OH)2. ceramics.