Novel one-step synthesis and characterization of bone-like carbonated apatite from calcium carbonate, calcium hydroxide and orthophosphoric acid as economical starting materials

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Novel one-step synthesis and characterization of

bone-like carbonated apatite from calcium carbonate,

calcium hydroxide and orthophosphoric acid as

economical starting materials

Doan Pham Minh, Ngoc Dung Tran, Ange Nzihou, Patrick Sharrock

To cite this version:

Doan Pham Minh, Ngoc Dung Tran, Ange Nzihou, Patrick Sharrock. Novel one-step synthesis and

characterization of bone-like carbonated apatite from calcium carbonate, calcium hydroxide and

or-thophosphoric acid as economical starting materials. Materials Research Bulletin, Elsevier, 2014, 51,

p. 236-243. �10.1016/j.materresbull.2013.12.020�. �hal-01611993�

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Novel

one-step

synthesis

and

characterization

of

bone-like

carbonated

apatite

from

calcium

carbonate,

calcium

hydroxide

and

orthophosphoric

acid

as

economical

starting

materials

Doan

Pham

Minh

*

,

Ngoc

Dung

Tran,

Ange

Nzihou,

Patrick

Sharrock

Universite´ deToulouse,MinesAlbi,UMRCNRS5302,CentreRAPSODEE,CampusJarlard,F-81013Albicedex09,France

1. Introduction

Calcium hydroxyapatite(Ca10(PO4)6(OH)2,Ca-HA)isfoundto

bethemainmineralphaseofhumanhardtissues[1].However, pure Ca-HA neveroccursinanybiological systembutco-exists withitsderivatives[2].Theseareformedbypartialreplacementof calcium(Ca2+_),_{orthophosphate}_(PO

43!)orhydroxide(OH!)groups

byotherspeciessuchasMg2+_,_Na+_,_K+_,_F!_,_Cl!_,_and_CO

32![3,4].

Amongthem,carbonatedapatite(CAP)istheclosestbiomimetic solid resembling the minerals in calcified tissues [4–6]. The amount of carbonate present in human bone mineral reaches typically4–8wt%[3,7].Recentstudiesshowedthatsubstituted Ca-HA,suchasCAP,weremoreeffectivethanpureCa-HAforskeletal implants[7–9].Thisleadstoagrowinginterestinthedevelopment ofsuchsubstitutedmaterials.

Manyhydroxyapatitetypeshavebeensynthesized, character-ized and examined for use as implantable bone-compatible biomaterials [10–12].Insynthesis proceduresfor pure Ca-HA, atmospheric carbon dioxide can be incorporated in the precipitated solids which lead tophosphate substitution with carbonateions.ThisyieldsCa-HAwithlowcarbonatecontentand

non-stoichiometric Ca/P ratios [13,14]. Onlythe more closely stoichiometricCa-HAwithstandsthehightemperaturerelatedto ceramic sintering or plasma-spraying, without decomposition intolimeortricalciumphosphate(TCP).CAPcontaininghigher carbonateamountscanbesynthesizedbymulti-stepsynthesis proceduresincludingtheprecipitation ofsoluble calciumsalts (Ca2+₎_with_PO

43!inthepresenceofCO32!(liquidroute)[15]or

theheatinguptomorethan10008Cofacalciumphosphateunder carbondioxide flux (thermal route)[16]. These methods have disadvantages of added manipulations for the elimination of waste counter ions or heating step under controlled carbon dioxide.CAPstartingfromcalciumcarbonateasthesourceofboth calcium cations (Ca2+₎ _and _carbonate _anions _(CO

32!) and

potassium dihydrogen orthophosphate (KH2PO4) under

atmo-sphericpressurewasbrieflymentionedbyLomo[17].Thisroute has recently been investigated under higher pressures of hydrothermalconditions[15,18,19].Despitetheseverereaction conditionsused(hightemperature,longreactiontimeandwater rinsing),theconversionofcalciumcarbonatewasnotcomplete

[19]. So, it is of current interest to find simple synthesis proceduresthatleadtoCAPwithcontrolledcarbonatecontents. In this study, the synthesis of CAP using calcium carbonate (calcite),calciumhydroxideandorthophosphoricacidwas investi-gatedundermoderateconditions.Orthophosphoricacidwaschosen becauseofitsstrongestacidityamongtheavailableorthophosphate sourcesandtheabsenceofalkalicationsinthefinalCAPproduct.

Keywords: A.Structuralmaterials B.Chemicalsynthesis C.Thermogravimetricanalysis D.Microstructure D.Thermalexpansion ABSTRACT

There is growing interest in the development of substitutedcalcium hydroxyapatite (Ca-HA) for biomedicalapplications.Carbonatedapatite(CAP)appearsasanimportantsubstitutedCa-HAbecauseof itsbetterbiocompatibilitycomparedtopureCa-HA.Thispaperreportsanovelpressure-mediated one-stepsynthesisofCAPstartingfromorthophosphoricacid,calciteandcalciumhydroxideasavailableand high-puritystartingmaterials.Undermoderatesynthesisconditions(808Cand<13bar),CAPwith differentcarbonatecontentscouldbeobtained.Theratioofcalcite/calciumhydroxidemixtureplayeda crucialroleforboththeadvancementofreactionandthecarbonatecontentinsertedinCAP’sstructure. At808C,thetotaldecompositionofcalciterequiredacalcite/calciumhydroxidemixturecontainingat leastahalfofcalcite.CAPscontaining2.25–4.17wt%ofcarbonateinsertedinitsstructurewereobtained asafunctionofthecompositionofcalcite/calciumhydroxidemixture.Theresultsopenasimplebut effectivewayforthesynthesisofhighqualitybiomimeticCAP.

* Correspondingauthor.Tel.:+33563493258;fax:+33563493043. E-mailaddresses:doan.phamminh@mines-albi.fr,doanhoa2000@yahoo.fr

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Variousmolar ratiosof calcium carbonatetocalcium hydroxide wereusedinordertocontroltheamountofcarbonateinsertedinthe apatitic structure. The results showed that CAP with different carbonatecontentscouldbeobtainedina one-stepsynthesisat 808Candpressuresbelow13atm,withoutawashingstep. 2. Materialsandmethods

Calcitepowder(CaCO3,98wt%)fromFisherScientific,calcium

hydroxide powder (Ca(OH)2, 98wt%) from Acros Organics and

orthophosphoricacid(H3PO4,85wt%inwater)fromMerckwere

used as received. CAP synthesis was carried out in a 250mL stainlesssteelreactor(TopIndustrial)whichwasequippedwithan electricalheatingjacketandamagneticstirrer.Foreachreaction, calcite or mixture of calcite/calcium hydroxide containing 100mmol of calcium and 45mL of water were fed into the reactor.After closing, 66.7mmol of orthophosphoric acid were quicklyinjectedintothereactor.Thisledtothestartingmolarratio ofcalciumtophosphorusof1.67(Table1).Duringthereaction,the stirringratewassetat800rpmandthereactorwasthermostated at808Cfor48h.Thisreactiontemperaturewaschosenbecause supplementaryexperimentsshowedthatthetemperatureslower than808Cwerenotsufficientforagooddecompositionofcalcite particles.After48hofreaction,thereactorwasfreelycooleddown toroomtemperature.Solidproductswereseparatedfromliquid phase by filtrationusing 0.45

m

mfilterpaper. Then they were driedat508Cfor48hbeforefurthercharacterizations.

Bothsolidandliquidphasesobtainedfromthefiltrationstepwere analyzedusingdifferentanalysisandcharacterizationtechniques. Elementalanalysiswascarriedoutusinginductivelycoupledplasma atomicemissionspectroscopy(ICP-AES,HORIBAJobinYvonUltima2 apparatus).Thermogravimetry(TG)wasperformedinaTA Instru-mentsSDTQ600analyzerwithaheatingrateof58Cmin!1_under_air

flux(100mLmin!1_)._{Thermo-mechanical}_analysis_(TMA)_was_carried

outinaSETARAMSetSys16/18analyzerwith5gconstantloadonthe powdersample.X-raydiffraction(XRD)dataofthesolidscollected usingaPhillipsPanalyticalX’pertProMPDdiffractometerwithaCu K

a

(1.543A˚) radiation source. Fourier transform infrared (FTIR) spectroscopywasperformedusingaShimadzu8400Sspectrometer. Particlesizedistribution wasdeterminedbylaserscatteringin a Mastersizer2000(MalvernInstruments Ltd.,Malvern,UK) inthe particlesizerangeof0.020–2000

m

m.Scanningelectronmicroscopy (SEM) was carried out on a Philips XL30 ESEM apparatus (FEI Company) which was coupled with an energy-dispersive X-ray spectroscopy(EDXanalysis).

3. Results

3.1. Elementalanalysis

Theliquidseparatedfromthefiltrationofthereactionmixture wasacidifiedwithconcentrated nitricacidtoavoidall further

re-precipitation. Solidproducts weremineralized using concen-trated nitric acid. The resulting homogeneous solutions were analyzed by ICP-AES technique and results are presented in

Table 2. In all cases, the contents of soluble calcium and phosphorusintheliquidphasewerelowerthan1.4%oftheinitial quantitiesofcalciumandphosphorusintroducedinthereactor.So, theprecipitationof orthophosphatespecieswasquitecomplete and most of the calcium existed in solid phases after 48h of reaction.Becauseoftheabsenceofanycounterionsintheliquid phase,nowashingstepwasrequiredforthepurificationofthefinal solidproducts.Thisbringsasignificantadvantageofthepresent synthesisprocessincomparisonwiththesynthesisusingsoluble calcium salts and/oralkali orthophosphates, since thewashing stepisusuallyarduouswhensmallparticlesareformed.

Themolarratiosofthebulksolidswerehigherthanthatofthe stoichiometric Ca-HA (1.67). This result is discussed in more detailsinthesectionofFTIRanalysis.

3.2. Decompositionofcalciumcarbonate

Thereaction ofcalcite withorthophosphoric acidledtothe formationofcarbondioxidewhichcanexistinbothgasandsoluble states. Pressure in the reactorincreasedwith theformation of carbondioxideingasphase.Takingintoaccountthevolumeofgas phaseinthereactor,thepartialpressureofwatervaporat808C andusingtheidealgaslaw,thequantityofcarbondioxideingas phase could be calculated from the final pressure at 48h of reaction.Fig.1indicatestherelativeadvancementofthereaction viathecalculatedamountsofcarbondioxide.Thedecomposition ofcalcitereachedatleast43,89,83and88%inthesynthesesusing 25, 50, 75 and 100% of calciteas calcium source, respectively,

Table1

Compositionofthestartingreactantmixturesanddesignationofsolidproducts; othercondition:synthesistemperatureof808C;stirringrateof800rpm;initial waterof45g.

Calciumsource Phosphate

source Solidproduct designation CaCO3 powder,mmol Ca(OH)2 powder,mmol H3PO4, mmol 100 0 66.7 CAP100/0 75 25 66.7 CAP75/25 50 50 66.7 CAP50/50 25 75 66.7 CAP25/75 Table2

ResultsofelementalanalysisusingICP–AEStechnique;CaLiq_and_PLiq_:_amount_of calciumandphosphorusremainedintheliquidphase;Ca/P:molarratioofbulk solidproducts.

Solid CaLiq_(%) _PLiq_(%) _Ca/P

CAP100/0 1.4 1.1 1.88 CAP75/25 0.9 1.2 1.84 CAP50/50 1.4 1.2 1.82 CAP25/75 1.3 1.2 1.74

10.9

44.4

62.3

87.9

0

10

20

30

40

50

60

70

80

90

100 CO

2 ga s

, m

m

ol

Fig.1.Quantityofcarbondioxideingasphase(COgas2 )calculatedfromthefinal reactionpressureasafunctionofthemolarpercentageofcalciteintheinitial mixtureofcalciteandcalciumhydroxide.

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whichdoesnotconsidertheamountofcarbondioxidedissolvedin theliquidphase.

3.3. XRDcharacterization

Inordertoidentifycrystallinephasesofthesolidproducts,XRD characterizationwasperformedandtheresultsarepresentedin

Fig.2.Forthesolidproductstartingfrom100%calciumcarbonate (CAP100/0), alldiffractionpeakscouldbeattributedtocalcium hydroxyapatite (Ca-HA). CAP might be also present but they seemedtoberelativelylowincomparisonwiththatofCa-HA.No

evidenceofcalciumcarbonatewasobserved,indicatingthatthe decompositionoftheinitialcalcitemustbequitecompleteforthis synthesis.

BothCAP75/25andCAP50/50products,whichwereformedfrom the mixture of calcite and calcium hydroxide, had similar XRD patternscomparedtothatofCAP100/0.Mostoftheirpeakscouldbe alsoattributedtoCa-HA.Traceamountsofoctocalciumphosphate (OCP,Ca8(HPO4)2(PO4)4"5H2O)couldbealsofound.Infact,thefinal

pHofthereactionmixturewasaround7–8.ThispHisfavorablefor theformationofOCPbecauseinthispHrange,themainsoluble phosphatespeciesareH2PO4!and HPO42![26].Thisresultwas

supportedbypreviousworkonthesynthesisofCa-HAundersimilar conditions,atatmosphericpressure[26].Asobservedpreviouslyfor CAP100/0,calcitewasalmostabsentinthesetwosolidproducts, whichwasinagreementwiththeresultsinFig.1.

Ontheotherhand,theresultschangednotablyforCAP25/75. Calcite, Ca-HA and OCP werefound tobe themain crystalline phases present in this sample. A small amount of crystalline calciumhydroxidewasfound,asillustratedbythelowintensityof itsmaindiffractionpeakat39.068.Calciumhydroxidehashigher solubility and basicity than calcite. Thus, orthophosphoric acid introduced in the mixture of calcite and calcium hydroxide preferentially consumes the calcium hydroxide. When a high amountofcalciumhydroxideisusedsuchasforthesynthesisof CAP25/75,thepHofreactionmixtureremainsathighvalue,which disfavorsthedecompositionofcalcite.

Inallcases,XRDresultsrevealedthepresenceofCAPatlevels muchlowerthanthoseofCa-HA.TheidentificationofCAPbyXRD isdelicatebecauseinmostcases,themaindiffractionpeaksofCAP areclosetothoseofCa-HAandwouldrequireRietveldrefinement forincreasedresolution.Othercharacterizationtechniqueswere usedtohighlighttheformationofCAP.

3.4. IRcharacterization

IRspectraofthesolidproductsarepresentedinFig.3.Inthe wavenumberrangefrom4000to1700cm!1_(not_presented),_there

10 15 20 25 30 35 40 45 50 55 60 65 70 Counts (a.u) 2 CAP100/0 CAP75/25 CAP50/50 CAP25/75 * * ** * *_* * ** * * * * * **_{*** * * * * * *}_{* * *} ** * * * * * * * * * * * * * ***_{** * *} _* * * * * * * * * * _* *_** ** * * *_** * * * * * * * * * * * * * *

Fig.2.XRDpatternsofthesolidproducts;(*)Ca-HA(JCPDSstandardNo 01-072-1243);(&)calcite(JCPDSstandardNo00-047-1743);(^)OCP(JCPDSstandardN0 00-026-1056);(")calciumhydroxide(JCPDSstandardNo00-050-0008).

550 750 950 1150 1350 1550 Trans m itt ance (a. u. ) Wavenumber (cm1₎ CAP100/0 CAP75/25 CAP50/50 CAP25/75 1415 1450 CO CO Phosphates Phosphates CO 1545 870 880

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wasonly a very broadweak peak around 3500cm!1_, _which _is

attributedtothestretchingofmolecularwater.Thevibrationsof orthophosphategroupsarecharacterizedbytheabsorptionbands inthewavelengthrangesof 650–550cm!1_and _1300–910_cm!1

[20].AccordingtoXRDresults,calcitewasonlynotablypresentin CAP25/75asconfirmedbyitsnetabsorptionbandat711cm!1_.

Thisisoneprincipalbandofcalciteandisnotsuperimposedwith bandsoforthophosphatesgroupsorcarbonatesgroupspresentin apatiticstructure[21].

The presence of carbonate groups inserted in the apatitic structure to form CAP wasclearly revealed by IR spectra. The replacement of PO43! groups present in Ca-HA’s structure by

CO32! groups led to the formation of B-type CAP, which is

characterizedbythebi-modalpeak at1450/1415cm!1_and _the

peakat870cm!1_[22,23]_._Thus,_B-type_CAP_was_present_in_all_solid

productsbutitscontentwashigherinCAP100/0,CAP75/25and CAP50/50 than that in CAP25/75. The higher intensity at 1415cm!1_compared_to_that_at₁₄₅₀_cm!1_of_CAP25/75_was_due

totheprincipalpeakat1389cm!1_of_calcite_that_remained_in_this

product.TheformationofB-typeCAPexplainedthemolarratioof Ca/P in Table 1, which was slightly higher than that of the stoichiometricCa-HA. Ontheotherhand,A-typeCAPis formed when OH! _groups _of _Ca-HA’s _structure _are _replaced _by _CO

32!

groups,whichhavethecharacteristicbandsat1545and880cm!1

[22,23].So,A-typeCAPwasonlyformedinCAP100/0,CAP75/25 andCAP50/50,andwaspracticallynotpresentinCAP25/75. 3.5. Thermalanalysis(TG)

Fig.4showstheresultsofthethermalanalysisoftheproducts inthewiderangeoftemperaturesfrom25to14008C.Aweightloss smallerthan1wt%tookplacebelow1008Cwhichcorrespondsto theremovalofsurfacemoisture.Thenextweightlossof1–2wt%, which occurredcontinuouslyfrom 1008C toabout 340–3808C, could be attributed to the removal of lattice molecular water

[24,25].Theweightlossinthetemperaturerangeof380–5008C wasattributedtothecondensationofHPO42!groups,presentin

nonstoichiometricapatitesuchasOCP,toformcalcium pyrophos-phate(Ca2P2O7),whichwasobservedforallproducts.Remaining

calcite decomposedat 610–6708C [26]. DTGsignalof both the condensationofHPO42!groupsandthedecompositionofcalcite

increasedwiththeincreaseofcalciumhydroxidecontentinthe initial reactant mixture. This showed the unfavorable effect of calcium hydroxide contentfor the decomposition of calcite,as explained in XRD section. According to XRD results, unreacted calciumhydroxideremainedonlynotablyinCAP25/75.Itsthermal decompositionischaracterizedbytwoseparatedpeaks[27].The firstweightlossstartingat3258Cwaspartiallysuperimposedwith thecondensationofHPO42!groups,andthesecondweightloss

startingat6008Cwaspracticallysuperimposedwiththethermal decompositionofcalcite,highlightedbyitsDTGcurve.

ThedecarbonationofCAPtookplaceinthetemperaturerange of670–12508C.AccordingtoIRresults,A-typeCAPwaspractically not present in CAP25/75. Thus, we deduce that the thermal decarbonationofA-typeCAPandB-typeCAPtookplaceat870– 12508Cand670–8708C,respectively.Finally,thelastweightloss at1250–13408CwasattributedtothedehydrationofOH!_groups

presentinCa-HA’sstructure[28].

86 88 90 92 94 96 98 100 0 200 400 600 800 1000 1200 1400 TG (w t% ) Temperature (°C) CAP100/0 CAP75/25 CAP50/50 CAP25/75 (A) 0 200 400 600 800 1000 1200 1400 DT G (a. u) Temperature (°C) CAP100/0 CAP75/25 CAP50/50 (B) CAP25/75

Fig.4.TG(A)andDTG(B)curvesofthesolidproductsanalyzedinthetemperaturerangeof25–14008Cunderairatmosphere.

Table3

CarbonatecontentsinthesolidproductsdeterminedbyTGanalysis. Product CO32!(wt%) CaCO3 (610–6708C) B-typeCAP (670–7808C) A-typeCAP (780–12508C) CAP (670–12508C) CAP100/0 0.30 0.63 3.5 4.17 CAP75/25 0.75 0.69 2.73 3.42 CAP50/50 1.20 0.63 1.62 2.25 CAP25/75 <4.7a 0.54 0 0.54

a _{Superimposition}_of_TG_signals_of_CaCO

3decompositionandCa(OH)2 dehydra-tion,givinganapproximatevalue.

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From TG curves in Fig. 4, the content of different types of carbonate present in the solid products could be calculated (Table 3). Asobservedby XRDcharacterization,theincrease of calciumhydroxidecontentintheinitialmixtureofcalciumsource ledtotheincreaseofremainingcalcitecontent.Inallcases,the carbonatecontentsofB-typeCAPwereclosetoeachother.Onthe otherhand,thecarbonatecontentofA-typeCAPdecreasedwith theincreaseofcalciumhydroxidecontentintheinitialmixtureof calciumsource,whichwasingoodagreementwithIRresults.The totalamountof carbonateobtained inCAP100/0and CAP75/25 approximated theamount of carbonate present in themineral phaseofbone,dentinandenamel,whichisintherangeof4–8%

[3,7,22].Itisevidentthatthecompositionoftheinitialmixtureof calciumsourceisanefficientparametertocontrolthecarbonate contentfortheone-stepsynthesisofCAP,sincetheconversionof bothinitialcalciumandorthophosphatesourcesintodesiredsolid productswaspracticallytotalinthesynthesisofCAP100/0,CAP75/

25,CAP50/50.Takingintoaccountthequasi-complete decompo-sitionofcalcite,thehighselectivityinapatiticcompoundsandthe weightofthesolidproductsrecoveredafterfiltrationanddrying steps,theyieldofthesynthesisprocedureindesiredCAPpowder could be calculated which reached at least 97% for CAP100/0, CAP75/25andCAP50/50.

3.6. SEM,particlesizedistribution

TheresultsofSEMobservationareillustratedinFigs.5and6.In theleft-handsideofFig.5(100

m

mscale),particlesofsizesranging fromseveralnmtohundredsof

m

mcouldbeobservedforCAP100/ 0,CAP75/25andCAP50/50.Theimagesontheright-handsideof

Fig.5(10

m

mscale)revealedthatthelargerparticlesconsistedof theagglomerationofsmallerones.

Theagglomeratesofthesesolidshadaflat-needle-like morphol-ogy.Thiswaspreviouslyobservedforapatiticcalciumphosphates

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synthesized underhydrothermal conditions [19,29] or at atmo-sphericpressure [30]. Theformation ofthese particles could be attributedtothegrowthofCAPparticlesalongthec-axisdirection, whereCO32!groupsreplacedOH!groups[31,32].ForCAP50/50,in

additiontothepresenceofparticlesofflat-needle-likemorphology, particles ofsheet structure couldbe alsoobserved,which were attributedtoapparentmorphologyofOCP,whichcontainsHPO42!

groups[33].

SEMimagesofCAP25/75areshowninFig.6.At100

m

mscale correspondingtoamagnificationof200times(Fig.6(A)),particle sizesvariedalsoina largerangeasobservedinFig.5 forother solids. At higher magnification, calcium phosphate particles of sheet-likestructurewereclearlyobserved,whichwasattributedto thepresenceatlargeamountofOCP(Fig.6(B)).Remainingcalcite particles were also frequently present, as confirmed by EDX

analysis(Fig.6(C)).Weobservedalsothattheremainingcalcite particleswereusuallycoveredbycalciumphosphatelayers.This explainedwhytheFTIRsignalofcalcitewasnotsignificantdespite itshighcontentinCAP25/75(Fig.3).

TobetterquantifytheresultsofSEMobservations,particlesize distribution was examined and Fig. 7 shows the volume distribution as a function of particle size. The initial calcite powderhad aGaussiandistributionintheparticle sizeranging from6to70

m

m.Ontheotherhand,theinitialcalciumhydroxide haddifferentcategoriesofparticlesizes,whichvariedfrom0.3

m

m toabout240

m

m.

The volume distribution of all four solid products can be classified into two categories. The first one consisted of fine particlesrangingfrom0.35toabout1.5

m

m.Thevolumeoccupied bythiscategorywasnotsignificantanddidnotexceed1.4%ofthe totalvolumeofthesolids.Thesecondoneincludedparticleslarger than1.5

m

m,whichstretchedupto310,400,450and710

m

mfor CAP100/0, CAP75/25, CAP50/50 and CAP25/75, respectively, indicatingdifferentlevelsofagglomerationofparticlesforeach product. The higher the calcium hydroxide content was, the higherthepHofcalcite/calciumhydroxidemixturewas,andthe more the acid–base reaction was violent leading to stronger agglomeration. In all cases, these results confirmed SEM observationsofFigs.5and6.

3.7. Thermo-mechanicalanalysis

Used as biometerials, apatitic calcium phosphate based productscanbestabilizedathightemperatureinordertoavoid allfurtherevolutionofmaterials.Thissectionwasdedicatedtothe study of the thermo-mechanical behavior of CAP100/0 as the selectedproduct.

Thermalshrinkageisdefinedas(L!L0)/L0or

D

L/L0,whereL0is

theinitiallengthofsample,andListhelengthofsamplemeasured at temperatureT or timet.Fig.8(A)shows thenon-isothermal analysisoftheCAP100/0sampleinthetemperaturerangeof30– 10008C.Thetemperatureof8808Cwasfoundtobethecritical pointforthethermalshrinkageofCAP100/0wheretheshrinkage accelerated. Only a slight thermal effect on the shrinkage was observedbelow8808Cwhichwassmallerthan0.02%.Ontheother hand,itbecamesignificantabove8808Candreached!0.46%when thetemperatureroseto10008C(Fig.8(A)).

Fig. 8(B) illustrates the isothermal shrinkage at different temperaturesbeforeandafterthecriticalpoint.Theinitialvalue of

D

L/L0ofeachcurvecorrespondedtotheshrinkageofthesample

Fig.6.SEMimagesofCAP25/75.

0 10 20 30 40 50 60 70 80 90 100 0.1 1 10 100 1000 Vo lu me a cc umu la tio n, % Particle size, µm CAP100/0 CAP75/25 CAP50/50 CAP25/75 Initial CaCO3 Initial Ca(OH)2

Fig.7.Volumedistributionasafunctionofparticlesize(inbase10logarithmic scale).

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during the heating time to reach the desired temperature. No significantthermaleffectwasobservedat600and8008Cforan isothermal time of 300min, confirming that CAP100/0 was thermo-mechanicallystableatthesetemperatures.Asexpected, theisothermalshrinkagetookplaceat10008Candreached!3.85% after300min.

The thermal shrinkage can be attributed to the sintering phenomenon wherein denser mass is formed and chemical reaction, i.e. the decarbonation or dehydration. Fig. 9 shows SEMimagesofCAP100/0sampleafterTMA.Theisothermaltimeof

300minat600and8008Chadnosignificanteffectonthesurface morphologyofCAPparticles,comparedtothesolidbeforethermal treatment(Fig.5fortheCAP100/0sample).At10008C,particlesof flat-needle-like morphology became only partially rounded. Probably, this morphology distortion of CAP particles corre-spondedtothefirststep ofsintering wherein theformationof thecontactareasbetweenadjacentparticlesstarted[34].

TG analysis showed that the heating of CAP solids led to different chemical reactions. We are interested in the thermal stabilityofcarbonateanionspresentinapatiticstructureofCAP.As

-4.0

-3.0

-2.0

-1.0

0.0

0

200

400

600 800 1 000

Sh

rin

ka

ge

(

%

)

Temperature (°C)

880 o_C

(A)

-4

-3

-2

-1

0

50 100 150 200 250 300

Sh

rin

ka

ge

(

%

)

Time (min)

(B) ₆₀₀_o_C 1000 o_C 800o_C

Fig.8.TMAofCAP100/0;(A)non-isothermalanalysiswiththeheatingrateof108Cmin!1_;_(B)_isothermal_analysis.

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showninFig.10forIRspectraofCAP100/0samplebeforeandafter TMA, the decarbonation of both A-type (bands at 1545 and 880cm!1₎_and_B-type_(bands_at_1450,₁₄₁₅_and₈₇₀_cm!1₎_CAP

wasonly partialafter theplateau time of 300min at different temperatures.Thisdemonstrates thepossibilitytostabilizeCAP usingthermaltreatmentwhileconservingitscarbonatecontent. 4. Conclusions

Forthefirsttime,carbonatedapatite(CAP)ofhighpuritywas successfullysynthesized by a one-step synthesis process using orthophosphoric acid, and a mixture of calcite and calcium hydroxideassimplestartingreactantsundermoderateconditions (808C, <13bar). Carbon dioxide was formed as the only by-productandnowashingstepwasrequiredforthepurificationof finalproducts.BothA-typeandB-typeCAPwereformedunderthe synthesisconditionsused.

Thetotal contentof carbonateinserted in apatitic structure couldbecontrolledby varyingthecalcitecontentin theinitial mixture of calcium source. We obtained CAPcontaining 2.25– 4.17wt%ofcarbonatewiththeinitialmixtureofcalciumsource containing 50–100% calcite. For a complete decomposition of

calcite, an initial mixture of calcite and calcium hydroxide containingatleast50%ofcalcitewasmandatory.

TheCAP100/0samplepreparedfrom100%calciteand contain-ing4.17%carbonate,wasfoundtobethermo-mechanicallystable upto8808C.ThisoffersagoodpossibilityforthesynthesisofCAP forbiomedicalapplications.

Acknowledgments

Theauthorsacknowledge gratefullycolleaguesatRAPSODEE center,NathalieLyczko,ChristineRollandandPhilippeAccartfor differentcharacterizationmeasurements.

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550 750 950 1150 1350 1550 Tr an sm itt an ce ( a. u .) Wavenumber (cm-1₎ 105o_C 600 o_C 800 o_C 1000 o_C 1545 1450 1415 870 880

Fig.10.IRspectraofCAP100/0before(1058C)andafter(600,800and10008C) TMA.