Controlling the Structure of Hydroxyapatite Ceramics with

Research ArticleControlling the Structure of Hydroxyapatite Ceramics withDual Pores Prepared Using Freeze-Casting

Z Cheng 1 Y C Yang2 and Z P Wu3

1School of Material Engineering Shaanxi Polytechnic Institute Xianyang 712000 China2School of Mechanical and Precision Instrument Engineering Xirsquoan University of Technology Xirsquoan 710048 China3School of Material Science and Engineering Xirsquoan University of Technology Xirsquoan 710048 China

Correspondence should be addressed to Z Cheng subrinaing126com

Received 20 March 2018 Revised 5 June 2018 Accepted 11 June 2018 Published 17 July 2018

Academic Editor Valery Khabashesku

Copyright copy 2018 Z Cheng et al )is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

A hydrogen peroxide solution in deionized water was used as the pore-forming agent in freeze-casting to fabricate a hy-droxyapatite porous ceramic containing both lamellar and spherical macrospores )e influence of the volumetric content ofH2O2 in the mixed solvent on the morphology of the porous ceramic was analyzed and the effect of the solvent cooling speed onthe microstructure and pore morphology of the pore ceramic was also investigated )e results show that H2O as the sole pore-forming agent led to the formation of a HA scaffold with only lamellar pores When 3 (vv) of H2O2 solution was used bothlamellar and spherical pores were observed However when the concentration of H2O2 reached 9 (vv) only spherical poreswere formed in the porous ceramic )e average lamellar spacing and the spherical pore size of the pore ceramic composed of HAdecreased with the increase of the cooling speed

1 Introduction

Porous ceramics are widely used as carriers for catalysts insound absorption and insulation heat preservation filteringand separation feedback sensors and general biology [1] In2000 Fukasawa et al reported the first preparation of porousceramics by freeze-casting [2] After this seminal study mostresearch focused on the development of pore-forming agentsused in freeze-casting in order to control the pore structureZhang et al used a water-based slurry as the pore-formingagent to prepare porous ceramics with lamellar pores usingthe freeze-casting method and achieved a porosity of 94[3] Soon et al used a camphor-based slurry as the pore-forming agent and adopted freeze-casting to obtain a dendriticporous ceramic [4] Subsequently Wang et al conductedresearch on a tert-butanol-based slurry as the pore-formingagent obtaining a porous columnar ceramic with cellularmorphology [5] )e pore structure of porous ceramics isdetermined by the pore-forming agent that is by the crystal

morphology of the liquid-phase solvents )us the porestructure of porous ceramics is strongly dependent on thesize and shape of crystalloids solidified from the liquid-phasesolvents In the freeze-casting process the liquid-phaseslurry should be carefully investigated Generally a slurrywith a moderate solidification temperature small rate ofvolume change during the solidification process low liquidviscosity and high vapor pressure in the solid state is optimalfor obtaining high-porosity ceramics [6] To date only threepure substances water camphene and tertiary butanolwere found to meet those requirements and are commonlyused as pore-forming agents [6]

To obtain ceramics with diverse pore structures thatmeet the requirements of different applications the devel-opment of different kinds of pore-forming agents is veryimportant Recently the use of slurries based on solutes suchas NaCl sucrose and polyethylene glycol mixed with wateras pore-forming agents was reported and porous ceramicswith pores of diverse morphologies were obtained [7 8] For

HindawiJournal of NanotechnologyVolume 2018 Article ID 7531464 5 pageshttpsdoiorg10115520187531464

example Mohamed et al used 14-dioxane and glycerin inwater as the pore-forming agent However the resultingporous ceramics still contained lamellar pores with a mor-phology similar to those obtained when using a water-basedslurry [9] To date there are no reports on the manufactureof porous ceramics with spherical and lamellar pores usingcomplex solvents as pore-forming agents

It is known that hydrogen peroxide can disproportionateto release oxygen gas )e authors believed that if the hy-drogen peroxide was mixed with water spherical holesinduced by oxygen and lamellar pores induced by watercould be obtained simultaneously generating spherical andlamellar pores at the same time Based on this ideaH2O2H2O mixtures with different H2O2H2O ratios wereused as the pore-forming agent to prepare porous ceramicsFinally the size and shape of the pores were controlledsuccessfully by tuning of the cooling speed

2 Materials and Methods

21 Starting Materials Hydroxyapatite (HA) powder witha grain size ranging from 05 to 10 microm (Ca10(PO4)6(OH)2Alfa Aesar USA) was used as the raw material Hydrogenperoxide (H2O2 Tianjin Bodi Chemical Co Ltd) anddistilled water in different proportions were used as sol-vent Carboxymethyl cellulose (Ningbo Cellulose De-rivative Plant China) and sodium polyacrylate (Mw 8000Alfa Aesar) were used as a binder and dispersing agentrespectively

22 Preparation of Scaffold Firstly H2O2H2O solutionswith H2O2 content of 3 4 and 9 (vv) were freshly pre-pared )en add dispersing agent and adhesive to the so-lutions at 1 and 2 of solute mass respectively mixed withHA powder after which a sizing agent with 25 (vv) ofsolid content was prepared )e agent was stirred usinga magnetic apparatus for 30min at 25degC and subsequentlyground in a roller-type ball mill (self-made equipment) for24 h at 25degC

)e ceramic slurry was poured into the mold(φ12mmtimes 30mm) which comprised two sections )e sideand top were made of the thermal insulation material and thebottom was made of metal with high thermal conductivity)is mold design was chosen to ensure that the freeze di-rection is from the bottom to top thus guaranteeing anoriented freezing of the ceramic slurry (Figure 1) )e elec-tronic probe of a freeze-drier (VFD2000G Boyikang Co LtdBeijing China) was used to measure the cryogenic temper-ature of the sizing agent to obtain the freezing curve Afterdemolding the frozen sample was lyophilized at ndash20 to 0degCfor 24 h and subsequently sintered at the temperature of1250degC for 2 h )e heat preservation period was 2 h

23 Characterization of the HA Scaffold A JSM-6700scanning electron microscope (SEM JEOL Japan) wasused to observe the morphology of the porous ceramic PCIView software (JEOL Japan) was used to measure pore sizein the electromicrographs From each group 5 samples were

selected each of which was tested at 2 to 3 points and theweighted average was used as the result Fentonrsquos reagentwas used to test the disintegration quantity of H2O2 in theceramic slurry [10]

3 Results and Discussion

31 Morphology of Porous Ceramics Manufactured UsingSolvents with Different H2O2 Contents Figure 2 shows themorphology of the porous HA ceramics obtained throughfreeze-casting based on different ratios of H2O2H2O (0 3and 9) As shown Figures 2(a) 2(c) and 2(e) indicate themacroscopic morphology of the porous ceramic whileFigures 2(b) 2(d) and 2(f) are the corresponding SEMimages )e dark zones are the pore channels and the lightzones are the pore walls Figure 2 shows that pure waterproduces porous ceramics with lamellar pores By contrastwhen the mixture with 3 of H2O2 was used lamellar poresand spherical pores were obtained However a further in-crease of the H2O2 content to 9 results in a porous ceramicwith only spherical pores

Because it is inherently unstable H2O2 easily de-composes when exposed to light heat and other physical orchemical factors Due to this we measured the content ofH2O2 before and after ball milling of the slurry As shown inTable 1 H2O2 decomposed and oxygen was released duringthe ball milling process )e oxygen accumulated as foam inthe ceramic slurry and thus spherical pores formed in theporous ceramic As shown in Figure 2 and Table 1 whena ceramic slurry with 3 (vv) of H2O2 was used sphericalpores formed in the resulting porous ceramic slurryHowever due to the small amount of H2O2 the amount ofbubbles or spherical pores formed in the slurry was in-sufficient to form large pores in the final ceramics Con-sequently the slurry still contained a large amount of waterand lamellar pores formed as the main pores in the porousceramic after freeze-casting (Figures 2(c) and 2(d)) Whenthe H2O2 content in the ceramic slurry was increased to 9(vv) the content of oxygen produced by H2O2 de-composition also increased In this case large pores wereformed in the final ceramic and large numbers of sphericalpores were observed (Figures 2(e) and 2(f ))

Cold plate

Metal

Heat flow

Thermal-resistantwall

Figure 1 Schematic diagram of the directional freeze-castingapparatus

2 Journal of Nanotechnology

32 Influence ofCoolingSpeedon theMorphologyof thePorousCeramic To further adjust the size of the spherical andlamellar pores we set the solid content and the H2O2H2Oratio in the ceramic slurry to 25 and 4 (vv) respectivelyand attempted to tune the pore structure of the HA ceramicby controlling the cooling speed during the freeze-dryingprocess Figure 3 shows SEM images of the porous HAceramics with two kinds of pores obtained at differentcooling speeds As can be seen the average diameter of

lamellar spacing and the spherical pores inside the ceramicsdecreased with the increase of cooling speed

)e influence of the cooling speed on pore size can beexplained using the Deville model [11] as shown in Figure 4)e blue pores in Figure 4(a) represent ceramic particleswhile the gray solvent represents the liquid phase of theslurry Firstly under conditions of local undercoolingminute ice crystals formed in the solvent (Figure 4(b)) andsubsequently grew gradually When the solidification speed

5 mm

(a)

Lamellar macropores

800 microm

(b)

5 mm

(c)

Spherical macropores

Lamellar macropores

800 microm

(d)

5 mm

(e)


800 microm

(f )

Figure 2 Morphology of HA scaffolds prepared from a 25 (vv) suspension by freeze-casting (a) and (b) water-based (c) and (d) 3 (vv)H2O2 (e) and (f) 9 (vv) H2O2

Table 1 )e content of H2O2 in the ceramic slurry before and after milling

H2O2 (vv))e content of H2O2 before

milling (ml))e content of H2O2 after

milling (ml))e content of decomposed H2O2 during

milling (ml)3 675 413 2629 225 1519 731

Journal of Nanotechnology 3

was lower than the critical speed ceramic particles wereeasily repelled and the space was occupied by the liquidphase thus forming the pore structure (Figure 4(c)) Atlower temperatures the cooling speed was so rapid that icecrystallization was finished before the ceramic particlescould be repelled by the frontier of the liquid-phase solid-ification Hence at rapid cooling speeds the average lamellarspacing was small On the contrary an increase of thefreezing speed reduced the expansion of O2 bubbles andmutual integration of the oxygen pores was also prevented tosome degree Consequently the spherical pore spacing of theHA ceramic was decreased

Figure 5 shows the relation between the cooling speedlamellar spacing and spherical pore spacing of the two kinds

of ceramic pores At a cooling speed of 13degCmin (Figure 4(a))the cooling rate of the ceramic slurry and the solvent nucle-ation speed were low Consequently the ice crystals were ableto repel the ceramic particles and the interspersed space wasoccupied by liquid-phase solidification Hence the averagelamellar spacing was large

As the cooling speed increased the consolidation speed ofthe slurry also increased In this case the ice crystals were notable to repel the ceramic particles and the spherical porespacing of the ceramic decreased as shown in Figures 4(b) and4(c) As shown in Figure 5 as the cooling speed increased theaverage lamellar spacing of the porous ceramics decreased from405 to 161μm and the spherical pore size increased from 596to 1215μm

800 microm

(a)

800 microm

(b)

800 microm

(c)

800 microm

(d)

Figure 3 SEM images of porous HA scaffolds prepared at different cooling speeds (a) 13degCmin (b) 22degCmin (c) 48degCmin (d)55degCmin

(a) (b) (c)

Figure 4 Lamellar pore-forming process in the aqueous slurry


After fitting the data (1) and (2) were obtained asfollows

LM minus15865Ln CR( 1113857 + 43807 R2

09863 (1)

LS minus167Ln CR( 1113857 + 41448 R2

09872 (2)

where LM and LS represent the average lamellar spacing andspherical pore size of the porous ceramic respectively andCR represents the cooling speed

Equations (1) and (2) can be used to adjust the coolingspeed to achieve an accurate average lamellar spacing andspherical pore sizes of the porous ceramic

4 Conclusions

In this paper a novel hydroxyapatite scaffold with lamellarand spherical pores was prepared by freeze-casting using anaqueous H2O2 solution as the pore-forming agent and theinfluence of H2O2 content in the aqueous phase on featuresof the HA porous ceramic was investigated In addition tothe lamellar pores induced by ice crystal formation sphericalpores were also formed in the HA scaffold due to the releaseof oxygen gas during the decomposition of H2O2 When 3(vv) of H2O2 solution was used a highly porous HA scaffoldcontaining both lamellar and spherical pores was obtainedHowever when the H2O2 volume fraction was 9 onlyspherical pores were observed )e average lamellar spacingand spherical pore sizes of the HA porous ceramic decreasedas the cooling speed increased

Data Availability

)e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

)e authors declare that they have no conflicts of interest

Acknowledgments

)is project was supported by the Scientific Research Pro-gram of the Shaanxi Provincial Education Department (no17JK0058) and the Research Fund Project of ShaanxiPolytechnic Institute (no ZK16-07)

References

[1] S Deville ldquoFreeze-casting of porous ceramics a review ofcurrent achievements and issuesrdquo Advanced EngineeringMaterials vol 10 no 3 pp 155ndash169 2008

[2] T Fukasawa M Ando T Ohji et al ldquoSynthesis of porousceramics with complex pore structure by freeze-dry pro-cessingrdquo Journal of the American Ceramic Society vol 84no 1 pp 230ndash232 2001

[3] H Zhang I Hussain M Brust et al ldquoAligned two-and three-dimensional structures by directional freezing of polymersand nanoparticlesrdquo Nature Materials vol 4 no 10pp 787ndash793 2005

[4] Y M Soon K H Shin Y H Koh et al ldquoCompressivestrength and processing of camphene-based freeze cast cal-cium phosphate scaffolds with aligned poresrdquo MaterialsLetters vol 63 no 17 pp 1548ndash1550 2009

[5] L Hu C-A Wang Y Huang et al ldquoControl of pore channelsize during freeze casting of porous YSZ ceramics withunidirectionally aligned channels using different freezingtemperaturesrdquo Journal of the European Ceramic Societyvol 30 no 16 pp 3389ndash3396 2010

[6] S Deville and G Bernard-Granger ldquoInfluence of surfacetension osmotic pressure and pores morphology on thedensification of ice-templated ceramicsrdquo Journal of the Eu-ropean Ceramic Society vol 31 no 6 pp 983ndash987 2011

[7] E Munch J Franco S Deville et al ldquoPorous ceramic scaffoldswith complex architecturesrdquo JOM vol 60 no 6 pp 54ndash582008

[8] C Pekor and I Nettleship ldquo)e effect of the molecular weightof polyethylene glycol on the microstructure of freeze-castaluminardquo Ceramics International vol 40 no 7 pp 9171ndash9177 2014

[9] Q Fu M N Rahaman F Dogan et al ldquoFreeze casting ofporous hydroxyapatite scaffolds II Sintering microstructureand mechanical behaviorrdquo Journal of Biomedical MaterialsResearch Part B Applied Biomaterials vol 86 no 2pp 514ndash522 2008

[10] C Walling ldquoFentonrsquos reagent revisitedrdquo Accounts of ChemicalResearch vol 8 no 4 pp 125ndash131 1975

[11] S Deville E Saiz and A P Tomsia ldquoIce-templated porousalumina structuresrdquo Acta Materialia vol 55 no 6pp 1965ndash1974 2007

Cooling rate (degCmin)

Lam

ella

r spa

cing

(microm

)

Lamellar poresSpherical pores

Sphe

rical

por

e siz

e (microm

)

1000900800700600500400300200100

550500450400350300250200150100

1 2 3 4 5 6

Figure 5 Influence of cooling speed on the average lamellarspacing and the spherical pore size


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example Mohamed et al used 14-dioxane and glycerin inwater as the pore-forming agent However the resultingporous ceramics still contained lamellar pores with a mor-phology similar to those obtained when using a water-basedslurry [9] To date there are no reports on the manufactureof porous ceramics with spherical and lamellar pores usingcomplex solvents as pore-forming agents

It is known that hydrogen peroxide can disproportionateto release oxygen gas )e authors believed that if the hy-drogen peroxide was mixed with water spherical holesinduced by oxygen and lamellar pores induced by watercould be obtained simultaneously generating spherical andlamellar pores at the same time Based on this ideaH2O2H2O mixtures with different H2O2H2O ratios wereused as the pore-forming agent to prepare porous ceramicsFinally the size and shape of the pores were controlledsuccessfully by tuning of the cooling speed

2 Materials and Methods

21 Starting Materials Hydroxyapatite (HA) powder witha grain size ranging from 05 to 10 microm (Ca10(PO4)6(OH)2Alfa Aesar USA) was used as the raw material Hydrogenperoxide (H2O2 Tianjin Bodi Chemical Co Ltd) anddistilled water in different proportions were used as sol-vent Carboxymethyl cellulose (Ningbo Cellulose De-rivative Plant China) and sodium polyacrylate (Mw 8000Alfa Aesar) were used as a binder and dispersing agentrespectively

22 Preparation of Scaffold Firstly H2O2H2O solutionswith H2O2 content of 3 4 and 9 (vv) were freshly pre-pared )en add dispersing agent and adhesive to the so-lutions at 1 and 2 of solute mass respectively mixed withHA powder after which a sizing agent with 25 (vv) ofsolid content was prepared )e agent was stirred usinga magnetic apparatus for 30min at 25degC and subsequentlyground in a roller-type ball mill (self-made equipment) for24 h at 25degC

)e ceramic slurry was poured into the mold(φ12mmtimes 30mm) which comprised two sections )e sideand top were made of the thermal insulation material and thebottom was made of metal with high thermal conductivity)is mold design was chosen to ensure that the freeze di-rection is from the bottom to top thus guaranteeing anoriented freezing of the ceramic slurry (Figure 1) )e elec-tronic probe of a freeze-drier (VFD2000G Boyikang Co LtdBeijing China) was used to measure the cryogenic temper-ature of the sizing agent to obtain the freezing curve Afterdemolding the frozen sample was lyophilized at ndash20 to 0degCfor 24 h and subsequently sintered at the temperature of1250degC for 2 h )e heat preservation period was 2 h

23 Characterization of the HA Scaffold A JSM-6700scanning electron microscope (SEM JEOL Japan) wasused to observe the morphology of the porous ceramic PCIView software (JEOL Japan) was used to measure pore sizein the electromicrographs From each group 5 samples were

selected each of which was tested at 2 to 3 points and theweighted average was used as the result Fentonrsquos reagentwas used to test the disintegration quantity of H2O2 in theceramic slurry [10]

3 Results and Discussion

31 Morphology of Porous Ceramics Manufactured UsingSolvents with Different H2O2 Contents Figure 2 shows themorphology of the porous HA ceramics obtained throughfreeze-casting based on different ratios of H2O2H2O (0 3and 9) As shown Figures 2(a) 2(c) and 2(e) indicate themacroscopic morphology of the porous ceramic whileFigures 2(b) 2(d) and 2(f) are the corresponding SEMimages )e dark zones are the pore channels and the lightzones are the pore walls Figure 2 shows that pure waterproduces porous ceramics with lamellar pores By contrastwhen the mixture with 3 of H2O2 was used lamellar poresand spherical pores were obtained However a further in-crease of the H2O2 content to 9 results in a porous ceramicwith only spherical pores

Because it is inherently unstable H2O2 easily de-composes when exposed to light heat and other physical orchemical factors Due to this we measured the content ofH2O2 before and after ball milling of the slurry As shown inTable 1 H2O2 decomposed and oxygen was released duringthe ball milling process )e oxygen accumulated as foam inthe ceramic slurry and thus spherical pores formed in theporous ceramic As shown in Figure 2 and Table 1 whena ceramic slurry with 3 (vv) of H2O2 was used sphericalpores formed in the resulting porous ceramic slurryHowever due to the small amount of H2O2 the amount ofbubbles or spherical pores formed in the slurry was in-sufficient to form large pores in the final ceramics Con-sequently the slurry still contained a large amount of waterand lamellar pores formed as the main pores in the porousceramic after freeze-casting (Figures 2(c) and 2(d)) Whenthe H2O2 content in the ceramic slurry was increased to 9(vv) the content of oxygen produced by H2O2 de-composition also increased In this case large pores wereformed in the final ceramic and large numbers of sphericalpores were observed (Figures 2(e) and 2(f ))

Cold plate

Metal

Heat flow

Thermal-resistantwall

Figure 1 Schematic diagram of the directional freeze-castingapparatus





5 mm

(a)

Lamellar macropores

800 microm

(b)

5 mm

(c)


Lamellar macropores

800 microm

(d)

5 mm

(e)


800 microm

(f )






milling (ml)3 675 413 2629 225 1519 731






800 microm

(a)

800 microm

(b)

800 microm

(c)

800 microm

(d)


(a) (b) (c)




LM minus15865Ln CR( 1113857 + 43807 R2

09863 (1)

LS minus167Ln CR( 1113857 + 41448 R2

09872 (2)



4 Conclusions


Data Availability




Acknowledgments


References













Lam

ella

r spa

cing

(microm

)


Sphe

rical

por

e siz

e (microm

)

1000900800700600500400300200100

550500450400350300250200150100

1 2 3 4 5 6





Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls







5 mm

(a)

Lamellar macropores

800 microm

(b)

5 mm

(c)


Lamellar macropores

800 microm

(d)

5 mm

(e)


800 microm

(f )






milling (ml)3 675 413 2629 225 1519 731






800 microm

(a)

800 microm

(b)

800 microm

(c)

800 microm

(d)


(a) (b) (c)




LM minus15865Ln CR( 1113857 + 43807 R2

09863 (1)

LS minus167Ln CR( 1113857 + 41448 R2

09872 (2)



4 Conclusions


Data Availability




Acknowledgments


References













Lam

ella

r spa

cing

(microm

)


Sphe

rical

por

e siz

e (microm

)

1000900800700600500400300200100

550500450400350300250200150100

1 2 3 4 5 6





Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls








800 microm

(a)

800 microm

(b)

800 microm

(c)

800 microm

(d)


(a) (b) (c)




LM minus15865Ln CR( 1113857 + 43807 R2

09863 (1)

LS minus167Ln CR( 1113857 + 41448 R2

09872 (2)



4 Conclusions


Data Availability




Acknowledgments


References













Lam

ella

r spa

cing

(microm

)


Sphe

rical

por

e siz

e (microm

)

1000900800700600500400300200100

550500450400350300250200150100

1 2 3 4 5 6





Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls





LM minus15865Ln CR( 1113857 + 43807 R2

09863 (1)

LS minus167Ln CR( 1113857 + 41448 R2

09872 (2)



4 Conclusions


Data Availability




Acknowledgments


References













Lam

ella

r spa

cing

(microm

)


Sphe

rical

por

e siz

e (microm

)

1000900800700600500400300200100

550500450400350300250200150100

1 2 3 4 5 6





Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls






Advances in



Journal of

Chemistry















Journal of





Volume 2018









Journal of


Na

nom

ate

ria

ls




Documents

Controlling the Structure of Hydroxyapatite Ceramics with