BIOMATERIALS

UDC 666.3.022:542.65:546.41’33’18

CALCIUM PHOSPHATE CERAMIC BASED ON POWDER SYNTHESIZED

FROM A MIXED-ANIONIC SOLUTION

T. V. Safronova,1, 2 A. V. Knot’ko,1 T. B. Shatalova,1 P. V. Evdokimov,1 V. I. Putlyaev,1

and M. S. Kostin1

Translated from Steklo i Keramika, No. 1, pp. 27 – 34, January, 2016.

Ceramic based on calcium phosphates is obtained from powders synthesized from water solutions of calcium

nitrate, ammonium hydrophosphate, and ammonium pyrophosphate as well as from a mixed-anionic solution con-

taining simultaneously ammonium pyro- and orthophosphate in the molar ratio (NH4)2HPO

4�(NH

4)4P

2O

7= 1.

The ceramic obtained contains biocompatible bioresorbable phases and can be used to make implants used in

regenerative methods for the treatment of bone tissue defects. This represents the first time synthesis from

mixed-anionic solutions is used to obtain powders of calcium phosphates with significant quantities of each

anion.

Key words: brushite, monetite, amorphous calcium pyrophosphate, ceramic, thermo-hydrolysis, calcium

pyrophosphate, tricalcium phosphate, biomaterial.

Ceramic based on calcium phosphates for bone implants

remains at the focus of research in the field of medical inor-

ganic materials science [1]. Materials based on calcium

phosphates are of interest because the main component of an

inorganic constituent of bone tissue is hydroxyapatite

Ca10

(PO4)6(OH)

2. The advancement of regenerative ap-

proaches to the medical treatment of bone tissue in humans

makes it necessary to develop bioresorbable materials based

on calcium phosphates for the fabrication of porous matrices

in which osteogenesis could occur after implantation. Cal-

cium phosphates with molar ratio Ca�P < 1.67 can be

resorbed upon implantation, since they are soluble in water

[2]. The following phases can be present in ceramic material

after firing: hydroxyapatite Ca10

(PO4)6(OH)

2(Ca�P = 1.67);

tricalcium phosphate Ca3(PO

4)2

(Ca�P = 1�5); calcium pyro-

phosphate Ca2P

2O

7(Ca�P = 1.0); and, calcium polyphos-

phate Ca(PO3)2

(Ca�P = 0.5).

The production of ceramic materials presupposes the use

of highly disperse powders obtained by chemical methods,

predominately precipitation from solutions. Variation of the

conditions of synthesis of the powders, for example, varia-

tion of the ionic composition of the solutions, is widely used

to control the properties of ceramic materials.

A great deal of experience has been accumulated on the

cationic modification of the precipitated powders for obtain-

ing conventional oxide ceramic based on Al2O

3or ZrO

2[3]

and for ceramic based on calcium phosphates [4 – 6].

Coprecipitation from solutions has also been used to obtain

anion-substituted calcium phosphates, when teraethoxysilane

is used to introduce the anion SiO4

4�

[7, 8] and the interaction

of Ca(OH)2

and CO2

gas is used to introduce the anion CO3

2�

[9].

The synthesis of calcium phosphates from calcium ni-

trate and sodium, potassium, and ammonium hydro-ortho-

phosphates at pH = 9 in the presence of magnesium ions with

ratio Mg�Ca = 0.11 and pyro-phosphate ions (in the form

Na4P

2O

7) in the amount 15% of the total phosphorus content

has been investigated [10]. In this investigation the condi-

tions chosen for synthesis (short duration (30 sec) and rela-

tively low temperature (25°C)) promoted the formation of

amorphous precipitates. The ratio Ca�P = 1.67 set at synthe-

sis was not preserved in the material after firing; the forma-

tion of phases with the ratio Ca�P = 1.5 or even Ca�P = 1

was observed.

Glass and Ceramics, Vol. 73, Nos. 1 – 2, May, 2016 (Russian Original, Nos. 1 – 2, January – February, 2016)

25

0361-7610�16�0102-0025 © 2016 Springer Science+Business Media New York

1M. V. Lomonosov Moscow State University, Moscow, Russia.

2E-mail: t3470641@yandex.ru.

DOI 10.1007/s10717-016-9819-6

Research on the synthesis of calcium phosphates from

solutions containing simultaneously the orthophosphate and

pyrophosphate ions in the ratio Ca�P = 1 for the components

of the initial solutions is not presented in the literature. At the

same time the experience gained in the production of ce-

ramic based on calcium pyrophosphate (Ca�P = 1) from

powders synthesized from ammonium pyrophosphate

(NH4)4P

2O

7and calcium nitrate is reflected in the literature,

specifically, in our research [11]. The use of soluble sodium,

potassium, or ammonium hydro-orthophosphates, depending

on the pH, in the reaction zone in the presence of interaction

with soluble calcium salts at the Ca�P ratio set for the initial

solutions makes it possible to obtain powders of basic or

acidic hydrated calcium phosphates with different Ca�P ra-

tios. The phase composition of the ceramic fabricated from

these powders can be represented by hydroxyapatite, �-tri-

calcium phosphate for the initial ratio Ca�P = 1.5 [12] as

well as �-tricalcium phosphate and �-calcium pyrophosphate

for the initial ratio Ca�P = 1 [13].

In summary, the aim of the present work was to investi-

gate the properties of ceramic based on calcium phosphate

powders synthesized with the ratio Ca�P =1 for the initial

components from a water solution of calcium nitrate and a

mixed solution containing ammonium hydro-orthophosphate

and pyrophosphate. An additional goal was to determined the

effect of the composition of a phosphate-containing solution

containing orthophosphate and�or pyrophosphate ions on the

properties of the powder after synthesis and on the properties

of ceramic fabricated from these powders.

EXPERIMENTAL PART

The powders were synthesized using calcium nitrate

tetrahydrate (GOST 4142–77, chemically pure), ammonium

hydro-orthophosphate (GOST 3772–74, chemically pure),

and ammonium pyrophosphate (TU 6-09-1288–76, chemi-

cally pure) from the Labtekh Company (Moscow). The fol-

lowing reactions were used to calculate the quantities of the

initial salts:

Ca(NO3)2

� 4H2O + (NH

4)2HPO

4=

CaHPO4

� 2H2O� + 2NH

4NO

3+ 2H

2O; (1)

3Ca(NO3)2

� 4H2O + (NH

4)4P

2O

7+ (NH

4)2HPO

4+

xH2O = CaHPO

4� 2H

2O� + Ca

2P

2O

7� xH

2O� +

10H2O + 6NH

4NO

3, 2 � x � 4; (2)

2Ca(NO3)2

� 4H2O + (NH

4)4P

2O

7+ xH

2O =

Ca2P

2O

7� xH

2O� + 4NH

4NO

3+ 8H

2O, 2 � x � 4. (3)

The conditions for synthesis of the powders are pre-

sented in Table 1. The synthesized powders were labeled as

follows: ortho — powder obtained via the reaction (1);

mix — powder obtained via the reaction (2); pyro — powder

obtained via the reaction (3). All syntheses were conducted

at room temperature. The solution containing Ca(NO3)2

was

added to the solutions containing phosphate ions.

The precipitate obtained was separated from the mother

liquor on a Büchner filter and dried at room temperature. The

dried powders were degassed in acetone in a planetary mill

with zirconium dioxide milling bodies and mass ratios mill-

ing bodies : powder : acetone = 5 : 1 : 1.

To obtain a ceramic and investigate the thermal evolution

of the obtained powders the samples pressed without using

an additional binder with specific pressing pressure 50 MPa

were fired in the interval 800 – 1000°C with step 100°C,

soaking time 2 h at the final temperature, and heating rate

5 K�min.

A Rigaku D�Max-2500 with a rotating anode (Japan)

was used to perform x-ray phase analysis (XPA) of the syn-

thesized powder and heat-treated samples. The ICDD PDF2

database was used for qualitative determination of the

phases. The photographs were obtained in reflection using

CuK�

radiation (angles 2� = 2 – 60° with steps 0.02°, acqui-

sition rates of the spectra 5 °�min). Thermal analysis (TA)

was performed with an STA 409 PC Luxx thermal analyzer

(NETZSCH, Germany) with heating rate 10 K�min in the

temperature interval 40 – 1000°C. The sample mass was at

least 10 mg. The microstructure of the synthesized powder

samples and the heat-treated ceramic samples was investi-

gated by means of scanning electron microscopy (SEM) per-

formed with a LEO SUPRA 50VP scanning electron micro-

scope (Carl Zeiss, Germany; auto-emission source); the pho-

tography was performed in a low-vacuum regime (40 Pa N2

)

with accelerating voltage 20 kV (VPSE secondary electron

detector) and voltages 3 – 20 kV (SE2 detector).

26 T. V. Safronova et al.

TABLE 1. Conditions for Synthesis of Powders and Sample Designations

No. Designation

Solutions containing Ca2+ ions Solutions containing phosphate and pyrophosphate ionsPlanned molar content of phases

in the synthesized powder, %

V, mlCa(NO

3)2,

mol�literV, ml

(NH4)2HPO

4,

mol�liter

(NH4)4P

2O

7,

mol�literCaHPO

4� 2H

2O Ca

2P

2O

7� xH

2O

1 ortho 250 1 250 1 0 100 0

2 mix 375 1 250 0.5 0.5 50 50

3 pyro 250 1 250 0 0.5 0 100

RESULTS AND DISCUSSION

XPA shows that the phase composition of the ortho pow-

der synthesized via the reaction (1) is represented by brushite

and ammonium nitrate, which is a co-product of the reaction.

The pyro powder synthesized via the reaction (3) contains an

x-ray amorphous product whose composition, based on the

fact that the qualitative and quantitative composition of the

solutions used, can comprise a mixture of hydrated calcium

pyrophosphate and the co-product ammonium nitrate. The

phase composition of the mix powder synthesized via the re-

action (2) includes brushite, an x-ray amorphous product,

and ammonium nitrate. The XPA data on the synthesized

powders after disaggregation in acetone are presented in

Fig. 1. After disaggregation brushite in the ortho powder

transforms into monetite (reaction (4)) via the action of ace-

tone under intense mechanical action:

CaHPO4

� 2H2O = CaHPO

4+ 2H

2O. (4)

The pyro powder remains x-ray amorphous after dis-

aggregation. The brushite in the mix powder transforms via

the reaction (4) only partially. Brushite, monetite, and an

x-ray amorphous product were found in the mix powder.

The phase composition of all synthesized powders after

disaggregation suggests the formation of calcium pyrophos-

phate after heat treatment at high temperature. This expecta-

tion relies on the data in [10, 12] and the fact that the compo-

sition of the initial components is known, and all products

found in the powders either decompose (NH4NO

3) or trans-

form upon heating via the reactions (4) – (7):

2CaHPO4

� 2H2O = Ca

2P

2O

7+ 5H

2O; (5)

2CaHPO4

= Ca2P

2O

7+ H

2O; (6)

Ca2P

2O

7� xH

2O = Ca

2P

2O

7+ xH

2O. (7)

Photomicrographs of the powders after synthesis are pre-

sented in Fig. 2. The particles of the brushite powder (ortho)

possess its characteristic plate-like morphology (Fig. 2a ).

The length and width of the plates lies in the interval

2 – 10 m. The thickness of the plates equals 200 – 500 nm.

The pyro powder particles (Fig. 2c ) of size 100 – 200 nm,

whose shape is close to isometric, are aggregated. It was

shown previously by means of transmission electron micros-

copy that the particles of such powder, which are close to

isometric, contain 20 – 40 nm mesopores [11]. The aggre-

gates of the particles appear in the image as large blocks

reaching 10 – 20 m in size. In the photomicrograph of the

mix powder (Fig. 2b ) it is hardly possible to find particles

with plate-like morphology characteristic for brushite,

though the XPA data show brushite to be present in the pow-

der. Individual sections where the particle aggregates are

similar to those found for the pyro powder can be found in

the photomicrograph. On the whole the mix powder is repre-

sented by aggregates reminiscent of carnation flowers and

consisting of small plates. The size of these aggregates is

6 – 10 m.

It can be supposed that the co-product, viz. ammonium

nitrate, crystallizes from the mother liquor filling the meso-

pores and voids between the particles in their aggregates.

The density of the samples after pressing falls into the

range 40 – 50% in terms of the density of �-calcium pyro-

phosphate (3.09 g�cm3 ) and increases from pyro to ortho

samples.

The thermal analysis data (Fig. 3) attest that the total

mass loss upon heating the powder to 1000°C equals 35% for

ortho and 40% for pyro powder. The total mass loss for the

Calcium Phosphate Ceramic Based on Powder Synthesized from a Mixed-Anionic Solution 27

2 , deg�

Fig. 1. XPA data on the synthesized ortho, mix, and pyro powders

after disaggregation in acetone: �) peaks corresponding to brushite

CaHPO4

� 2H2O, card PDF2 No. C11-293; +) monetite CaHPO

4,

card PDF2 No. C9-80; �) ammonium nitrate, card PDF2 No. C8-452.

2 m 2 m 2 m

a b c

Fig. 2. Photomicrographs of the synthesized ortho (a), mix (b ), and pyro powders (c).

mix powder equals 37.5%. Loss of physically bound ad-

sorbed water occurs for all powders in the interval from 40 to

220°C. The subsequent mass loss for brushite is associated

with successive transformation via monetite into pyrophos-

phate (reactions (5) and (6)); for the amorphous product,

which is most likely represented by hydrated calcium

pyrophosphate, it is associated with its transformation into

pyrophosphate via the reaction (7). The decomposition of the

co-product of the reaction — ammonium nitrate — also

makes its own contribution to the mass loss. The course of

the curve for mix powder suggests that the mass loss upon

heating of this powder is due to the dehydration of brushite

as well as hydrated calcium pyrophosphate.

The XPA data obtained after heating at 800°C attest that

the phase composition of the ceramic samples obtained from

the ortho, mix, and pyro powders is represented by the �-cal-

cium pyrophosphate (Fig. 4 – 6). The phase composition of

the ceramic obtained from ortho powder after heat-treatment

at 900 and 1000°C does not change, which agrees with the

previously obtained data [13, 14]. The phase composition of

ceramic obtained from pyro powder after firing at 900°C is

also represented by �-calcium pyrophosphate (Fig. 4c and d ).

However, after firing at 1000°C �-tricalcium phosphate was

found in the phase composition of ceramic obtained from

pyro powder; this contradicts the previously obtained data

[11]. The formation of �-tricalcium phosphate in ceramic ob-

tained from pyro powder in the present investigation can be

explained by the difference in the preparation protocol for

the powder used to obtain ceramic as compared with the pro-

tocol used in [8]. In the present work the synthesized and

disaggregated powders were used as quickly as possible after

synthesis and therefore contained large quantities of ad-

sorbed or occluded water, which during subsequent heating

permitted thermohydrolysis of calcium pyrophosphate via

the reaction

3Ca2P

2O

7+ H

2O = 2Ca

3(PO

4)2

+ 2HPO3. (8)

The phase composition of ceramic obtained from mix

powder after firing at 900 and 1000°C is represented mainly

28 T. V. Safronova et al.

m� 0, %m

Fig. 3. Thermal analysis data for the ortho, mix, and pyro powders

after synthesis: m) sample mass at the running temperature; m0

) ini-

tial sample mass.

2 , deg�

Fig. 4. XPA data after firing at temperature 800 – 1000°C of ce-

ramic samples fabricated from ortho powders:�) peaks correspond-

ing to �-Ca2P

2O

7(card PDF2 No. C9-346).

2 , deg�

Fig. 5. XPA data after firing at temperature 800—1000°C of ce-

ramic samples fabricated from mix powders: �) peaks correspond-

ing to �-Ca2P

2O

7, card PDF2 No. C9-346; �) peaks corresponding

to �-Ca3(PO

4)2, card PDF2 No. C9-169.

2 , deg�

Fig. 6. XPA data after firing at temperature 800—1000°C of ce-

ramic samples fabricated from pyro powders: �) peaks correspond-

ing to �-Ca2P

2O

7, card PDF2 No. C9-346; �) peaks corresponding

to �-Ca3(PO

4)2, card PDF2 No. C9-169.

by �-tricalcium phosphate (Fig. 4b, d, and e). The formation

of this phase in ceramic obtained from mix powder can also

be a consequence of the thermohydrolysis via the reaction (8)

upon heating. In addition, the appearance of �-tricalcium

phosphate even at 900°C can be due to the fact that the struc-

ture of the synthesized mix powder, which includes ortho-

and pyrophosphate ions, is capable of retaining water mole-

cules in its environment up to higher temperatures. The water

can also remain in closed mesopores upon heating to temper-

atures significantly above the boiling point.

Curves of the change in the linear dimensions (diameter)

of the pellets formed as a function of the heat-treatment tem-

perature reflect compaction processes that occur in the parts

pressed from ortho, mix, and pyro powders (Fig. 7). The

highest relative linear shrinkage (33% (�3)) and density

(2.32 g�cm3 (�0.1)) are observed for samples made from

pyro powder after firing at 1000°C. The lowest relative linear

shrinkage (7% (�3)) and density (1.83 g�cm3 (�0.1)) are seen

after firing after 1000°C for ceramic made from ortho pow-

der. For ceramic made from mix powder after firing at

1000°C the shrinkage was equal to 16% (�3)) and the density

2.15 g�cm3 (�0.1)). It was noted previously that during

sintering, because of its structural particularities the pyro-

phosphate anion, which is spatially larger, diffuses more

slowly than the orthophosphate anion [15]. The course of the

temperature dependences of the linear dimensions of the

samples and their relative arrangement in Fig. 7 most likely

illustrates the effect of the sintering activity of the powders,

which is also related with the particle size and morphology.

The photomicrographs of the ceramic samples fabricated

from ortho powder (Fig. 8) confirm grain growth and poros-

ity reduction with increasing firing temperature. For exam-

ple, the grain size of ceramic equals 1 – 2 m after firing at

800°C and 2 – 6 m after firing at 1000°C. No particles or

grains with plate-like morphology could be found in the

microstructure of ceramic samples fabricated from the ortho

powder after firing in the interval 800 – 100°C.

The evolution of the microstructure of the ceramic fabri-

cated from the mix powder is displayed in Fig. 9. After firing

at 800°C the particles of the powder blank are in the sintering

state, they are weakly confined next to one another, and the

particle size does not exceed 1 m. The microstructure of the

ceramic after firing at 900°C can be described as a three-di-

mensional network formed by particles whose contact with

one another is characterized by a larger area than at the tem-

perature 800°C. The grain size is about 1 m. After firing at

900°C the grains of the ceramic form a quite dense sinter.

The grain size equals 1 – 4 m.

The photomicrographs of the ceramic fabricated from the

pyro powder and fired at 800 – 1000°C also attest to the stan-

dard development of the microstructure of the ceramic with

increasing firing temperature. The grain size after firing at

800°C and 900°C is quite difficult to evaluate. It can only be

presumed that at 800°C the particle size is much less than

1 m. At 900°C we observe sintering in individual aggre-

gates of particles. The grain size of the ceramic after firing at

1000°C does not exceed 1 m.

It is evident that the material obtained from a solution

containing simultaneously the ortho- and pyrophosphate ions

possesses a more uniform microstructure than the material

obtained from the ortho powder. In all probability this fact is

associated with the influence of the pyrophosphate ion on the

Calcium Phosphate Ceramic Based on Powder Synthesized from a Mixed-Anionic Solution 29

Fig. 7. Relative diameter of the ceramic samples fabricated from

ortho, mix, and pyro powders versus the firing temperature: D) di-

ameter of the sample at the running temperature; D0

) initial diame-

ter of the sample.

2 m 2 m 2 m

a b c

Fig. 8. Photomicrographs of fracture surfaces of ceramic samples, fabricated from ortho powder, after firing for 2 h at 800°C (a), 900°C (b ),

and 1000°C (c).

character of the growth of brushite particles in the form of

plates when calcium nitrate interacts with ammonium hydro-

phosphate (see Fig. 2b ). At the same time the grain size of

such material is larger than that of the ceramic obtained from

pyro powder.

In summary, the use of a mixed anion solution in synthe-

sis, presupposing the ortho- and pyrophosphate ions to be

present simultaneously, can serve as a tool for controlling the

microstructure and phase composition of the ceramic ma-

terial.

CONCLUSIONS

The properties of biocompatible and bioresorbable ce-

ramic materials from powders synthesized from ammonium

orthophosphate, ammonium pyrophosphate, and their mix-

ture were investigated. It was shown that the composition of

the phosphate-containing solution affects the phase composi-

tion and microstructure of the ceramic after firing. In the

presence of the pyrophosphate ion it is possible to obtain

powders of calcium phosphate ions that at a prescribed value

of the initial ratio Ca�P = 1 in the ceramic material form after

firing at 800°C a phase with a large ratio Ca�P = 1.5, specifi-

cally, a �-tricalcium phosphate phase. The formation from

calcium phosphates with the ratio Ca�P = 1 of a phase with a

higher ratio Ca�P = 1.5 could be due to thermohydrolysis in

the presence of occluded water in closed mesopores when

the powdered material is heated. It should be noted that the

grain size tends to decrease in the sequence ortho, mix, and

pyro ceramic samples. Owing to the known properties of the

components of their phases the ceramic materials obtained

can be used as a biocompatible and bioresorbable structural

base for tissue engineering in order to restore lost bone tissue.

The equipment used in this work was acquired under the

development program at Moscow University.

The research was supported by the Russian Science

Foundation under Grant No. 15-19-00103.

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Calcium Phosphate Ceramic Based on Powder Synthesized from a Mixed-Anionic Solution 31