Combustion Synthesis and Photoluminescence Characteristics of Akermanite: A Novel

Biomaterial

V.B. Bhatkar Department of Physics Shri Shivaji College

AKOT (MS) 444101 India bhatkar_vinod@yahoo.com

N.V.Bhatkar Department of Zoology

Shri Shivaji College AKOT (MS) 444101 India bhatkarneha@hotmail.com

Abstract— Silicate based bioceramics are the promising candidates

as biomaterials for tissue engineering. The combustion synthesis

method provides the control on the morphology and the particle size

of the synthesised material. This paper discusses the combustion

synthesis of Akermanite (Ca2MgSi2O7 and Sr2MgSi2O7), which has

been shown to have good In vitro and in vivo bioactivity by the

earlier studies. Both Ca2MgSi2O7 and Sr2MgSi2O7 have akermanite

structure. Ca2MgSi2O7 and Sr2MgSi2O7 were prepared using urea

and ammonium nitrate. The combustion synthesis using Urea and

Ammonium Nitrate was found to be cost effective and efficient

method of synthesis. The photoluminescence study of Ca2MgSi2O7:

Eu2+

and Sr2MgSi2O7: Eu2+

shows host specific intense emission of Eu

2+.

Keywords- Biomaterials, Silicates, Akermanite, Combustion synthesis, Photoluminescence

I. INTRODUCTION It is essential to develop biocompatible, bioactive,

bioresorbable and durable materials for orthopaedic and dental implants, that are capable of bearing high stress and loads, and that invoke positive cellular and genetic responses for the rapid repair, modification, regeneration and maintenance of the affected tissue in the human body [1]. Silicate-based bioceramics, including silicate bioglass 45S5 [2, 3], wollastonite (CaSiO3) [4-6], akermanite (Ca2MgSi2O7) [3], diopside (Ca2MgSi2O6) [7] and merwinite (Ca3MgSi2O8) [8] ceramics, have been shown to have excellent apatite forming abilities in stimulated body fluids. Other studies showed that these silicate ceramics also possess good in vivo bioactivity [9-12]. In vitro and in vivo investigations of a calcium magnesium silicate (Ca2MgSi2O7) bioceramic for bone regeneration showed that akermanite extract promoted proliferation and osteogenic differentiation These results suggest that akermanite might be a potential and attractive bioceramic for tissue engineering [13-16]. A bioactive, degradable, and cytocompatible akermanite (Ca2MgSi2O7) scaffold with high porosity (63.5-90.3%) and pore interconnectivity with a corresponding compressive strength between 1130 and 530 kPa has been reported [17].

Akermanite and wollastonite have also been studied for drug delivery [18].

Akermanite belongs to the Tetragonal crystal system with Space Group, P421m where a = 7.8288(8) c = 5.0052(5) Z = 2 and point group: 42m. It is optically transparent to translucent and colorless. The X-ray powder pattern is characterized by strong lines at 2.87 (100), 3.09 (30), 1.764 (30), 2.039 (20), 2.488 (18), 3.73 (14), 5.55 (12). Most of the silicates have high melting points. Moreover, they can appear in crystalline as well as glassy form. Synthesis of silicates is rather tricky for these reasons. Conventionally, solid-state diffusion methods have been used for the synthesis of silicates. Akermanite ceramics prepared by sintering akermanite powder compacts at 1370° C for 6 h, is previously reported [19]. Pure akermanite (Ca2MgSi2O7) powders with polycrystalline particles with dimensions of 5–40 μm, were synthesized by sol–gel method [20].

The combustion synthesis as a preparation process to produce homogeneous, very fine crystalline, unagglomerated, multicomponent oxide ceramic powders without the intermediate decomposition and/or calcining steps has attracted a good deal of attention [21, 22]. The combustion synthesis is based on the exothermic reaction between fuel and oxidiser. The combustion process has several advantages over the other methods in terms of simplicity, cost-effectiveness, energy saving, purity and homogeneity. The combustion-derived powders have narrow size distribution with average agglomerate particle sizes in the range of 0.5-5 μm. The fine particle nature of the combustion derived powder is attributed to the low exothermicity of the combustion reaction and evolution of large amount of gases (NH3, H2O, CO2), which help to dissipate the heat thereby preventing the oxides from sintering.

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II. MATERIALS AND METHODS

In this study (Ca2-x Srx)MgSi2O7: Eu2+ with x- values 0, 0.5, 1.5 and 2 were synthesised by combustion method. The detailed description of the methods can be found in our earlier works [21, 22]. Ingredients used were metal carbonates, Silicic acid (SiO2.xH2O), and the dopant salts. The dopant europium concentration was 2 mole % of the AE ion. The equivalent amount of Eu2O3 was dissolved in 3M nitric acid for this purpose. Urea was used as a fuel and ammonium nitrate as oxidizer. Fuel to oxidizer ratio, optimised as described by Bhatkar et. al., [21, 22] was used. The details of the molar ratio of ingredients used in the synthesis of all compounds are given in Table 1.

TABLE I. DETAILS OF THE MOLAR RATIO OF INGREDIENTS

S/N Name of the

compound Starting Materials

1 Ca2MgSi2O7: Eu2+ CaCO3, Mg(NO3)2

SiO2.xH2O Eu (NO3)2

NH4NO3 UREA

Mole ratio:-> 1.96, 1

2 0.04 30 35

2 Ca1.5Sr0.5MgSi2O7: Eu2+

CaCO3, SrCO3,

Mg(NO3)2

SiO2.xH2O Eu (NO3)2

NH4NO3 UREA

Mole ratio:-> 1.46, 0.5, 1,

2 0.04 30 35

3 Ca0.5Sr1.5MgSi2O7: Eu2+

CaCO3, SrCO3,

Mg(NO3)2

SiO2.xH2O Eu (NO3)2

NH4NO3 UREA

Mole ratio:-> 0.5, 1.46, 1,

2 0.04 30 35

4 Sr2MgSi2O7: Eu2+ SrCO3, Mg(NO3)2

SiO2.xH2O Eu (NO3)2

NH4NO3 UREA

Mole ratio:-> 1.96, 1

2 0.04 30 35

All constituents in stoichiometric proportions, along with

fuel and oxidizer were mixed together and small quantity of double distilled water was added. The mixture on thoroughly mixing was transferred to a pre-heated furnace at 500 oC. On rapid heating the mixture evaporates and ignites at 450 oC, with the evolution of a large amount of gases, to yield silicates. Entire process completes within few minutes. The as-prepared phosphors did not show intense emission, probably the activator Eu is not incorporated in divalent form. The phosphors were reheated, in the reducing atmosphere provided by heating in a closed box with charcoal, at 900 oC for 1 hour. Photoluminescence spectra were recorded on Hitachi F-4000 spectro-fluorimeter with spectral slit width of 1.5 nm, at room temperature. To confirm the structure of the synthesized phosphors, powder photographs were obtained using Philips diffractometer, PW 1710.

III. RESULTS AND DISCUSSION Akermanite belongs to the Tetragonal crystal system with

Space Group, P421m where a = 7.8288, c = 5.0052, Z = 2 and point group 42m. It is optically transparent to translucent and colorless. The X-ray powder pattern is characterized by strong lines at 2.87 (100), 3.09 (30), 1.764 (30), 2.039 (20), 2.488 (18), 3.73 (14), 5.55 (12). Both Ca2MgSi2O7 and Sr2MgSi2O7

have akermanite structure. In this type of structure there is one calcium or strontium site, which is coordinated by eight oxygen ions. The review of the data from the earlier studies [24, 25] on the nature of emission and the decay time for these phosphors, are summarized in Table 2.

TABLE II. REVIEW OF THE DATA (NATURE OF EMISSION AND THE DECAY TIME)

S/N

Name of the compound

Emission Peak (nm)*

Emission Peak(nm)

@

T50 (K)

Decay time (μs)

Reference

1 Ca2MgSi2O7: Eu2+ 535 545

475 bb 285 280

0.2 1.1

Blasse1968 Poort 1997

2 Ca1.5Sr0.5MgSi2O7: Eu2+

525 448 280 1.0 Poort 1997

3 Ca0.5Sr1.5MgSi2O7: Eu2+

490 450 280 0.8 Poort 1997

4 Sr2MgSi2O7: Eu2+ 470 475

457 305 300

0.3 0.7

Blasse1968 Poort 1997

@ This work for PL measurements at room temperature. * Literature values at 4.2 K G. Blasse, et al., Philips Res. Repts, 23, 189, 1968. S. M. H Poort,., et al., J. Phys. Chem. Solid., 58, 1451, 1997

The PL properties of individual compounds are discussed

below:

Fig. 1 shows the excitation spectra for Ca2MgSi2O7:Eu2+, Ca1.5Sr0.5MgSi2O7:Eu2+, Ca0.5Sr1.5MgSi2O7:Eu2+, and Sr2MgSi2O7:Eu2+. The emission spectra are shown in fig. 2. Efficient excitation by 385 nm, was observed for Eu2+ doped disilicates as seen from fig. 1. The emission spectrum of Ca2MgSi2O7:Eu2+, (curve a), is a broad band at 475 nm. Ca1.5Sr0.5MgSi2O7:Eu2+ (curve b) exhibit blue emission around 448 nm. Emission in Ca0.5Sr1.5MgSi2O7:Eu2+ (curve c) peaks at 450 nm. Emission in Sr2MgSi2O7:Eu2+ (curve d) peaks around 457 nm.

Figure 1, PL spectra for Eu2+ doped disilicate phosphors

a) (Red) Ca2MgSi2O7 excitation for 447 nm emission b) (Blue) Ca1.5Sr0.5MgSi2O7 excitation for 448 nm emission c) (Black) Ca0.5Sr1.5MgSi2O7 excitation for 478 nm emission d) (Green) Sr2MgSi2O7 excitation for 460 nm emission.

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0

20

40

60

80

100

400 450 500 550

Inten

sity (

arb.

units

)

Wavelength (nm)

Excitation spectrum of all the samples (Fig 1, curves a-d) shows considerable intensity at 385 nm. At room temperature the intensity of emission, except for Sr2MgSi2O7:Eu2+ is considerably quenched, which is consistent with the literature data. The luminescence process of Eu2+-activated phosphor is characterized by the 4f6 5d→4f7 transition of Eu2+ acting as an activator center. The absorption and emission due to the transition between 4f7 and 4f

6 5d states of Eu2+, strongly depend on host material cations replaceable by Eu2+ in the host matrix and the crystal field acting on Eu2+.

IV. CONCLUSIONS Both Ca2MgSi2O7:Eu2+ and Sr2MgSi2O7:Eu2+ has

akermanite structures. The combustion synthesis using urea and ammonium nitrate is the easy, time saving and cost effective method for the synthesis. The photoluminescence study of these materials shows the strong emission of Eu2+ in alkaline earth silicates which agrees well with the literature. The biocompatibility of the akermanite has already been reported by many researchers. The luminescence properties of these biomaterials may be useful in their use as biomarkers and in the controlled drug delivery.

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Figure 2. PL spectra for Eu2+ doped disilicate phosphors

a) (Black) Ca2MgSi2O7 emission for 385 nm excitation b) (Green) Ca1.5Sr0.5MgSi2O7 emission for 385 nm excitation c) (Blue) Ca0.5Sr1.5MgSi2O7 emission for 385 nm excitation d) (Red) Sr2MgSi2O7 emission for 385 nm excitation

V.B. Bhatkar et al. / (IJAEST) INTERNATIONAL JOURNAL OF ADVANCED ENGINEERING SCIENCES AND TECHNOLOGIES Vol No. 5, Issue No. 2, 184 -186

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