Dy3+ Occupancy in Zirconia Lattice Affects Tetragonal to Cubic Phase

Article pubs.acs.org/crystal

Dy3+ Occupancy in Zirconia Lattice Affects Tetragonal to Cubic Phase Transitions in Zirconia Toughened Alumina Systems V. Ponnilavan and S. Kannan* Centre for Nanoscience and Technology, Pondicherry University, Puducherry 605 014, India S Supporting Information *

ABSTRACT: Current research on zirconia toughened alumina (ZTA) systems employed in total hip joint replacement applications is focused on the usage of alternative stabilizers to improve their properties. Herein, a wide range of dysprosium (Dy3+) additions to ZTA systems have been formed through an in situ method. Dy3+ induced significant structural changes in ZrO2 rather than the αAl2O3 component of the composite. Dy3+ tends to occupy along the a = b-axis of the ZrO2 lattice to stabilize tetragonal zirconia (tZrO2), whereas its enhanced accumulation directed the formation of cubic zirconia (c-ZrO2). As a consequence of phase transition, a different behavior in the emission characteristics was also noticed. However, t- → c-ZrO2 phase transition was not found to affect the paramagnetic behavior of Dy3+ added ZTA systems. The structural stability of the Dy3+ added ZTA systems was preserved until 1500 °C, and moreover it was also determined that optimum Dy3+ content is essential for enhanced mechanical stability of the composite.

1. INTRODUCTION The in vivo failure of individual ceramic components explicitly metastable tetragonal zirconia (t-ZrO2) and alumina (α-Al2O3) has shifted the focus toward the development of zirconia toughened alumina (ZTA) systems in total hip and joint replacements.1−3 α-Al2O3 is associated with the problems of intrinsic brittleness and high fracture rate, whereas the failure of t-ZrO2 is related to the drawback of its gradual transformation to monoclinic zirconia (m-ZrO2) in vivo.4,5 ZTA systems developed as an alternative to individual t-ZrO2, and α-Al2O3 components possess the salient features of high chemical wear resistance and toughening. Experimental observations revealed that ZTA systems undergo minimum degradation to yield to mZrO2 on comparison with t-ZrO2.6,7 The main reasons pointed out for the failure of ZTA systems during the aging phenomenon are the type of stabilizer and its content, the residual stress, and the grain size.8 In spite of several mechanisms proposed for the t-ZrO2 → m-ZrO2 transition, the basic principle of action occurs in a sequential manner such as initiation from the surface and penetration into the bulk of the material, inducing surface uplift, creation of cracks, penetration of water, and an increase in bulk volume leading to t-ZrO2 → m-ZrO2 transition. Attempts were made to improve the properties of ZTA systems through the incorporation of stabilizers in the form of yttria (Y2O3) and ceria (CeO2). Among the investigated systems, CeO2 stabilized ZTA systems have been proven to yield better mechanical features and also displayed resistance to t-ZrO2 → m-ZrO2 transition during aging tests.2,9,10 In vivo © XXXX American Chemical Society

studies also indicated the success of CeO2 stabilized ZTA systems in dental prostheses.11 Nevertheless, to monitor the satisfactory performance of all these investigated systems in vivo essentially requires a revision of surgery. In a further step ahead, we intend to propose a ZTA system that is aimed to resolve two important issues, namely, a mechanically stable component and the periodical monitoring of implant performance without invasive surgical techniques. To meet this demand, it has been planned to use of Dy2O3 as a stabilizer in ZTA systems. The paramagnetic property of dysprosium (Dy3+) has been explored as an magnetic resonance imaging (MRI) contrasting agent in the form of Dy2O3, Dy(OH)3, and its ionic liquid state.12,13 It has also been presented that Dy3+ displays the highest magnetic moment with good transverse relaxivity (r2) among all lanthanides and hence used as a most effective T2 MRI contrast agent.14 The aim of this present study was to form a series of dysprosium additions to the ZTA systems through a citrate nitrate sol−gel process. The structural and phase transition behavior of the resultant systems were investigated, and the mechanical and magnetic features were tested using appropriate techniques. Received: September 14, 2016 Revised: November 21, 2016

A

DOI: 10.1021/acs.cgd.6b01355 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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of dysprosium atom at the zirconium site, while the oxygen site was fixed. The fractional atomic coordinates and isotropic thermal parameters (Uiso) of dysprosium and zirconium atoms were also accounted during the refinement of occupancy factors. The mechanical behavior of the systems was investigated in accordance with the procedure described in our previous reports.19,20

2. MATERIALS AND METHODS 2.1. Powder Synthesis. A citrate assisted sol−gel technique was adopted for the powder synthesis. Analytical grade ZrOCl2·8H2O, Al(NO3)3·9H2O, and Dy(NO3)3 precursors procured from SigmaAldrich, India, were used without further purification. The precursor ratios of Al(NO3)3 and ZrOCl2 were kept constant for all the synthesis, whereas the Dy(NO3)3 concentrations were varied to obtain five different combinations. The precursor concentrations along with their sample codes are mentioned in Table 1, and the same codes will be used throughout the manuscript.

3. RESULTS ZTA systems are proven to possess improved structural and mechanical stability than pure Al2O3 systems, which is due to the considerable presence of metastable t-ZrO2. However, these salient features are expected to change by the influence of two major factors, namely, thermal treatment and type of stabilizers. X-ray diffraction (XRD) measurements at varied temperatures were performed to determine the structural and thermal stability of DZTA systems and the associated results are depicted in Figures 1−3. XRD patterns indicated the presence

Table 1. Precursor Concentrations Used during the Synthesis molar concentrations of precursors sample code

Al(NO3)3

ZrOCl2

Dy(NO3)3

ZTA DZTA10 DZTA20 DZTA30 DZTA50 DZTA100

0.750 0.750 0.750 0.750 0.750 0.750

0.250 0.250 0.250 0.250 0.250 0.250

0.025 0.050 0.075 0.125 0.250

In brief, discrete stock solutions of Dy(NO3)3, Al(NO3)3, and ZrOCl2 with appropriate molar concentrations were prepared separately in deionized water. In the preliminary step, ZrOCl2 was transferred to Al(NO3)3 solution under constant stirring conditions, and this was followed by the slow addition of Dy(NO3)3 after 10 min. After 15 min, a required amount of citric acid solution was transferred to the continuously stirred nitrate mixtures. The reaction temperature was maintained at 60 °C at constant stirring conditions until the formation of a transparent viscous gel. This viscous gel was transferred into the hot air oven and dried at 120 °C for 24 h. The dried gels were ground to fine powders using an agate mortar and pestle and were used for further analysis. 2.2. Characterization Techniques. The as-dried powders were subjected to thermal treatment at specific temperatures with a dwell time of 4 h to investigate the structural and thermal stability. The phase analysis of the composite powders was analyzed using a high resolution X-ray diffractometer (RIGAKU, ULTIMA IV, JAPAN) with Cu Kα radiation (λ = 1.5406 Å) produced at 40 kV and 30 mA to scan the diffraction angles (2θ) between 10 and 70° with a step size of 0.02° 2θ per second. The Raman spectroscopic analysis of the powders was carried out by using backscattering geometry of Raman microscope (RENISHAW, UNITED KINGDOM). All the powder samples were excited at a wavelength of 785 nm by semiconductor diode laser (0.5% of power) with the data acquiring time of 30 s. Magnetic hysteresis measurements were carried out using a vibrating sample magnetometer (VSM, LAKE SHORE 7404) under an applied magnetic field at room temperature with a maximum field of 15 k Gauss. The surface morphology of the sintered specimens was analyzed through a scanning electron microscopy (SEM, FEI QUANTA-FEG 200, USA). The emission spectrum was recorded with an excitation wavelength of 350 nm using a fluorescence spectrofluorometer (FLUOROMAX-4, HORIBA SCIENTIFIC). Quantitative phase analysis through Rietveld refinement for the selected compositions was performed using the GSAS-EXPGUI software package. For the analysis through Rietveld refinement, an average of three scans were recorded for each powder sample in the range between 10° (2θ) and 70° (2θ) with a step size of 0.02° 2θ. The standard crystallographic data for the refinement of c-ZrO2, t-ZrO2, and α-Al2O3 were obtained from Wyckoff,15 Howard et al.,16 and Newnham et al.,17 respectively. The refinement procedure was performed in accordance with an earlier report by the authors,18,19 and the structural refinement was carried out with the following approach. The refinement parameters were background with shifted Chebyshev polynomial, scale factors, lattice parameters, and occupancy factors. Dy3+ occupancy at the ZrO2 lattice was refined by the inclusion

Figure 1. XRD spectra recorded at 1100 °C. The diffraction lines of standard α-Al2O3, t-ZrO2 and c-ZrO2 and corresponding to JCPDS Card Nos. 01-080-0786, 01-079-1765, and 01-071-4810, are also plotted.

of t-ZrO2, cubic zirconia (c-ZrO2), and α-Al2O3 at various temperatures, and the observed phases were confirmed by matching with their respective standard Joint Committee on Powder Diffraction Standards (JCPDS) Cards Nos., namely, 01-079-1765 for t-ZrO2, No. 01-071-4810 for c-ZrO2, and No. 01-080-0786 for α-Al2O3. At 1100 °C (Figure 1), XRD patterns displayed the discrete presence of ZrO2 in all the compositions with no traces of Al2O3 formation. The gradual t- → c-phase transition of ZrO2 is witnessed as a function of enhanced Dy3+ content. A similar kind of result was also observed in a ZrO2− SiO2 system with Gd3+ additions.21 The presence of discrete ZrO2 and the absence of any peak that corresponds to either Dy2O3 or other form of Dy3+ crystallization infer the complete solubility of Dy3+ in the ZrO2 matrix. Besides, the presence of Al2O3 in the amorphous state at 1100 °C is understood by its high activation energy required for α-Al2O3 crystallization that generally occurs at 1200 °C. Diffraction patterns of DZTA systems recorded at 1200 and 1500 °C (Figures 2 and 3) displayed similar profile, however, with an exception of enhanced crystallization with respect to the rise in temperature. Crystallization of α-Al2O3 is witnessed at 1200 °C, and its intensity is not altered by the Dy3+ additions. The observation of t-ZrO2 for low Dy3+ additions (DZTA10) and c-ZrO2 formation for high Dy3+ additions (DZTA20 and above) were witnessed at 1200 °C, and the corresponding phases remained stable throughout the elevated B



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Figure 2. XRD spectra recorded at 1200 °C. The diffraction lines of standard α-Al2O3, t-ZrO2, and c-ZrO2 and corresponding to JCPDS Card Nos. 01-080-0786, 01-079-1765, and 01-071-4810 are also plotted.

Figure 4. Raman spectra of all the compositions recorded at 1200 °C (a) and at 1500 °C (b). Figure 3. XRD spectra recorded at 1500 °C. The diffraction lines of standard α-Al2O3, t-ZrO2, and c-ZrO2 and corresponding to JCPDS Card Nos. 01-080-0786, 01-079-1765, and 01-071-4810 are also plotted.

ZrO2 at 1200 °C and cubic form at 1500 °C, and this inference justifies the influence of both dopant concentration and calcination temperature to effect t- → c-ZrO2 phase transitions. The observed gradual reduction in the intensity of t-ZrO2 active modes and the emergence of c-ZrO2 active modes in DZTA10 with the influence of progressive calcination is witnessed at 1200 and 1500 °C. A sharp shift toward the lower frequency region of c-ZrO2 active modes with respect to the increased Dy3+ content is witnessed at both 1200 and 1500 °C. Such a shift in cubic fluorite of ZrO2 structure with simultaneous enhancement in the Dy3+ content indicates its accommodation at the ZrO2 lattice. In accordance with the XRD results obtained at 1500 °C, the Raman spectra display highly scattered vibrations for DZTA100 that account for its structural distortions induced by the excess Dy3+. Rietveld refinement was performed to gain quantitative information on phase compositions, lattice data, and Dy3+ occupancy at the ZrO2 lattice. The refined diffraction patterns of selective compositions are presented in Figure 5, and the reliability factors of refined XRD patterns are presented in Table S1 (Supporting Information 1). The crystallization of tetragonal unit cell with P42/nmc (137) space setting for t-ZrO2, cubic unit cell with Fm3̅m (225) space setting for c-ZrO2 and hexagonal crystal setting with a space group R3̅c for α-Al2O3 were confirmed from the refined powder diffraction patterns. The phase fractions of the compositions at various calcination temperatures obtained from refinement are shown in Table 2. The unique presence of t-ZrO2 and c-ZrO2 is respectively

heat treatments until 1500 °C. The absence of any monoclinic zirconia (m-ZrO2) trace in any of the compositions throughout investigated temperatures indicates the role of Dy3+ in the stabilization of both t-ZrO2 and c-ZrO2. The crystalline nature of α-Al2O3, t-ZrO2 and c-ZrO2 improved with progressive heat treatments. The presence of secondary phases has been detected in case of DZTA100 that possessed high Dy3+ content (indicated by arrow mark in Figure 3) and hence any further investigation on that particular composition was not proceeded. Raman spectroscopy is an effective technique to determine the vibrations of metal oxygen bonds, and any kind of disorder or defects in a crystal structure are corroborated. Figure 4a,b depicts the corresponding Raman spectra of all the compositions at 1200 and 1500 °C. The number of Raman active modes differs with respect to the polymorphs of same metal oxides in which c-ZrO2 displays one active mode in the range between 600 and 620 cm−1, whereas t-ZrO2 polymorph (P42/nmc (137)) exhibits six active modes approximately at 147, 261, 318, 463, 611, and 641 cm−1.22,23 Raman spectra displayed active modes of t-ZrO2 polymorph at lower Dy3+ content and c-ZrO2 polymorph for high Dy3+ content. Six Raman active modes of t-ZrO2 are witnessed in DZTA10 at both 1200 and 1500 °C. DZTA20 perceived active modes of tC



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The structural parameters of c-ZrO2 and t-ZrO2 determined from the refinement of all the compositions at selective temperatures are presented in Table 3. However, structural parameters of both t-ZrO2 and c-ZrO2 indicated significant variations with respect to the Dy3+ content in the compositions. In the case of t-ZrO2 stabilization, Dy3+ additions induced simultaneous enhancement in a = b-axis and reduction in the caxis parameters at 1100 °C. A similar trend of reduction in the a = b-axis and enhancement in c-axis parameters were also observed during progressive heat treatments. In the case of cZrO2 stabilization, a gradual enhancement in the overall a = b = c-axis parameters is observed with enhanced Dy3+ additions, and a similar trend was also observed during progressive heat treatments. The enhancement in the lattice data is justified by the substitution of larger sized Dy3+ for the smaller sized Zr4+ at the ZrO2 structure. Table 4 illustrates the changes in unit cell volume and Dy3+ occupancy at the ZrO2 lattice of all the investigated compositions at 1400 °C. Unit cell volume of ZrO2 and occupancy level of Dy3+ at the ZrO2 lattice indicated an enhancing trend as a function of Dy3+ content. The unit cell volume of α-Al2O3 (Table 4) did not show any significant changes as a function of Dy3+ content. Figure 6 emphasizes a gradual increment in the a = b-axis parameters until 20 wt % of Dy3+ additions, whereas an abrupt upsurge in a = b-axis parameters that is equal to c-axis parameter is witnessed right from the 30 wt % of Dy3+ additions to the ZTA systems. The graphical representation of the cell volume (Figure 6) also exhibits good coherence with the lattice data due to the diverse level of Dy3+ inclusions at the ZrO2 lattice. The emission bands (Figure S1, Supporting Information 2) are observed in all samples at the wavelength range of 440− 460, 470−490, and 560−590 nm. Bands detected at ∼483 and ∼583 nm correspond to the typical fluorescence of Dy3+, whereas a band at 452 nm is assigned to the emission by the host lattice.24 The fluorescence band detected at blue (483 nm) and yellow regions are due to the magnetic dipole (4F9/2 → 6 H15/2) and electric dipole (4F9/2 → 6H13/2) transition of Dy3+.25 It has been stated that electric dipole transition is more hypersensitive to the host lattice, whereas a magnetic dipole is less sensitive.26,27 DZTA10 and DZTA20 compositions displayed triplet emission band at yellow region, however with reduced intensity as a function of Dy3+ content. The compositions beyond DZTA20 exhibited a single band at the yellow region, and this particular band also displayed a large shift compared to the bands determined for DZTA10 and DZTA20. The band that pertains to the magnetic dipole transition of Dy3+ indicated a shift between low (DZTA10 and DZTA20) and high (DZTA30 and DZTA100) Dy3+ contents in ZTA. However, these shift are minor as compared to the shift observed in the yellowish region. Beyond DZTA20, no

Figure 5. Refined diffraction patterns of selective DTZA compositions. Panels a and b correspond to DZTA10 and DZTA30.

confirmed in DZTA10 and all the other DZTA compositions at 1100 °C. Both at 1200 °C and higher temperatures all the compositions displayed the presence of two distinct phases, namely, ZrO2 and α-Al2O3. Common changes are perceived in all temperatures in the context of t- → c-ZrO2 phase transitions that are influenced by Dy3+ content. The t-ZrO2 is detected for DZTA10 at all calcination temperatures. The effect of temperature on t- → c-ZrO2 phase transition is witnessed in DZTA20 that indicated t-ZrO2 at 1100 °C and its transformation to c-ZrO2 at 1400 °C. The general trend on the gradual enhancement in phase content of α-Al2O3 is witnessed with respective increments in the heat treatment. Since the first instance of α-Al2O3 crystallization occurred at 1200 °C, progressive heat treatments have led to its improved crystal growth and hence displayed its enhanced phase content as a function of temperature increments. Besides, it is also due to the entry of Dy3+ into the crystal lattice of ZrO2; the Dy3+ additions led to the simultaneous increments in the phase content of ZrO2. In other words, a gradual decline in the phase content of α-Al2O3 is noticed with enriched Dy3+ additions.

Table 2. Phase Fractions of the Compositions at Various Calcination Temperatures Determined from Refinement phase fractions (wt %) 1100 °C sample code ZTA DZTA10 DZTA20 DZTA30 DZTA50 DZTA100

c-ZrO2

100 100 100

1200 °C

t-ZrO2

α-Al2O3

68.50 100 100

31.50

c-ZrO2

49.80 52.60 75.50 D

1400 °C

t-ZrO2

α-Al2O3

c-ZrO2

42.20 50.90

57.80 49.10 50.20 47.40 24.50

88.80 45.10 56.20 60.60

t-ZrO2

α-Al2O3

64.30

35.60 11.20 54.90 43.80 39.40



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Table 3. Structural Parameters of c-ZrO2 and t-ZrO2 Determined from the Refinement of All the Compositions at Selective Temperatures lattice parameter values (Å) 1100 °C

1200 °C

t-ZrO2

c-ZrO2

sample code

a=b

c


3.5986(2) 3.6002(4) 3.6184(4)

5.1759(3) 5.1757(4) 5.1556(3)

t-ZrO2

a=b=c

a=b

c

3.5936 3.6144 (4) 3.6404 (4)

5.1873 5.1826 (4) 5.1328 (4)

5.1456(4) 5.1568(4) 5.1882(3)

Table 4. Changes in Unit Cell Volumes and Dy3+ Occupancy at the ZrO2 Lattice of All the Compositions at 1400 °C


t-ZrO2

c-ZrO2

67.71 (5) 68.05 (4) 137.04 (5) 138.31 (3) 140.79 (4)

α-Al2O3 254.95 254.96 255.13 255.08 254.99

(5) (3) (2) (2) (4)

a=b=c

5.1439 5.1556 5.1714 5.2022

(4) (4) (4) (4)

t-ZrO2

c-ZrO2

a=b

c

3.6204 (4)

5.1527 (4)

a=b=c

5.1438 5.1445 5.1701 5.1894

(5) (4) (4) (4)

significant variations in the emission intensity were noticed. Nevertheless, emissions assigned for host in all the compositions are mainly due to the oxygen vacancies in the host material. Any influence in magnetic behavior due to Dy3+ additions in ZTA were analyzed by performing VSM measurements at room temperature under the applied magnetic field in range of −10000 to +10000 Oe. The paramagnetic behavior of all the DZTA compositions is confirmed from the obtained magnetization-hysteresis curves (Figure S2, Supporting Information 3), and the results indicated good coherence with the literature values.28 The microstructures of the DZTA specimens sintered at 1500 °C are presented in Figure 7. The morphology of all the specimens displayed the presence of two distinct phases by the virtue of observed two different grains that displayed contrast colors. Grain size and its distribution tend to differ with the varied level of Dy3+ additions in ZTA systems. DZTA10 displayed closely packed small sized grains along with their uniform distribution, however, with the intergranular voids observed in random areas. The presence of intergranular porosity indicates the poor densification of the material. It is also noted from the morphologies that enhanced Dy3+ additions resulted in the grain size enhancement as it is witnessed in DZTA30 and DZTA50. Grains in DZTA30 displayed a well-defined growth with a negligible amount of intergranular voids, however with a less uniformity observed in all the grains. DZTA50 demonstrated a morphology that possessed spherical sized grains, and most of the grains were observed to be elevated from the boundary layer. Again, the existence of intergranular voids is also seen in DZTA50 by virtue of the observed loosely packed grains.

volume (Å3) sample code

1400 °C c-ZrO2

Dy3+ occupancy at t-ZrO2* and c-ZrO2 lattice 0.14 0.23 0.57 0.70 1.00

Figure 6. Influence of lattice parameters and unit cell volume of ZrO2 with respect to Dy3+ additions in the ZTA system at 1200 °C.

Figure 7. Microstructures of the DZTA specimens sintered at 1500 °C. Figure 8, panels a, b, and c correspond to DZTA10, DZTA30, and DZTA50. E



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Figure 8. Load vs. displacement profile recorded for selective DZTA specimens. Panels a, b, and c correspond to DZTA10, DZTA30, and DZTA50.

stability compared to the existing ZTA composites. Dy3+ additions mainly affected the ZrO2 crystal structure rather than the α-Al2O3, and moreover t- → c-ZrO2 phase transition has been induced due to the gradual accumulation of Dy3+ at the ZrO2 lattice. Dy3+ additions did not induce any structural alterations in α-Al2O3 rather than its delayed crystallization. Usually, α-Al2O3 crystallizes ∼1200 °C through sequential phase transition via intermediate phases in the order of γ → δ → θ → α.35 The prior crystallization of ZrO2 ahead of α-Al2O3 in the in situ systems observed in the present study is due to the comparably low activation energy required for ZrO2 crystallization rather than α-Al2O3. A majority of the existing literature reports on ZTA systems indicate their formation via the mechanical mixing of individual α-Al2O3 and t-ZrO2 components.12,36,37 It is observed that Zr4+ is insoluble in Al2O3 lattice; rather it tends to segregate at the grain boundaries of α-Al2O3.38 Moreover, the solubility of Zr4+ at the boundaries is limited by grain size factor and hence Zr4+ precipitates as ZrO2 beyond critical grain size.39 Characterization studies exhibited good uniformity in the refined lattice data of α-Al2O3 and moreover attempt to refine the possibility of either Dy3+ or Zr4+ occupancy at the α-Al2O3 lattice also yielded negative values, and these inferences reveal the fact that α-Al2O3 crystallization is an independent process and are not altered by the additives. Some reports have focused on the alternative additives to stabilize ZTA systems instead of the routinely utilized yttria. Cerium additions in ZTA systems are reported to form solid solution of Zr1−xCexO2 with unperturbed Al2O3 phase.40 TiO2 additions beyond 3 wt % to the ZTA systems resulted in the formation of a thermodynamically stable Al2TiO5 component.41 Meanwhile, MgO additions indicated its reaction with Al2O3 to yield a thermodynamically stable MgAlO4 spinel along with associated t- → m-ZrO2 phase transition beyond 1100 °C.42 Here it is perceived that Dy3+ additions tend to disrupt ZrO2 rather than the Al2O3 component in the composite except its delayed crystallization. It has been determined from the quantitative analysis that progressive Dy3+ additions led to the lattice expansion of ZrO2, which is quite understood by the replacement of higher sized Dy3+ (0.91 Å) for the lower sized Zr4+ (0.79 Å). The t- → cZrO2 transition is mainly induced by the excess Dy3+ content in the ZTA system, and to a certain extent the temperature effect also plays a crucial role in this phase transition. For instance, 20 wt % of Dy3+ content preserves t-ZrO2 at 1200 °C, whereas cZrO2 is stabilized at 1400 °C. Beyond 20 wt % of Dy3+, all the systems intend to stabilize c-ZrO2 that is invariable of the temperature effect. The lattice data of the corresponding t-ZrO2 and c-ZrO2 JCPDS standards are given as a- = b- = 3.596 Å, c- = 5.184 Å and a- = b- = c- = 5.100 Å. Here, the Dy3+ inclusions at the ZrO2 induced gradual lattice expansion along the a- = b-

Mechanical properties for selected DZTA specimens sintered at 1500 °C were determined through nanoindentaion. The recorded load vs. displacement profile is shown in Figure 8, and the resultant average hardness and Young’s modulus values were determined based on the Oliver−Pharr method (Table 5).29 The load profile of an indentation curve determines the Table 5. Mechanical Properties of DZTA Specimens Determined from Nanoindentation sample code

hardness (GPa)

Young’s modulus (GPa)

DZTA10 DZTA30 DZTA50

09.34 (±2.86) 20.37 (±1.43) 12.63 (±3.37)

190.68 (±36.80) 248.40 (±14.69) 235.22 (±32.69)

density of a material in qualitative manner, while the unloading curve correlates with the stiffness of material that is proportionate to the material’s hardness. Pop-ups were observed in the loading profile of DZTA10 and DZTA50, and the chance of such pop-ups arises from the presence of voids in interlayer or porosity material. The displacement profile that corresponds to the penetration depth was also not found to be uniform in the case of both DZTA10 and DZTA50 compositions. These facts are justified by the microstructures of both the compositions that display intergranular porosity in the case of DZTA10 and loosely packed grains in the case of DZTA50. However, DZTA30 demonstrate uniform loading and unloading profile and also constant displacements, which suggest its better densification and also absence voids or porosity in interlayers. Such a phenomenon has been reflected in the mechanical data in which DZTA30 recorded maximum values of hardness and Young’s modulus. Moreover, the attained data are found to be higher than the values reported in the literature.30,31

4. DISCUSSION The X-ray analysis of pure ZTA that was formed void of any stabilizer indicated the distinct presence of t-ZrO2 at 1100 °C, whereas the crystallization of α-Al2O3 along with the observed t- → m-ZrO2 phase transition is witnessed at 1200 °C. In the present study, it was a planned motive to use Dy3+ as a stabilizer for two major reasons: (i) to preserve t-ZrO2 phase at higher temperatures and (ii) to explore the magnetic characteristics of Dy3+ as a contrast agent. Incorporation of Dy3+ in ZTA composites enhanced the thermal stability of the composites until 1500 °C by preventing t-ZrO2 degradation, and this has been indicated from the characterization studies. ZTA composites formed by the mechanical mixing of commercial metastable t-ZrO2 and α-Al2O3 induced m-ZrO2 formation around 1500 °C.32−34 Herein, the in situ method of Dy3+ additions to ZTA composites displayed good thermal F



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axis, whereas the c-axis remains unperturbed for the whole range of Dy3+ additions. During continuous inclusions of Dy3+ at the ZrO2 lattice, a critical stage of lattice expansion is reached for 30 wt % Dy3+ additions where the a- = b- axis and c-axis becomes equal, thus leading to the complete transformation of c-ZrO2. Both the observed t-ZrO2 and c-ZrO2 stabilization induced by progressive Dy3+ additions is correlated with the generation of oxygen vacancies. It has been reported that doping higher sized element intends to segregate around the ZrO2 boundaries and causes strong resistance to the phase transformations.43 Most of the metastable t-ZrO2 systems induced by stabilizers are due to the creation of oxygen vacancies that arise from the mismatch in the oxidation state between Zr4+ and stabilizers in the form of Ca2+, La3+, Y3+, or Mg2+.23,44−46 With more of the oxygen vacancies induced by stabilizer the maximum stability of t-ZrO2 system is achieved and thereby resisting the t- → m-ZrO2 transition at room temperature. In other words, the susceptibility to undergo t- → m-ZrO2 transition is enhanced by the reduction of oxygen vacancies. The ratio of O/Zr could also affect t-ZrO 2 stabilization, and the reported limit for O/Zr ratio is 1.63 to 1.98.47 Garvie et al. stated that reducing the particle size could influence t-ZrO2 stabilization at room temperature, and the proposed size was ∼300 Å.48 It is thus ascertained from the present results that both tetragonal/cubic forms of ZrO2 achieved in the DZTA system are due to the creation of oxygen vacancies rather than the role of particle size, which is rather a challenging task to preserve in nanoscale at 1500 °C. Raman results of DZTA systems showed good concurrence with the XRD results. The presence of t-ZrO2 for low Dy3+ additions is confirmed from their characteristic six Raman active modes, and the formation of c-ZrO2 for high Dy3+ additions is indicated from their characteristic single Raman active mode determined ∼600 cm−1. The bands typical of α-Al2O3 were determined at 380 and 410 cm−1, and these bands remain unperturbed by the varied level of Dy3+ additions, thus articulating the absence of structural defects induced by Dy3+ content. A gradual Raman shift toward the lower wavelength of ZrO2 with simultaneous enhancement in the Dy3+ additions further confirms the occupancy of higher sized Dy3+ at the ZrO2 lattice. The inclusion of Dy3+ in the ZTA system is also expected to disturb its emission characteristics. The magnetic dipole (MD) and electric dipole (ED) transition of Dy3+ has been confirmed by their corresponding emissions at 480 and 580 nm. A strong shift observed at 580 nm with progressive Dy3+ additions is correlated with the observed t- → c-ZrO2 transition. It has been shown that the ED transition of Dy3+ is more hypersensitive to the host, and any structural changes to the host structure is expected to influence the emission characteristics. Moreover, XRD and Raman results also confirmed the influence of Dy3+ to induce t- → c-ZrO2 transition. This evidence further supports the aforementioned reason for the shift in emission bands. Fu et al.23 reported that the presence of oxygen vacancies acts as a sensitizer to induce the effective energy transfer between host and Dy3+. The observed emission at 452 nm assigned for emission due to the host lattice agrees with the results of Fu et al.23 In high Dy3+ content, the corresponding emission intensities of ED and MD transitions are observed with a declining trend, and this infers the quenching effect. A similar observation is also reported by Diaz-Torres et al.26 in which the quenching effect is observed with 2 mol wt % of Dy3+ ion in ZrO2. However, in the DZTA system a quenching effect is

observed starting from DZTA50 onward, and this could be expected due to the presence of Al2O3. Surface morphologies of the DZTA specimens sintered at 1500 °C indicates the optimum Dy3+ content necessary to achieve improved mechanical properties. All the microstructures indicated two different contrast colored grains with their corresponding uniform distribution throughout the matrix, thus signifying the uniform existence of ZrO2 and Al2O3 in the composite achieved by the in situ synthetic route. DZTA10 showed the uniform distribution of small sized grains, however with the observed excess intergranular porosity throughout the matrix. The high Dy3+ content (DZTA50) displayed an excessive grain growth with almost spherical sized grains, however with the observed lack of pinning between two different phases. DZTA30 that possessed medium Dy3+ content demonstrated controlled grain growth, and moreover the apparent pinning between two different phases has led to the absence of voids in the microstructure. Besides, results from the quantitative X-ray analysis ensure the presence of excess αAl2O3 content in DZTA30, which is the highest among the systems investigated for mechanical analysis. It is also reported that pure Al2O3 always displays better hardness values than ZrO2 systems.30 Thus, it could be stated that excess Dy3+ content in ZTA systems is expected to affect the grain growth and stability of the microstructure. Indentation values shows a strong correlation with the SEM results by the determined enhanced hardness and Young’s modulus data for DZTA30 composition. The presence of pores in the interlayer is witnessed from the observed pop-ups in the loading profile and different penetration depths of DZTA10 and hence recorded low values from the indentation results.

5. CONCLUSIONS In the present study, it has been shown that Dy3+ could be used as a stabilizer to improve the structural stability of ZTA composites. The critical role of Dy3+ in the ZrO2 component of the ZTA composite was articulated from the analytical techniques involving XRD, Raman spectra, quantitative studies through Rietveld refinement, and photoluminescence spectra. 20 wt % of Dy3+ occupancy at the ZrO2 lattice is determined as a critical limit to retain t-ZrO2, whereas beyond this value cZrO2 formation is accomplished. Moreover, it was also deduced that the α-Al2O3 component of the composite remains unperturbed due to Dy3+ additions. The structural stability of Dy3+ added ZTA composites is maintained until 1500 °C, and the results also signified the fact that optimum Dy3+ content in the composite is essential to achieve good mechanical stability. The paramagnetic features displayed by the investigated systems are currently being explored for their corresponding in vitro MRI and luminescence features, and the outcome will be reported shortly.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01355. Table S1: Reliability factors of refined powder XRD patterns; Figure S1: Emission spectra of DZTA compositions with an excitation at 350 nm; Figure S2: Magnetization curves of DZTA compositions determined at 1400 °C (PDF) G



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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 0091-413-2654973. ORCID

S. Kannan: 0000-0003-2285-4907 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial assistance received from DST-SERB [Reference: EMR/2015/002200 dated 20.01.2016] India is acknowledged. The facilities availed from Central Instrumentation Facility (CIF) of Pondicherry University are also acknowledged.

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