Gadolinium Doping in Zirconia-Toughened Alumina Systems and

Article pubs.acs.org/IC

Gadolinium Doping in Zirconia-Toughened Alumina Systems and Their Structural, Mechanical, and Aging Behavior Repercussions V. Ponnilavan,† Pavan Poojar,‡ Sairam Geethanath,‡ and S. Kannan*,† †

Centre for Nanoscience and Technology, Pondicherry University, Puducherry 605 014, India Medical Imaging Research Centre, Dayananda Sagar Institutions, Bangalore, India

‡

S Supporting Information *

ABSTRACT: A series of Gd3+ dopings in zirconia-toughened alumina (ZTA) systems were undertaken to explore the resultant structural, morphological, hydrothermal aging, and mechanical behavior and imaging contrast abilities. The results from the characterization techniques demonstrate the significance of Gd3+ in preserving the structural stability of ZTA systems. ZTA undergoes phase degradation with 10 wt % Gd3+ at 1400 °C, while the 100 wt % Gd3+ yields GdAlO3 even at 1200 °C. Gd3+ doping at the intermediate level preserves the structural stability of ZTA systems until 1400 °C. Gd3+ occupies the ZrO2 lattice, and its gradual accumulation induces tetragonal ZrO2 (t-ZrO2) to cubic ZrO2 (c-ZrO2) phase transition. α-Al2O3 crystallizes at 1200 °C and remains unperturbed except for its reaction with the free Gd3+ ions to yield GdAlO3. Aging studies and mechanical tests signify the impeccable role of Gd3+ in ZTA systems to resist phase degradation. Further, the imaging contrast ability of ZTA systems due to Gd3+ doping is verified from the in vitro magnetic resonance imaging (MRI) tests. Ti4+ and Zr4+, the stabilizing effect of Ti4+ is limited by its smaller ionic size in comparison with Zr4+.20−22 A recent report by the authors emphasizes the optimum level of Dy3+ additions in ZTA systems results in improved mechanical features.23 It is shown that Dy3+ accommodation at the ZrO2 lattice effects a tto c-ZrO2 phase transition in ZTA systems while the Al2O3 component of the composite remains unperturbed during the whole process. In a further extension of the existing reports, the present study is aimed at exploring the influence of gadolinium (Gd3+) doping in a ZTA system. The rationale behind the Gd3+ doping relies on not only its role as a stabilizer but also its inherent magnetic resonance imaging (MRI) contrast features that could be tailored to observe the in vivo performance of the ZTA implant in a noninvasive manner. Gd3+ with seven unpaired 4f electrons displays remarkable magnetic properties, which enables the use of Gd3+-based complexes as MRI contrast agents in the medical industry. Gd3+-based complexes display both longitudinal (T1) and transverse (T2) relaxivity under a magnetic field that facilitates their use as both T1 and T2 MRI contrast agents.24,25 In this context, a wide range of Gd3+ doping in ZTA systems and the resultant structural and mechanical stability, aging, and imaging contrast features have been investigated in the present study.

1. INTRODUCTION Salient features of ZTA, for example, excellent corrosion resistance, biocompatibility, and mechanical stability, have made zirconia-toughened alumina (ZTA) systems inevitable in orthopedic industries.1−4 The respective shortcomings of brittleness and phase degradation encountered by Al2O3 and ZrO2 are annulled by their effective combination in ZTA systems.5−9 The majority of the ZTA systems use Y3+ as a stabilizer to retain metastable tetragonal ZrO2 (t-ZrO2) in the composite.10−12 The toughening role of ZrO2 in preventing the crack growth of Al2O3 in ZTA systems is well established.13−15 Further, in vitro and in vivo studies have also demonstrated the high resistance displayed by ZTA systems in tetragonal ZrO2 (tZrO2) to monoclinic ZrO2 (m-ZrO2) phase degradation in comparison with the monolithic ZrO2.16,17 Nevertheless, hydrothermal degradation tests still ensure the occurrence of t- to m-ZrO2 transitions in ZTA systems.18 The gradual formation of m-ZrO2 fractions during in vitro and in vivo studies implies a great effect in the loss of mechanical properties and subsequently affects the premature removal of the implant from the host site. In this context, several investigations are currently being performed to improve the properties of ZTA systems with the major focus centering on countering the phase degradation phenomenon. Recent years have witnessed the use of stabilizers such as CeO2, TiO2, and Dy2O3 to improve the phase stability of ZTA systems. The studies on CeO2-stabilized ZTA (Ce-ZTA) systems demonstrate better mechanical features along with improved resistance to phase degradation in both in vitro and in vivo models.18,19 Despite the similarities in the valence of © 2017 American Chemical Society

2. EXPERIMENTAL METHODS 2.1. Powder Synthesis. A citrate-assisted sol−gel process was used for the powder synthesis. Analytical grade Gd(NO3)3·6H2O, Received: May 23, 2017 Published: August 28, 2017 10998

DOI: 10.1021/acs.inorgchem.7b01291 Inorg. Chem. 2017, 56, 10998−11007

Article

Inorganic Chemistry ZrOCl2·8H2O, and Al(NO3)3·9H2O were used as the precursors along with the use of citric acid (C6H8O7) as a fuel to catalyze the reaction process. In this current study, five different compositions were synthesized with progressive amounts of Gd3+ additions to the constant concentrations of Zr4+ and Al3+ precursors. Precursor concentrations along with the sample codes are presented in Table 1.

to avoid any immersion in water during the experiments. Autoclaving was performed up to 60 h, and the resultant specimens were subjected to XRD analysis to determine structural changes during hydrothermal exposure. 2.5. In Vitro MRI Tests. The T1 and T2 relaxivities of selective GZTA samples (GZTA10, GZTA30, and GZTA50) were determined with a Siemens Magnetom Avanto 1.5 T MRI scanner. The obtained in vitro phantom images were used to enumerate the relaxivity values of GZTA samples at different concentrations. The enumeration of relaxation rate maps by fitting all the corresponding curves was done with Matlab (The Mathworks Inc., MA, USA) software. A fast spin echo (FSE) sequence with an echo train length of 16 and a repetition time (TR) value of 3000 ms, matrix size of 512 × 512, slice thickness of 5 mm, and number of slices 1 was used. The echo time (TE) was varied from 22 to 352 ms with a difference of 22 ms, resulting in 16 images with varied T1 and T2 weighting. The T1 and T2 relaxation times were determined from the obtained images.

Table 1. Precursor Concentrations Used during the Synthesis concentration of precursors (M) sample code

Al(NO3)3

ZrOCl2

Gd(NO3)3

ZTA GZTA10 GZTA20 GZTA30 GZTA50 GZTA100

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

3. RESULTS 3.1. Phase Analysis. The phase analysis of varied levels of Gd3+ doping in ZTA (GZTA) systems at selective temperatures was determined through XRD analysis. Polymorphs of ZrO2 (tZrO2 and c-ZrO2) and α-Al2O3 formation at various temperatures were confirmed by their corresponding matches with the standard ICDD Card Nos. 01 079 1765 for t-ZrO2, 01 0714810 for c-ZrO2, and 01-080-0786 for α-Al2O3. Depending on the Gd3+ content, two different polymorphs of ZrO2 are confirmed from the XRD patterns recorded at 1100 °C (Figure 1a). A Gd3+ content of less than 20 wt % retains t-ZrO2, whereas Gd3+ addition beyond 20 wt % articulates the c-ZrO2 formation. Further progressive Gd3+ additions result in the gradual shift of XRD reflections of ZrO2 toward lower diffraction angles. Neither Al2O3 crystallization nor the formation of secondary phases at 1100 °C implies poor crystalline behavior of the GZTA systems. However, a contrasting behavior in the crystallization of GZTA systems is noticed from the XRD patterns recorded at 1200 °C (Figure 1b). The X-ray reflections that correspond to the crystallization of α-Al2O3 are noticed in all of the GZTA systems at 1200 °C. Nevertheless, the phase behavior of the ZrO2 component in all systems has been found to be the same at both 1200 and 1400 °C. GZTA10 ensures the t- to m-ZrO2 transition, while all the remaining compositions retained c-ZrO2 along with the enhanced α-Al2O3 crystallization at 1400 °C (Figure 1c). The composition with high Gd3+ content (GZTA100) yields a minor amount of GdAlO3 above 1200 °C. 3.2. Quantitative Implications. The effects of diverse Gd3+ doping levels in ZTA systems were analyzed through the refinement of their respective XRD patterns. The refinement of the resultant diffraction patterns (Figure 2a−c) affirms the distinct presence of t-ZrO2, c-ZrO2, and α-Al2O3 phases, which respectively crystallize in tetragonal (P42/nmc (No. 137) space group), cubic (Fm3̅m (No. 225) space group) and hexagonal (R3̅c) unit cells. A gradual augmentation in the phase content of ZrO2 as a function of Gd3+ content is apparent at both 1200 and 1400 °C (Table 2). In contrast, the phase fractions of αAl2O3 display an enhanced trend with respect to the temperature increments, which infers the lack of available thermal energy for its enhanced crystallization. GZTA10 undergoes phase degradation at 1400 °C with a significant amount of m-ZrO2 formation. The compositions that range from GZTA20 to GZTA50 determine the unique presence of cZrO2 and α-Al2O3 at the investigated temperatures. However,

In a brief description of the synthesis, the discrete stock solutions of Gd(NO3)3, Al(NO3)3, and ZrOCl2 were mixed together with stirring at constant temperature. After 15 min, an appropriate amount of C6H8O7 was added to these cationic mixtures and the resultant mixtures were continuously stirred until the formation of a gel. The gel was dried at 120 °C overnight and ground into a fine powder. 2.2. Powder Characterization. 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 was used to analyze the phase behavior of powder samples after heat treatment at predetermined temperatures with a dwell time of 4 h. X-ray scans were performed in the 2θ range between 5 and 90° with a step size of 0.02° 2θ per second. The Raman spectra (Renishaw, United Kingdom) were recorded at a wavelength of 785 nm by semiconductor diode laser (0.5% of power) with a data acquisition time of 30 s to determine the vibrational modes of the powder samples. Rietveld refinement of the powder XRD patterns were performed in accordance with the procedure described in our previous reports.23 The standard crystallographic data for the refinement of t-ZrO2, m-ZrO2, α-Al2O3, and GdAlO3 were obtained from Howard et al.,26 Smith et al.,27 Newnham et al.,28 and du Boulay et al.,29 respectively. The surface morphologies of the sintered specimens at 1400 °C were determined using high-resolution scanning electron microscopy (SEM, FEI QUANTA-FEG 200, USA). 2.3. Mechanical Evaluation. Select mechanical properties such as hardness and Young’s modulus were determined using the nanoindentation (CETR, USA) technique at room temperature. The sample preparation for the aforementioned task is as follows. Initially the synthesized powders were thermally treated at 700 °C in order to eliminate all the volatile impurities that arise from the citrate−nitrate synthetic process. The resultant powders were ball-milled (Retsch, Germany) for 2 h and then pressed in the form of pellets (13 mm diameter and 1 mm thickness) using a semiautomatic hydraulic press (Kimaya Engineers, India) under an applied force of 10 N for 60 s. The specimens thus pressed were sintered at 1400 °C for 4 h and then finely polished using diamond paste before indentation tests. A triangular pyramid (Berkovich) diamond tip indenter (50 nm radius of curvature) with an indent load of 5 mN was used. A conventional depth-sensing test with a measurement cycle (load/unload−displacement curves) consisted of a loading segment followed by a dwell time at maximum load; finally an unloading segment was used to determine the selective mechanical properties. All of the specimens were analyzed by single indents with a maximum of 10 indents for each specimen at random locations. 2.4. Aging Studies. The degradation behavior of the systems was tested under hydrothermal conditions at different aging periods. For this purpose, the selected GZTA specimens were hydrothermally treated in an autoclave at 134 °C and 2 bar pressure conditions at different time intervals. The specimens were carefully placed on a grid 10999


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GZTA20, a progressive trend in the t- → c-ZrO2 phase transition is noticed. Figure 2d presents variation in the lattice parameters of c-ZrO2 and crystallite size of α-Al2O3 with respect to the increment in the level of Gd3+ doping. The refined lattice parameters of α-Al2O3 (Tables 3 and 4) display negligible variations with respect to the diverse Gd3+ doping levels. The crystallite size of α-Al2O3 was determined for a comprehensive understanding on the effect of Gd3+ additions. A gradual reduction in the crystallite size of α-Al2O3 (Figure 2d) with respect to the enhanced Gd3+ content is observed, and this confirms the influence of Gd3+ in inhibiting the crystal growth of α-Al2O3. Notwithstanding, the hexagonal crystal setting of α-Al2O3 is not affected by the presence of Gd3+. 3.3. Raman Spectroscopy. The vibrational modes that correspond to the two different polymorphs of ZrO2 have been determined from the Raman spectra recorded at 1300 °C (Figure 3). Six Raman-active modes (A1g + 2B1g + 3Eg) representative of t-ZrO2 have been determined at 140, 253, 314, 456, 593, and 631 cm−1 for GZTA10 and GZTA20.30,31 GZTA30 and GZTA50, having a higher Gd3+ content, reveal a single band at 564 cm−1, which is assigned to the cubic fluorite structure of c-ZrO2.32,33 The Raman-active modes of GdAlO3 for GZTA100 are confirmed from the bands detected at 140, 242, and 312 cm−1.34 α-Al2O3 crystallization is also verified from the Raman bands at 380 and 410 cm−1.23,35 In addition, deep observation of the active modes of t-ZrO2 reveals a strong shift toward higher frequency with progressive Gd3+ additions. Such a shift is attributed to the crystal strain induced by the Gd3+ occupancy at the t-ZrO2 lattice. In addition, a similar trend is also observed for c-ZrO2 stabilization: however, with poor indication due to its broad nature. 3.5. Morphological Analysis. The morphological analysis determined from SEM (Figure 4) clearly indicates the composite formation by the observed two different grains, distinguished by their contrast nature. The sizes and distributions of grains in the composite are highly influenced by the Gd3+ content. In the case of compositions with low Gd3+ content, small-sized grains with an average size of ∼900 nm are uniformly distributed throughout the matrix. When the Gd3+ content in ZTA is increased, a major growth in the light contrast grains with a comparably minimum growth in the dark contrast grains are evident. The invariable absence of voids witnessed in the microstructures signifies the improved densification behavior of the composites. Literature reports also evince the contrast nature of grains observed in the ZTA composites.12,36,37 EDS analysis was employed to confirm the elemental distribution in the microstructures, and the results are presented in Figure 5. The darker grains are dominated by Al3+ along with the Gd3+ and Zr4+ traces, whereas Gd3+ and Zr4+ are determined in abundance in the case of light grains. 3.6. Nanoindentation. Nanoindentation is a useful technique to determine the surface hardness and Young’s modulus of any solid material. The selective GZTA specimens sintered at 1400 °C and pure ZTA at 1100 °C were subjected to indentation tests. The morphological features of pure ZTA indicate their poor sintering ability at 1100 °C, which is also reflected in the load−displacement profiles (Figure S1 in the Supporting Information with pop-ups denoted on the loading profile) and the resultant mechanical data derived from the indentation curve are very less (Table 5). In GZTA specimens, owing to the presence of two different grains, multiple indents were performed throughout the specimen to avoid any misperception of grain location. The multiple indents displayed

Figure 1. Diffraction pattern of various GZTA compositions recorded at (a) 1100, (b) 1200, and (c) 1400 °C. The standard patterns of tZrO2, c-ZrO2, and α-Al2O3 corresponding to ICDD Card Nos. 01-0791765, 01-071-4810, and 01-080-0786 are also presented.

GZTA100 with maximum Gd3+ content ensures a gradual increase in the formation of GdAlO3 as a function of temperature increments (Table 2). The variations in the refined lattice parameters due to the combined influence of Gd3+ content and temperature effects are presented in Tables 3 and 4. The lattice parameters of the ZrO2 system indicate major changes as a function of Gd3+ content, and this trend is corroborated by the pragmatic shift observed in the X-ray analysis. The t-ZrO2 stabilization is noticed until GZTA20, and thereafter a trend in the reduction of tetragonal to cubic ratio of the ZrO2 component accompanied by the simultaneous, respective enhancement and reduction in the a = b and c axis lattice as a function of Gd3+ content is witnessed. Beyond 11000


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Figure 2. Refined diffraction patterns of (a) GZTA10, (b) GZTA30, and (c) GZTA100. (d) Lattice parameter variations in ZrO2 and crystallite size of α-Al2O3 with respect to the incremental Gd3+ additions.

Table 2. Quantitative Phase Content Derived from the Refinement of Varied Amounts of Gd3+ Substitutions in ZTA Systems after Heat Treatment at Two Different Temperatures phase fraction (wt %) 1200 °C sample code GZTA10 GZTA20 GZTA30 GZTA50 GZTA100

c-ZrO2

64.60 75.40 55.60

1400 °C

t-ZrO2/*GdAlO3

α-Al2O3

c-ZrO2/*m-ZrO2

t-ZrO2/*GdAlO3

α-Al2O3

Gd3+ occupancy at t-/c-ZrO2

52.70 58.10

47.30 41.90 35.40 24.60 28.60

*18.10

24.70 50.70

57.20 49.30 46.60 36.90 30.40

0.37 0.52 0.68 0.88 0.78

*15.80

53.40 63.10 51.30

*18.30

Table 3. Refinement Agreement Factors and Lattice Parameters for Different GZTA Systems after Heat Treatment at 1200 °C lattice params refinement agreement factor

α-Al2O3 (Å)

t-ZrO2 (Å)

sample code

χ

Rp

a = b axis

c axis

ZTA GZTA10 GZTA20 GZTA30 GZTA50 GZTA100

1.472 1.719 1.548 1.391 1.386 1.75

7.11 7.05 06.64 6.73 7.01 9.46

3.5948 (8) 3.6129 (2) 3.6507 (3)

5.1976 (9) 5.1780 (4) 5.1685 (1)

2

c-ZrO2 (Å) a = b = c axis

a = b axis

5.1792 (1) 5.2061 (3) 5.2119 (4)

4.7565(2) 4.7568 (2) 4.7568 (2) 4.7571 (3) 4.7576 (4)

c axis 12.9882 12.9886 12.9877 12.9882 12.9880

(2) (2) (1) (1) (3)

3.7. Aging Studies. Low-temperature degradation (LTD) is a key aspect when ZrO2-based materials are considered for implant applications. Due to the excess amount of ZrO2 in GZTA composites, LTD tests were performed for selective compositions at 134 °C up to a maximum of 128 h,17 and any structural deformations during the tests were determined through periodic X-ray analysis. Figure 7 displays the XRD patterns of compositions recorded at selected time intervals during autoclave tests. All of the investigated compositions

in Figure 6 demonstrate smooth loading profiles with good consistency among each other, and moreover no pop-ups are obvious in any of the specimens. The determined hardness and Young’s modulus values (Table 5) signify a more or less good uniformity, and the achieved values demonstrate good consistency with the literature values11,38−40 reported for pristine ZTA. Thus, the sintering ability of the GZTA specimens is improved with Gd3+ doping in the ZTA matrix. 11001


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Table 4. Refinement Agreement Factors and Lattice Parameters for Different GZTA Systems after Heat Treatment at 1400 °C lattice params refinement agreement factor

α-Al2O3 (Å)

t-ZrO2 (Å)

sample code

χ

Rp

a = b axis

c axis

GZTA10 GZTA20 GZTA30 GZTA50 GZTA100

1.838 1.793 1.334 1.797 1.515

7.05 7.17 6.98 8.96 7.90

3.6134 (2) 3.6504 (4)

5.1906 (4) 5.1652 (1)

2

c-ZrO2 (Å) a = b = c axis

a = b axis(Å)

5.1786 (4) 5.2061 (3) 5.2053 (4)

4.7618 (2) 4.7615 (1) 4.7612 (2) 4.7626 (3) 4.7627(4)

c axis (Å) 12.9870 12.9871 12.9876 12.9872 12.9867

(2) (2) (1) (1) (4)

with respect to the increase in Gd3+ concentration. The continuous increment in the relaxivity values recorded for increasing Gd3+ concentrations demonstrates the T1-weighted MRI contrast features of GZTA samples. T2-weighted MRI images (Figure 9b) display a steady reduction in brightness for increasing Gd3+ concentrations. Thus, the T2-weighted MRI contrast features of GZTA samples are also verified from the upsurge demonstrated in r2 relaxivity.

4. DISCUSSION The ZrO2 component plays a key role in improving the mechanical properties of ZTA. Reports explore the use of selective additives with varied oxidation states (divalent to tetravalent) to attain metastable t-ZrO2 at room temperature.23,41−43 Literature reports also evince the minimum level of t- → m-ZrO2 phase transitions in ZTA systems in comparison to pristine YSZ.17 The t-ZrO2 component of the pure ZTA system synthesized via a citrate-assisted sol−gel process undergoes structural degradation even at ∼1200 °C.23 Gd3+ additions in the present study were intended to alter the structural stability of ZTA systems. Indeed, Gd3+ delays the crystallization of Al2O3 that is implicit from the high enthalpy of formation (ΔH) of α-Al2O3 (1676 kJ/mol) in comparison to that of ZrO2 (1100 kJ/mol). The available thermal energy is thus consumed to host Gd3+ at the t/c-ZrO2 lattice and as a consequence delays the crystallization of Al2O3. GZTA10 with low Gd3+ content undergoes structural degradation to yield mZrO2 at 1400 °C. Further, addition of Gd3+ in an intermediate range (GZTA20, GZTA30, and GZTA50) preserves structural stability as either t-ZrO2 or c-ZrO2 at 1400 °C. Gd3+ occupancy at the ZrO2 lattice is confirmed from the gradual changes induced in the ZrO2 lattice parameters with respect to the simultaneous increment in Gd3+ additions as determined from the refinement study. The observed steady increase in the unit cell parameters (a = b axis) along with a

Figure 3. Raman spectra of various GZTA compositions recorded at 1300 °C.

show strong resistance against hydrothermal aging, as the X-ray reflections of t/c-ZrO2 patterns remains the same before and after aging tests. In addition, the traces of m-ZrO2 nucleation sites are completely absent under all the investigated conditions at varied time intervals. 3.8. Magnetic and MRI Contrast Behavior. Figure 8 presents the room-temperature magnetization (M) curves recorded as a function of applied field (H). The linear relationship obtained in all the curves implies the paramagnetic nature of all the GZTA compositions. Magnetization values reveal a strong increase with respect to the Gd3+ content, and this implies that the magnetic properties of GZTA systems strongly depend on the inherent magnetic features of Gd3+. T1 and T2 weighted MRI images with corresponding r1 and r2 relaxivities are shown in Figure 9. The T1-weighted MRI image (Figure 9a) indicates a gradual shift from dark to bright color

Figure 4. Microstructures of the GZTA specimens sintered at 1400 °C. Figures 4a, b and c correspond to GZTA10, GZTA30 and GZTA50. 11002


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Figure 5. EDS analysis of a GZTA30 specimen sintered at 1400 °C. The elemental analysis was determined from the two different grains, and the corresponding atomic percentages of individual elements are also presented.

equals the c axis parameters, as witnessed in the case of GZTA30. Thus, the critical amount of ∼20 wt % Gd3+ induces a t- → c-ZrO2 phase transition. In the meanwhile, the α-Al2O3 component of the composite remains unperturbed until a certain amount of Gd3+ addition, as witnessed until GZTA50. Further, Gd3+ additions lead to a steady expansion of the cZrO2 lattice until a saturation occupancy limit is attained and the excess Gd3+ triggers GdAlO3 formation. Nonetheless, the formation of GdAlO3 is inevitable in the case of GZTA100, which possesses high Gd3+ content. This observed inference is mainly due to the enhanced reaction kinetics between the free Gd3+ that is rejected by the ZrO2 lattice and Al2O3 to form GdAlO3. In other words, Gd3+ occupancy at the ZrO2 lattice already attains the saturation limit and only the excess Gd3+ tends to react with Al2O3 to form GdAlO3. Further, the kinetics of GdAlO3 formation is

Table 5. Hardness and Young’s Modulus of GZTA Specimens Determined from Nanoindentation sample code

hardness (GPa)

Young’s modulus (GPa)

ZTA (from literature) ZTA GZTA10 GZTA30 GZTA50

14−1811,38−40 2.42(±0.24) 20.90(±0.64) 19.79(±1.07) 19.23(±0.58)

298.8638 44.52 (±02.03) 279.55(±09.43) 262.58(±13.05) 266.70(±06.23)

simultaneous decrease in the c axis parameters of t-ZrO2 until a certain limit of Gd3+ addition (GZTA20) implies the ability of t-ZrO2 to host Gd3+ along the a = b axis. This enhancement in the unit cell parameters is justified by the substitution of largersized Gd3+ (0.94 Å) for the lower-sized Zr4+ (0.79 Å).44 Beyond GZTA20, the continuous accumulation of Gd3+ at the ZrO2 lattice attains a stage in which the expansion of the a = b axis

Figure 6. Load vs. displacement profile recorded for selective GZTA specimens. 11003


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Figure 7. XRD recorded for GZTA specimens at selected time intervals during low-temperature degradation (LTD) tests.

contrast, Gd3+ occupancy at the c-ZrO2 lattice is limited to 50 wt % in the present GZTA system. These observations imply that the size effects of the stabilizers play a critical role in the occupancy limit and the phase stability of c-ZrO2. This inference is also supported by comparison with the ZrO2− SiO2 binary oxide (ZS) system, in which a t- → c-ZrO2 phase transition is witnessed for both Gd3+ and Dy3+ additions.32,45 Gd3+ occupancy in the ZrO2 lattice is limited to 25 wt %, whereas 50 wt % occupancy of Dy3+ in the ZrO2 lattice is ensured in ZS systems. Thus, the restriction to accommodate excess Gd3+ rather than Dy3+ is due to the comparably large size difference between Zr4+ and Gd3+ in comparison to that for Zr4+ and Dy3+ (0.91 Å). It is also shown that the thermal stability of c-ZrO2 is superior for the Dy3+-doped ZS system in comparison to the Gd3+-doped ZS system. Hence, an analogous trend of limited Gd3+ occupancy in the present GZTA system ascertains the role of dopant size in the phase stability of cZrO2. Raman spectra also exhibit good corroboration with the XRD and quantitative analysis. The relative shift in the characteristic Raman bands of t-ZrO2 with Gd3+ additions are attributed to the strain induced in ZrO2 lattice by the occupancy of largersized Gd3+. It has been shown that qualitative evaluation of the oxygen displacements induced by the occupancy of a foreign ion in the ZrO2 lattice could be deduced from the Raman spectra.46 Accordingly, the oxygen displacements created by Gd3+ occupancy at the t-ZrO2 lattice indicates a reduced trend in the Raman intensity ratio between I4 and I6 with

Figure 8. Magnetization curve of various GZTA compositions.

enhanced by temperature effects as its content displays a gradual upsurge from 1200 to 1400 °C. The availability of free Gd3+ in GZTA100 to form GdAlO3 is justified from factors such as the sudden halt in X-ray peak shift, abrupt stoppage in the upsurge of unit cell parameters, and attainment of the saturation occupancy limit. In the case of Dy3+ additions in the ZTA (DZTA) system, the ZrO2 lattice preferentially accommodates even 100 wt % Dy3+ devoid of any structural distortions and moreover the phase stability of c-ZrO2 is preserved until 1400 °C.23 In

Figure 9. (a) T1- and (b) T2-weighted MRI images and corresponding relaxivity values of GZTA with different concentrations. 11004


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Inorganic Chemistry simultaneous increase in the Gd3+ accumulation. This fact implies the reduced oxygen displacements owing to the generation of oxygen vacancies due to the mismatch in valence between Gd3+ and Zr4+. Further, Raman spectra also confirm the presence of GdAlO3 in GZTA100. The scanning electron micrographs depict the good sintering ability of the GZTA specimens by revealing the negligible porosity in the microstructures. Further, the micrographs indicate the crucial role of Gd3+ in controlling the grain growth of the ZTA specimens, as the respective average grain sizes of GZTA10, GZTA30, and GZTA50 have been determined in the order of ∼1, ∼2, and ∼3 μm. Moreover, the obvious abundance of Gd3+ in ZrO2 grains rather than Al2O3 grains determined from EDS analysis indicates the critical role of Gd3+ in the expansion of ZrO 2 grains. As a consequence of this phenomenon, the expanded ZrO2 grains prefer to engulf αAl2O3, as shown by the elemental analysis determined through EDS. Literature reports emphasize the fact that the low tetragonal to cubic ratio of ZrO2 shows resilient resistance toward phase degradation under hydrothermal conditions that is proportional to the thermodynamic stability.47 The c- to t-ZrO2 ratio determined from the refinement indicates a low value for GZTA10 and a strong upsurge as a function of Gd3+ content. The strong resistance to hydrothermal degradation displayed by the investigated compositions signifies the crucial role of Gd3+. These observations confirm the superiority of GZTA systems in comparison to the pure ZTA system, which detects 15 wt % of m-ZrO2 after 60 h.17,18 Zhang et al. opined that the size of a cationic dopant exerts a strong influence on aging resistance, as dopant concentration above its solubility limit segregates at the grain boundaries and as a consequence acts as a strong barrier to phase degradation.47 It has also been reported that c-ZrO2 remains as a nontransformable phase of ZrO2 under hydrothermal conditions.47 The results from the aging tests validate the aforementioned phenomenon, as none of the investigated compositions underwent phase degradation until 64 h of hydrothermal testing, which is approximately equal to 250 years. The mechanical properties of the selective specimens did not exhibit significant difference with respect to varied levels of Gd3+ additions. Further, the indentation profiles display smooth loading and unloading curves that endorse the good sintering ability along with the negligible voids of the specimens at 1400 °C. Hardness and Young’s modulus values determined by the Oliver−Pharr method are higher than the corresponding values of ZTA specimens reported in the literature.48 However, the reasons for the improved mechanical properties displayed by the GZTA system need further examination. It is necessary to emphasize that the presence of Dy3+ yields only the T2weighted MRI contrast features25 while the Gd3+ additions to the ZTA system delivers both the T1- and T2-weighted MRI contrast characteristics.

the excess Gd3+ reacts with Al2O3 to form GdAlO3. Gd3+ doping in ZTA systems delivers dense and pore-free microstructures with good mechanical strength; moreover, the aging resistance of GZTA systems is verified from the LTD tests. The MRI contrast ability of the GZTA system has been documented through imaging studies.

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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.inorgchem.7b01291. Microstructure and load vs displacement profile recorded for the ZTA specimens sintered at 1100 °C (PDF)

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AUTHOR INFORMATION

Corresponding Author

*S.K.: e-mail, [email protected]; tel, 0091-413-2654973. ORCID

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

The authors declare no competing financial interest.

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

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REFERENCES

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5. CONCLUSION The current investigation ascertains the role of Gd3+ as an effective structural and mechanical stabilizer for ZTA systems. Hosting a larger-sized Gd3+ in the ZrO2 lattice causes a strong expansion of its unit cell. The accommodation of Gd3+ at minimum levels up to 20 wt % preserves metastable t-ZrO2, whereas further additions induce a t- → c-ZrO2 phase transition. Further expansion of the c-ZrO2 lattice is restricted by the saturation occupancy limit of Gd3+. Beyond the saturation limit, 11005


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