Zinc oxide (ZnO) is a valuable ceramic material to health when applied as antibacterial coating for metallic implants [1]. It is a semiconductor considered for various uses such as chemical sensors, solar cells, electrical devices. Moreover, it can be used in doped form with other oxides as ceramic coatings favoured with surface dimples like TiO₂/ZnO. It is characterized by photocatalytic and photo-oxidizing ability against chemical and biological species [2]. The doping of ZnO using transition metals improves properties in antibacterial coatings [3]. Some examples include Ta-ZnO [4], Al-ZnO [5], Mg-ZnO [6], Ti-ZnO [7], Cr-ZnO [8], Si-ZnO [9], Ni-ZnO [10, 11] and (Mn,Ni)-ZnO [12]. Mechanical milling (MM) and annealing is a technique that can be used to refine powder particles and induce feasible chemical reactions at lower temperatures [13]. The product of MM alone does not produce a stable product but rather a metastable powder sensitive to high temperatures. Previous studies shows that MM of atomised TiNi powder leads to unstable particles with suppressed shape memory effect properties [14]. However, MM can be used successfully to promote an effective transfer of O atoms between a transition metal and an oxide, the process completed by thermal treatment [15]. Gao et. al., [12] investigated the Mn, Ni doping on ZnO to enhance a positive correlated sensitivity to ethanol gas on sensing applications. Furthermore, Zn_1 − xTi_xO composite revealed a change in ferromagnetic and semiconducting nature signifying the incorporated Ti⁴⁺ ion into the lattice site of the ZnO [16]. Elsewhere, the hydrothermally grown Ni, Ti and NiTi alloy to ZnO nanowires resulted in a photoluminescence enhancement. Where ZnO nanowires were coated with NiTi, it showed superior plasmonic effect of the same magnitude as the plasmonic effect of Au and Ag coated ZnO [17]. ZnO is also an important material to manufacture crucibles for flame fired furnaces with high thermal conductivity that can reduce energy usage during melting. Doping of Cr and Mg to ZnO produces efficient catalyst for the oxygen evolution reaction, where Cr introduces Cr³⁺ ions into the ZnO lattice and advances the electrical conductivity and catalytic activity [18]. Due to their smaller ionic radius compared to Zn²⁺, Cr³⁺ ions substitute Zn²⁺ ions, inducing lattice strain and generating oxygen vacancies. On the other hand, Mg modulates the electronic structure and improves material’s performance while generating tensile strain into the ZnO lattice [18]. To the best of our knowledge, the effect of doping ZnO with metallic alloy with known properties such as nitinol (TiNi) is not known. The current investigation aims to study the product of alloying of TiNi with ZnO powder using the ball milling process and subsequent annealing. Additionally, the mixed metal oxide structural change and its formation mechanism was explored.

Zinc oxide (ZnO) with 99.9% purity and Titanium-nickel (Ti50at.%Ni) powders were mixed at 1:1 weight ratio and subjected to ball milling (BM) for 30 and 60 hours (h), respectively. The milling speed of 250 revolution per minute (RPM) and 10:1 ball-to-powder ratio (BPR) was used. The milled ceramic powders were annealed at 600 ºC in a tube furnace under argon (Ar) gas flowing for two hours. The surface morphology of the powders was characterized using Zeiss-Auriga field-emission scanning electron microscope (FE-SEM) coupled with a Robinson Backscatter Electron Detector (RBSD) and an Oxford Link Pentafet energy dispersive x-ray spectroscopy (EDS) detector. The structural development of the ZnO-TiNi composite was studied using a PANalytical X’pert PRO PW 3040/60 X-ray diffraction (XRD) machine fitted with a Cu K_α radiation source. Raman analysis was carried out using a Horiba Jobin-Yvon HR800 Raman microscopy equipped with an Olympus BX-41 microscope attachment. An Ar⁺ laser (514.5 nm) with a power setting of 1.2 mW from a Coherent Innova Model 308 was used as an excitation source. High-resolution transmission electron microscopy was employed to investigate the localized phase formation, crystal growth of the milled NPs. Electron micrographs, and selected area diffraction patterns (SAED) were collected with an FEI Tecnai G220 FE-TEM. ZnO-TiNi composite powders were heated up to 800°C using Al₂O₃ as a baseline during thermal analysis. A heating rate of 20°C min^− 1 under Ar gas with 20 ml/L standard flow rate was used.

Figure 1: SEM images of the (a) ZnO, (b) Ti50Ni atomised alloy powder, milled for (c-d) 30 h and (e,f) 60h ZnO-TiNi powders.

Figure 1a shows the agglomerated ZnO powder spherically shaped nanoparticles (NPs). The TiNi alloy powder shown in Fig. 1b. It is composed of mainly spherical particles and some porous/irregular shaped micron particles. Upon MM of the ZnO-TiNi powder mixture for 30 h (Fig. 1c,d), the particles were excessively welded due to easy adherence of NPs ZnO on the spherical TiNi particles. The spherical particles are flattened to pancake shape upon milling and becomes highly reactive due to larger surface area [19]. MM induce vacancies and generates fresh particle surfaces which cause the oxygen atoms transfer between ZnO and reactive TiNi particles [15, 20]. The porous interconnected particles in Fig. 1c coexisted with large, welded particles as revealed in Fig. 1d. Upon the 60 h, the ZnO-TiNi milled powder revealed agglomeration and clustering of particles that are welded together.

Figure 2: XRD pattern of the (a) ZnO, (b) TiNi, (c) 30 h milled and (d) 60 h milled ZnO-TiNi powders.

Figures 2a shows the XRD pattern of ZnO powder. The XRD analysis reveals the HCP intensity peaks with lattice parameters a = 3.249 Å; c = 5.205 Å with space group and number P63mc # 186 (PDF Code: 01-089-0510). Figure 2b is the XRD patten of the TiNi alloy. Its crystal structure is comprised of cubic (B2) austenite and monoclinic (B19′) martensite phases. The B2 phase has a lattice parameter a = 3.015 Å (Pm-3m # 221) while the B19′ resembles parameters are a = 2.889 Å; b = 4.120 Å; c = 4.622 Å (P21/m # 11). The 30h milled ZnO-TiNi powder mixture is shown in Fig. 2c. Only one predominantly broad peak has emerged indicative of the formation of a disordered phase. It has been shown that MM of the atomised TiNi powder destroys the shape meory effects (SME) order and induce the amorphous structured powder while, depending on the severity of particle deformation, annealing might not reverse the SME [14].

Moreover, it has also been shown that MM of micon-sized ZnO generates nanocrystalline particles [21–24], expands the lattice constants [24]. Since, ZnO NPs were used in this work, the effect of MM was critical for TiNi particle refinement and the promotion of solid state reaction, hence, the observed agglomeration in Fig. 1c-f. Upon 30 h milling, it is evident that the intensity peaks for both ZnO and TiNi have disappeared as shown in Fig. 2c. Similar behaviour occurred after 60 h of milling as shown in Fig. 2d. The destruction of the long-range order in ZnO and TiNi powders has been manifested.

Figure 3: Raman spectra of the (a) ZnO, (b) TiNi, (c) 30 h milled ZnO-TiNi and (d) 60 h milled ZnO-TiNi powders.

Figure 3a-d shows the Raman spectra of the ZnO, TiNi, 30 and 60 h milled ZnO-TiNi powders. The Raman analysis compliment the XRD results illustrated in Fig. 2a-d. The ZnO intensity peaks are found at 97, 201, 371, 376, 437, 579, 1047 and 1147 cm^− 1 (Fig. 3a). The peaks of ZnO exhibit a strong peak at ~ 437 cm^− 1 attributable to E2 (high) mode of the oxygen atoms movement in ZnO lattice structure or O vacancies [25–27]. The weak intensity peaks at 589, 1047 and 1147 cm^− 1 correspond to 2 LA, E1 (LO) and 2LO of ZnO nanoparticles [25–27].

Figure 3b illustrates the weak Raman intensity peaks of the atomised TiNi powder at 239 and 525 cm^− 1. The broad, weak intensity peaks are due to the phonon-phonon anharmonic interaction and fine grains crystal defects which causes internal stress leading to changed phonon vibration [28]. Figure 3c,d shows a similar single broad intensity peak representing the short-range order or the lack of crystallinity of the 30 h and 60 h ZnO-TiNi milled ceramic powder. This behaviour compliment the XRD analysis. The Raman peaks confirms that the milled powders have the combination of crystalline and amorphous particles. Broad intensity peaks are located at ~ 203 cm^− 1 in 30 h ZnO-TiNi while 60 h ZnO-TiNi is located at ~ 263 cm^− 1.

Figure 4: HR-TEM images and their corresponding SAED of (a,b) ZnO, (c,d) 30 h ZnO-TiNi and (e,f) 60 h ZnO-TiNi powders.

Figure 4a shows the HR-TEM image of the ZnO particles. They are nanorods particles with a diameter of 24 ± 0.05 nm with hexagon shapes. A selected area electron diffraction (SAED) shown in Fig. 4b indicates that the ZnO nanoparticles are crystalline in nature. The HR-TEM images and its corresponding SAED pattern of the 30 h milled ZnO-TiNi powder are shown Fig. 4c,d. Due to milling, none of the ZnO nanorods or TiNi spherical particles could be identified. These observations are in agrrement with the SEM images in Fig. 1c,d, and also correlates with the XRD and Raman structural analysis. MM has altered the stability of ZnO and TiNi particles. The SAED image of the 30 h ZnO-TiNi ceramic powder exposed decreased number of diffraction rings attributable to partial amorphization (mixed nanocrystalline-amorphous phases). This amorphous structure is more prevalent in the 60 h milled ZnO-TiNi powder as shown by the SAED in Fig. 4f. The HR-TEM EDS analysis is presented in Table 1. The average chemical composition of the 60 h milled ZnO-TiNi powder is composed of Ti-rich (59.50 wt.%), 19.49 wt.% Ni, 14.68 wt.% Zn and 6.31 wt.% O.

Table 1: TEM chemical analysis of the 60 h milled ZnO-TiNi powder

Figure 5: DSC curves of the 30 and 60 h milled ZnO-TiNi powders. Samples were heated to 1000°C, respectively.

The thermal stability of the milled powder was investigated through the DSC at 1000 ºC shown in Fig. 5. There is initial erratic behaviour at low temperature upon heating the 30 h milled powder showing the broad peak up to ~ 400 ºC indicative of the stress relief followed by a sharp crystallisation and exothermic peak at 543°C. This exothermic peak is symptomatic of a further chemical reaction of the oxygen transfer between unstable ZnO and TiNi powder. Furthermore, the 60 h milled ZnO-TiNi milled powder reveals a different thermal behaviour upon heating when compared with 30 h milled powder. There is no exothermic peak, instead a broad exothermic peak that ends at ~ 900°C. This behaviour implies that the milling effect on the powder was extensive. Thermal analysis confirms that milling generates defects and vacancies in ZnO and TiNi which promotes the O exchage [15]. Thermal analysis validates the instability of SME properties in TiNi powder [14]. Moreover, the thermal behaviour also nulifies the presence of free crystalline Ni paricles becouse it has been shown that Ni has an second order endothermic peak at 356°C [29].

Figure 6: XRD pattern of the annealed (a) 30 h milled and (b) 60 h milled ZnO-TiNi powders at 600°C. The annealing temperature was set above the exothermic reaction that took place at 543°C.

DSC analysis (Fig. 5) predicted the milled powder annealing temperatures. The milled powders were annealed at 600 ºC above 543°C to establish any phase formation on the 30 and 60 h milled powders. As a result, the XRD patterns of the 30 h and 60 h milled ZnO-TiNi powders at 600°C were conducted (Fig. 6a,b). The 30 h milled ZnO-TiNi XRD pattern is shown in Fig. 6a. The FCC phase with a lattice parameter a = 11.240 Å of the Ni₃Ti₃O prototype which belongs to Fd-3m #227 space group was identified by the PDF Code: 01-075-0400 was detected. The formation of this cubic superstructure proves that O transfer between ZnO and TiNi phases has occured. Moreover, another cubic phase (B2) with a lattice parameter a = 2.948 Å (CuZn prototype) with Pm-3m # 221 was detected (PDF Code: 00-002-1231) attributable to another mixed oxide phase. Annealing of milled powders has promoted the chemical reaction between the milled TiNi and ZnO powders, thereby producing the new metastable mixed oxide composite phases [30]. The DSC exothermic peak observed at 543°C is attributed to the formation of FCC superstructure. The 60 h-annealed powder also formed two tetragonal phases with different lattice parameters. This behaviour further confirms that MM of ZnO-TiNi promotes O transfer reaction completed during annealing. The tetragonal phase has a lattice parameters a = 4.593 Å; c = 2.959 Å (rutile type TiO_1.95) with P42/mnm # 136 while the other tetragonal phase is of the anatase type Ti_0.72O₂ with a = 3.783 Å; c = 9.497 Å (I41/amd # 141).

Figure 7: SEM images of the annealed (a-b) 30h-milled ZnO-TiNi and the (c-d) 60h-milled ZnO-TiNi powders annealed at 600°C.

The SEM images of the 30h-milled ZnO-TiNi powders are shown in Fig. 7a,b. The morphology reveals large agglomerations of ultrafine particles. Similarly, the annealed 60 h-milled ZnO-TiNi powder exhibit irregular shaped particle agglomerates (Fig. 7c). Furthermore, small spherical particles (nanospheres) has precipitated and randomly attached on the surface of the large agglometares (Fig. 7d). The bulk of the structure belongs to the tetragonal Ti-rich ceramic phase (TiO_1.95-type) while the nanospheres are attributed to the Zn-rich Ti_0.72O₂ structure.

The mixture of ZnO and TiNi atomised powders was mechanically milled for 30 and 60h. MM produced amorphous agglomerated powder particles. The XRD analysis revealed a predominatly single broad peak after 30 and 60 h, respectively. Raman analysis revealed similar broad intensity peak indicative of the short-range order or lack of crystallinity due to MM. TEM images showed the lattice fringes with decreased number of diffraction rings due to partial formation of amorphous particles consistent with the XRD and Raman results. Thermal analysis conducted on the 30h milled ZnO-TiNi powder revealed an exothermic peak at 543°C, while the peak has not emerged on the 60h milled ZnO-TiNi powder. After annealing at the 30 h milled ZnO-TiNi powder at 600°C, the FCC and B2 phases was formed. The FCC phase has a suppercell structure with a = 11.240 Å while cubic B2 phase had a = 2.948 Å lattice parameters. The annealed 60 h milled powder has recrystallized into tetragonal structures of the rutile type TiO_1.95 (a = 4.593 Å; c = 2.959 Å) and anatase type Ti_0.72O₂ (a = 3.783 Å; c = 9.497 Å), respectively. The O atoms was accepted from the ZnO duwe to excessive MM of the powder.