Ceramic Conversion Treatment of Commercial Pure Titanium with a Pre-Deposited Vanadium Layer

Titanium is characterized by poor wear resistance which restricts its application. Ceramic conversion treatment (CCT) is used to modify the surface; however, it is a time-consuming process. In this work, a thin vanadium layer was pre-deposited on the commercial pure titanium (CPTi) samples’ surface, and it increased the oxygen absorption significantly and assisted in obtaining a much thicker oxide layer than those samples without a V layer at the treatment temperatures of 620 °C and 660 °C. The oxidation of the samples pre-deposited with the V layer had a much higher oxidation rate, and V was evenly distributed in the oxide layer. After CCT, all samples had a low wear volume and stable coefficient of friction in comparison to the untreated CPTi sample. A slightly higher wear area in the wear track was observed on the V pre-deposited samples than those samples without vanadium, especially those with a thicker oxide layer (>4 µm). This might be associated with defects in a thicker oxide layer and insufficient support from a shallower oxygen diffusion zone or hard debris created at the initial stage. Vanadium in the oxide layer reduced the contact angles of the surface and increased the wettability significantly

Low-Cost Magnesium-Based Thermoelectric Materials: Progress, Challenges, and Enhancements

Magnesium-based thermoelectric (TE) materials have attracted considerable interest due to their high ZT values, coupled with their low cost, widespread availability, nontoxicity, and low density. In this review, we provide a succinct overview of the advances and strategies pertaining to the development of Mg-based materials aimed at enhancing their performance. Following this, we delve into the major challenges posed by the severe working conditions, such as high temperature and thermal cycling, which adversely impact the behavior and long-term stability of the TE modules. Challenges include issues like the lack of mechanical strength, chemical instability, and unreliable contact. Subsequently, we focus on the key methodologies aimed at addressing these challenges to facilitate the broader application of the TE modules. These include boosting the mechanical strength, especially the toughness, through grain refining and additions of second phases. Furthermore, strategies targeted at enhancing the chemical stability through coatings and modifying the microstructure, as well as improving the contact design and materials, are discussed. In the end, we highlight the perspectives for boosting the practical applications of Mg-based TE materials in the future

Developing advanced CrSi coatings for combatting surface degradation of N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) half Heusler thermoelectric materials

In this study, advanced oxidation-protective CrSi coatings were developed and deposited on N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric (TE) materials using an environmentally friendly closed field unbalanced magnetron sputtering PVD system. The oxidation behaviour of the CrSi-coated and uncoated samples was evaluated through static oxidation tests at 500 °C and 600 °C for 10h and 50 h, respectively. In addition, the samples were exposed to cyclic oxidation tests between 25 °C and 500 °C for 10, 30 and 50 cycles, with each cycle consisting of 1h of oxidation at 500 °C, to examine the coating ability to withstand thermal shock, which is involved in the service of TE devices. The surface morphology, cross-section layer structure, elemental distribution and phase constitution were analysed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX) and X-ray diffraction (XRD). The mass gain after the oxidation tests was measured and calculated for each sample to study the oxidation kinetics. The uncoated samples exhibited a considerable amount of oxidation, leading to change in the surface morphologies, and the formation of alternated layers including oxide products (Zr(Ti)O2, SnO2) and Ni3Sn4 compound for the N-type and (Zr(Ti)O2, SnO2, Sb2O4) and CoSb compound for the P-type material, after static and cyclic oxidation tests. The experimental work in this study has, for the first time, demonstrated that the CrSi coatings developed with very high thermal stability and oxidation resistance can effectively protect both the N-type and P-type materials from oxidation and sublimation even during cyclic oxidation tests. The strong affinity of Cr and Si to oxygen can trap oxygen, retard its inward diffusion and thus protect the TE material substrate from oxidation. This finding could pave the way towards the further development of long-life high-performance TE generators and devices, thus contributing to the net zero target.</p

Cyclic oxidation behaviour of N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric materials

In this study, the fabricated Hf-free N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric materials were subjected to cyclic oxidation testing at 500 °C for 10, 30, and 50 cycles. The oxidation behaviour of the materials was systematically investigated by evaluating mass gain to study the oxidation kinetics and by analysing surface morphology, phase constitution and elemental distribution to investigate the oxidation mechanism. The results indicated that both of the materials were severely oxidised during the cyclic oxidation testing, and the mass gain followed the parabolic kinetics and the parabolic rate constant (kp) being 0.006165 mg2cm−4s−1 and 0.000109 mg2cm−4s−1 for the N-type and the P-type TE materials, respectively. Alternated multilayers of Ni3Sn4+SnO2+(Zr,Ti)O2 and CoSb + SnO2+Sb2O4+(Zr,Ti)O2 were identified on the surface of the N-type and P-type materials, respectively, after the cyclic testing, which would deteriorate the thermoelectric performance of the materials. The outcome of this study strongly suggests that it is essential to improve the oxidation resistance and the thermal stability of the N-type (Zr,Ti)Ni(Sn,Sb) and P-type (Zr,Ti)Co(Sn,Sb) thermoelectric materials for high-temperature applications

Processing conditions and mechanisms for the plasma defect-engineering of bulk oxygen-deficient zirconia

In recent years, the utilisation of oxygen-deficient zirconia (ZrO2-α), commonly referred to as black zirconia, has garnered considerable attention due to its potential applications for solid oxide fuel cells (SOFCs), gas sensors, biomedical implant materials, and photocatalysis. However, current methods employed to manufacture ZrO2-α exhibit noticeable limitations regarding their scalability, environmental sustainability, and cost-effectiveness. Our recent work has successfully demonstrated the feasibility for bulk conversion of conventional white zirconia into oxygen-deficient black zirconia through direct current (DC) plasma treatment (i.e. plasma blackening). This study elucidates the conditions for plasma blackening and provides a unique mechanism for the bulk transformation of zirconia. A systematic investigation of different plasma technologies (DC, active-screen plasma), treatment configurations (contact conditions, cathode material, and cathode potential), and treatment parameters (voltage, temperature, duration) uncover the crucial variables that influence the feasibility and rate of the reduction process. The reduction of zirconia is shown to initiate from localised contacting points at the cathode-facing surface and grow, with a hemispherical shape, towards the anode-facing surface. A series of development stages are proposed for the process, namely: bulk oxygen vacancy conductance, surface activation, oxygen vacancy generation and a moving cathode front. The findings of this study provide insights into the underlying mechanisms involved in the bulk-reduction of zirconia and help to pave the way towards future scalable and cost-effective generation of oxygen-deficient zirconia

Uncertainty Sentence Sampling by Virtual Adversarial Perturbation

Active learning for sentence understanding attempts to reduce the annotation cost by identifying the most informative examples. Common methods for active learning use either uncertainty or diversity sampling in the pool-based scenario. In this work, to incorporate both predictive uncertainty and sample diversity, we propose Virtual Adversarial Perturbation for Active Learning (VAPAL) , an uncertainty-diversity combination framework, using virtual adversarial perturbation (Miyato et al., 2019) as model uncertainty representation. VAPAL consistently performs equally well or even better than the strong baselines on four sentence understanding datasets: AGNEWS, IMDB, PUBMED, and SST-2, offering a potential option for active learning on sentence understanding tasks