| 1. |
- Chen, Xi
(författare)
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Silicon Nanowire Field-Effect Devices as Low-Noise Sensors
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the past decades, silicon nanowire field-effect transistors (SiNWFETs) have been explored for label-free, highly sensitive, and real-time detections of chemical and biological species. The SiNWFETs are anticipated for sensing analyte at ultralow concentrations, even at single-molecule level, owing to their significantly improved charge sensitivity over large-area FETs. In a SiNWFET sensor, a change in electrical potential associated with biomolecular interactions in close proximity to the SiNW gate terminal can effectively control the underlying channel and modulate the drain-to-source current (IDS) of the SiNWFET. A readout signal is therefore generated. This signal is primarily determined by the surface properties of the sensing layer on the gate terminal, with sensitivity close up to the Nernstian limit widely demonstrated. To achieve a high signal-to-noise ratio (SNR), it is essential for the SiNWFETs to possess low noise of which intrinsic device noise is one of the major components. In metal-oxide-semiconductor (MOS)-type FETs, the intrinsic noise mainly results from carrier trapping/detrapping at the gate oxide/semiconductor interface and it is inversely proportional to the device area.This thesis presents a comprehensive study on design, fabrication, and noise reduction of SiNWFET-based sensors on silicon-on-oxide (SOI) substrate. A novel Schottky junction gated SiNWFET (SJGFET) is designed and experimentally demonstrated for low noise applications. Firstly, a robust process employing photo- and electron-beam mixed-lithography was developed to reliably produce sub-10 nm SiNW structures for SiNWFET fabrication. For a proof-of-concept demonstration, MOS-type SiNWFET sensors were fabricated and applied for multiplexed ion detection using ionophore-doped mixed-matrix membranes as sensing layers. To address the fundamental noise issue of the MOS-type SiNWFETs, SJGFETs were fabricated with a Schottky (PtSi/silicon) junction gate on the top surface of the SiNW channel, replacing the noisy gate oxide/silicon interface in the MOS-type SiNWFETs. The resultant SJGFETs exhibited a close-to-ideal gate coupling efficiency (60 mV/dec) and significantly reduced device noise compared to reference MOS-type SiNWFETs. Further optimization was performed by implementing a three-dimensional Schottky junction gate wrapping both top surface and two sidewalls of the SiNW channel. The tri-gate SJGFETs with optimized geometry exhibited significantly enhanced electrostatic control over the channel, thereby confined IDS in the SiNW bulk, which greatly improved the device noise immunity to the traps at bottom buried oxide/silicon interface. Finally, a lateral bipolar junction transistor (LBJT) was also designed and fabricated on a SOI substrate aiming for immediate sensor current amplification. Integrating SJGFETs with LBJTs is expected to significantly suppress environmental interference and improve the overall SNR especially under low sensor current situations.
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| 2. |
- Chen, Jian
(författare)
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Low Noise Oscillator in ADPLL toward Direct-to-RF All-digital Polar Transmitter
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In recent years all-digital or digitally-intensive RF transmitters (TX) have attracted great attention in both literature and industry. The motivation is to implement RF circuits in a manner suiting advanced nanometer CMOS processes. To achieve that, information is encoded in the time-domain rather than voltage amplitude. This enables RF design to also benefit from CMOS process scaling. In this thesis an improved architecture of a digitally-intensive transmitter is proposed and validated experimentally. The techniques to lower oscillator phase noise and all-digital phase-locked loop (ADPLL) quantization noise are discussed and proved by both simulation and measurements.The impact of device sizing on 1/f^2 phase noise is analyzed and validated by measurements. Seven oscillators in 180-nm CMOS with the same LC-tank, operation frequency and power consumption but different core device width are compared. The conclusion clarify the different suggestions on device sizing in the literature. It is illustrated that tail noise contribution is strongly positive dependent to core device sizing, while the contribution of core devices themselves is weakly dependent. Measurements demonstrate that there is a 14-dB phase noise increase when sizing core devices from 40 um to 280 um in the case of noisy tail current. If tail current is clean, the increase is only 4 dB. For 1/f^3 phase noise, the investigation reveals that the capacitance modulation is the dominant factor accounting for the 1/f or flick noise up-conversion, which is proved by measurements of 180-nm CMOS designs. A class-C oscillator with ensured start-up and constant amplitude is presented. It achieves a 3.9-dB phase noise reduction in theory and 5-dB reduction in measurements, compared to a conventional LC-tank oscillator operating at the same frequency and power. With the help of a digital bias voltage and bias current control loop, a 191 Figure-of-Merit (FoM) is achieved, showing the ability for low power and noise application. The previous oscillator optimization techniques have been applied in designing a digital controlled oscillator (DCO) for an ADPLL. A fine tuning varactor is proposed to reduce quantization noise, achieving a frequency step of only several hundreds Hz. In order to measure this small frequency step when the DCO is free-running, a method based on the narrow-band frequency modulation (FM) theory is proposed. The ADPLL wide-band FM is fulfilled by using a digital two-point modulation so that the modulation bandwidth is not limited by the ADPLL loop dynamic.Finally an all-digital polar TX is proposed based on an improved architecture. The ADPLL is used for FM while a one-bit low-pass Sigma Delta modulator using the phase modulated ADPLL output as the clock accomplishes amplitude modulation. A simple AND gate is adopted to increase the fundamental power as mixers. A class-D power amplifier stages diliver 6.8-dBm power to antenna through a on-chip band-pass pre-filter. The filter also acts as single-ended to differential-end conversion and matching network.
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| 3. |
- Chen, Li-Ju, 1977-
(författare)
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Essays on Female Policymakers and Policy Outcomes
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis consists of three papers, summarized as follows. "Female Policymakers and Educational Expenditures: Cross-Country Evidence" This paper investigates the influence of women in politics on decision-making using public educational expenditures as the outcome of interest. The results suggest that an increase in the share of female legislators by one percentage point increases the ratio of educational expenditures to GDP by 0.028 percentage points. The effect of female legislators on educational policies is strengthened accounting for forms of government, but not influenced by left-wing government, electoral rules, parliamentary system and non-marriage. Moreover, this study supports the hypothesis that the identity of the legislator matters for policy. "Women in Politics: A New Instrument for Studying the Impact of Education on Growth" This paper tests the growth model of distance to the technological frontier, which states that an economy closer to the technological frontier should invest more in skilled labor since innovation is a skill-intensive activity. In contrast to Vandenbussche, Aghion, and Meghir (henceforth VAM) (2006), I use the proportion of female legislators as an instrument for skilled labor, instead of lagged educational expenditures. The results with the new instrument are consistent with the theoretical prediction and the previous results of VAM (2006). "Do Gender Quotas Influence Women's Representation and Policies?" This paper investigates the effect of applying gender quotas on policy decisions. The results show that an increase in the share of female legislators by one percentage point increases the ratio of government expenditure on health and social welfare to GDP by 0.18 and 0.67 percentage points, respectively. The robustness check supports that the effect of quotas on female legislators is likely to be translated into the influence of female policymakers on social welfare.
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| 4. |
- Feng, Yi
(författare)
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Printed RFID Humidity Sensor Tags for Flexible Smart Systems
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Radio frequency identification (RFID) and sensing are two key technologies enabling the Internet of Things (IoT). Development of RFID tags augmented with sensing capabilities (RFID sensor tags) would allow a variety of new applications, leading to a new paradigm of the IoT. Chipless RFID sensor technology offers a low-cost solution by eliminating the need of an integrated circuit (IC) chip, and is hence highly desired for many applications. On the other hand, printing technologies have revolutionized the world of electronics, enabling cost-effective manufacturing of large-area and flexible electronics. By means of printing technologies, chipless RFID sensor tags could be made flexible and lightweight at a very low cost, lending themselves to the realization of ubiquitous intelligence in the IoT era.This thesis investigated three construction methods of printable chipless RFID humidity sensor tags, with focus on the incorporation of the sensing function. In the first method, wireless sensing based on backscatter modulation was separately realized by loading an antenna with a humidity-sensing resistor. An RFID sensor tag could then be constructed by combining the wireless sensor with a chipless RFID tag. In the second method, a chipless RFID sensor tag was built up by introducing a delay line between the antenna and the resistor. Based on time-domain reflectometry (TDR), the tag encoded ID in the delay time between its structural-mode and antenna-mode scattering pulse, and performed the sensing function by modulating the amplitude of the antenna-mode pulse.In both of the above methods, a resistive-type humidity-sensing material was required. Multi-walled carbon nanotubes (MWCNTs) presented themselves as promising candidate due to their outstanding electrical, structural and mechanical properties. MWCNTs functionalized (f-MWCNTs) by acid treatment demonstrated high sensitivity and fast response to relative humidity (RH), owing to the presence of carboxylic acid groups. The f-MWCNTs also exhibited superior mechanical flexibility, as their resistance and sensitivity remained almost stable under either tensile or compressive stress. Moreover, an inkjet printing process was developed for the f-MWCNTs starting from ink formulation to device fabrication. By applying the f-MWCNTs, a flexible humidity sensor based on backscatter modulation was thereby presented. The operating frequency range of the sensor was significantly enhanced by adjusting the parasitic capacitance in the f-MWCNTs resistor. A fully-printed time-coded chipless RFID humidity sensor tag was also demonstrated. In addition, a multi-parameter sensor based on TDR was proposed.The sensor concept was verified by theoretical analysis and circuit simulation.In the third method, frequency-spectrum signature was utilized considering its advantages such as coding capacity, miniaturization, and immunity to noise. As signal collision problem is inherently challenging in chipless RFID sensor systems, short-range identification and sensing applications are believed to embody the core values of the chipless RFID sensor technology. Therefore a chipless RFID humidity sensor tag based on near-field inductive coupling was proposed. The tag was composed of two planar inductor-capacitor (LC) resonators, one for identification, and the other one for sensing. Moreover, paper was proposed to serve as humidity-sensing substrate for the sensor resonator on accounts of its porous and absorptive features.Both inkjet paper and ordinary packaging paper were studied. A commercial UV-coated packaging paper was proven to be a viable and more robust alternative to expensive inkjet paper as substrate for inkjet-printed metal conductors. The LC resonators printed on paper substrates showed excellent sensitivity and reasonable response time to humidity in terms of resonant frequency. Particularly, the resonator printed on the UV-coated packaging paper exhibited the largest sensitivity from 20% to 70% RH, demonstrating the possibilities of directly printing the sensor tag on traditional packages to realize intelligent packaging at an ultra-low cost.
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| 5. |
- Hu, Qitao
(författare)
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Silicon Nanowire Based Electronic Devices for Sensing Applications
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Silicon nanowire (SiNW) based electronic devices fabricated with a complementary metal-oxide-semiconductor (CMOS) compatible process have wide-range and promising applications in sensing area. These SiNW sensors own high sensitivity, low-cost mass production possibility, and high integration density. In this thesis, we design and fabricate SiNW electronic devices with the CMOS-compatible process on silicon-on-insulator (SOI) substrates and explore their applications for ion sensing and quantum sensing. The thesis starts with ion sensing using SiNW field-effect transistors (SiNWFETs). The specific interaction between a sensing layer and analyte generates a change of local charge density and electrical potential, which can effectively modulate the conductance of SiNW channel. Multiplexed detection of molecular (MB+) and elemental (Na+) ions is demonstrated using a SiNWFET array, which is functionalized with ionophore-incorporated mixed-matrix membranes (MMMs). As a follow-up, polyethylene glycol (PEG) doping strategy is explored to suppress interference from the hydrophobic molecular ion and expand the multiplexed detection range. Then, the SiNW is downscaled to sub-10 nm with a gate-oxide-free configuration for single charge detection in liquid. We directly observe the capture and emission of a single H+ ion with individually activated Si dangling bonds (DBs) on the SiNW surface. This work demonstrates the unprecedented ability of the sub-10 nm SiNWFET for investigating the physics of the solid/liquid interface at single charge level.Apart from ion sensing, the SiNWFET can be suspended and act as a nanoelectromechanical resonator aiming for electrically detecting potential quantized mechanical vibration at low temperature. A suspended SiNW based single-hole transistor (SHT) is explored as a nanoelectromechanical resonator at 20 mK. Mechanical vibration is transduced to electrical readout by the SHT, and the transduction mechanism is dominated by piezoresistive effect. A giant effective piezoresistive gauge factor (~6000) with a strong correlation to the single-hole tunneling is also estimated. This hybrid device is demonstrated as a promising system to investigate macroscopic quantum behaviors of vibration phonon modes.Noise, including intrinsic device noise and environmental interference, is a serious concern for sensing applications of SiNW electronic devices. A H2 annealing process is explored to repair the SiNW surface defects and thus reduce the intrinsic noise by one order of magnitude. To suppress the external interference, lateral bipolar junction transistors (LBJTs) are fabricated on SOI substrate for local signal amplification of the SiNW sensors. Current gain and overall signal-to-noise ratio of the LBJTs are also optimized with an appropriate substrate voltage.
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| 6. |
- Li, Chen, 1993-
(författare)
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Non-coding RNAs and Extracellular Vesicles in Cutaneous Squamous Cell Carcinoma
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cutaneous squamous cell carcinoma (cSCC) ranks among the most widespread malignancies with metastatic potential. Investigating the molecular mechanism of tumorigenesis will enhance our understanding of cSCC. Aberrant expression of non-coding RNAs has been extensively reported in human cancers. Here, we summarize our work exploring the role of a microRNA (miRNA) (Paper I) and a long non-coding RNA (lncRNA) (Paper II and III) in cSCC. Additionally, we discuss the role of cSCC-derived extracellular vesicles (EVs) in tumor formation (Paper IV).In Paper I, we explored the function of miR-130a in cSCC. We reported that miR-130a expression was downregulated in cSCC under the regulation of the MAPK pathway. We demonstrated a tumor suppressor role of miR-130a in cSCC: ectopic overexpression of miR-130a suppressed malignant behaviors of human cSCC cells and inhibited primary tumor growth in cSCC xenograft models. Mechanistically, we revealed a link between MAPK and BMP/SMAD signaling pathways, which was mediated by the direct target of miR-130a, ACVR1. In Paper II, we investigated the role of lncRNA PVT1 in cSCC. Elevated PVT1 expression in cSCC, under MYC regulation, suggested it may contribute to keratinocyte transformation. Subsequently, we revealed that PVT1 exerted an oncogenic role in cSCC through regulating CDKN1A/p21 expression and preventing cellular senescence. We identified exon 2 as a crucial element for maintaining PVT1's oncogenic role. In Paper III, we further investigated the underlying mechanism for the oncogenic role of PVT1 in cSCC. Our data revealed that PVT1 is mainly distributed in the nuclei of cSCC cells and the exon 2 is essential for nuclear localization of PVT1. Furthermore, we identified several subunits of the transcription-export (TREX) complex as interacting partners of PVT1 and demonstrated that PVT1 modulated the function of the TREX complex in nuclear export of poly (A)+ RNAs.In Paper IV, we found that cSCC cells secreted more EVs than primary keratinocytes. Blocking cSCC EV production suppressed xenograft growth, indicating a crucial role of cSCC cell-derived EVs in tumor development. Transcriptome analysis on xenograft tissues suggested that cSCC cell-derived EVs contribute to extracellular matrix organization. Further experiments indicated that metastatic cSCC cell-derived EVs efficiently educated dermal fibroblasts into cancer-associated fibroblasts. Additionally, metastatic cSCC cell-derived EVs activated the TGFβ signaling pathway in dermal fibroblasts. Collectively, our study suggested that cSCC cell-derived EVs play a key role in regulating cSCC development through modulating cancer-stroma communication.
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| 7. |
- Li, Jun
(författare)
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Ultra-low latency communication for 5G transport networks
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The fifth generation (5G) mobile communication system is envisioned to serve various mission-critical Internet of Thing (IoT) applications such as industrial automation, cloud robotics and safety-critical vehicular communications. The requirement of the end-to-end latency for these services is typically within the range between 0.1 ms and 20 ms, which is extremely challenging for the conventional cellular network with centralized processing. As an integral part of the cellular network, the transport network, referred to as the segment in charge of the backhaul of radio base stations or/and the fronthaul of remote radio unit, plays an especially important role to meet such a stringent requirement on latency.This thesis investigates how to support the ultra-low latency communications for 5G transport networks, especially for backhaul networks. First, a novel passive optical network (PON) based mobile backhaul is proposed and tailored communication protocols are designed to enhance the connectivity among adjacent base stations (BSs). Simulation results show that an extremely low latency (less than 1 ms packet delay) for communications among the BSs can be achieved, which thereby can be used to support fast handover for users with high mobility (e.g., vehicles).Furthermore, the thesis presents a fog computing enabled cellular networks (FeCN), in which computing, storage, and network functions are provisioned closer to end users, thus the latency on transport networks can be reduced significantly. In the context of FeCN, the high mobility feature of users brings critical challenges to maintain the service continuity with stringent service requirements. In the meanwhile, transmitting the associated services from the current fog server to the target one to fulfill the service continuity, referred to as service migration, has been regarded as a promising solution for mobility management. However, service migration cannot be completed immediately, and may lead to a situation where users experience loss of access to the service. To solve such issues, a quality-of-service (QoS) aware service migration strategy is proposed. The method is based on the existing handover procedures with newly introduced distributed fog computing resource management scheme to minimize the potential negative effects induced by service migration. The performance of the proposed schemes is evaluated by a case study, where realistic vehicle mobility pattern in the metropolitan network scenario of Luxembourg is used to reflect the real world environment. Results show that low end-to-end latency (e.g., 10 ms) for vehicular communication can be achieved with typical vehicle mobility. During service migration, both the traffic generated by migration and other traffic (e.g., control information, video) are transmitted via mobile backhaul networks. To balance the performance of the two kinds of traffic, a delay-aware bandwidth slicing scheme is proposed in PON-based mobile backhaul networks. Simulation results show that, with the proposed method, migration data can be transmitted successfully within a required time threshold, while the latency and jitter for non-migration traffic with different priorities can be reduced significantly.
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| 8. |
- Li, Lin, 1989-
(författare)
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Somatic and occupational outcomes in adult ADHD : epidemiology studies based on real-world data
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Attention-deficit/hyperactivity disorder (ADHD) is one of the most common neurodevelopmental disorders, characterized by inattention or hyperactivity–impulsivity, or both. ADHD is a multifactorial disorder influenced by the complex interplay between genetic and environmental risk factors, but a detailed understanding of the causal status of these factors is lacking. ADHD is associated with many psychiatric disorders, but somatic comorbidity in ADHD has received less attention in the research literature. Pharmacological treatment is effective in reducing the core symptoms of ADHD, but the effects on occupational outcomes remain unclear. The overarching aim of this thesis is to extend previous knowledge on the early risk factors of ADHD, and to increase the awareness and the understanding on somatic and occupational outcomes of ADHD in adults.In Study I, we combined a systematic review and mate-analysis with a population based cohort of 971,501 individuals born between 1992 and 2004 in Sweden. The meta-analysis revealed a positive association between maternal pre-pregnancy overweight/obesity and risk of ADHD in offspring. However, these associations gradually attenuated toward the null when adjusted for measured confounders,unmeasured factors shared by cousins and unmeasured factors shared by siblings. In Study II, by using a Swedish population-based twin study with 17,999 individuals aged 20–47 years, we found both inattention and hyperactivity/impulsivity was associated with higher consumption of high-sugar food and unhealthy dietary habits, although these associations were generally weak. Further, the observed associations was explained by both genetic and non-shared environmental factors.In Study III, we explored the prospective associations between ADHD and a broad range of cardiovascular diseases in 5,389,519 adults from Sweden, and found that ADHD may be a novel and independent risk factor for cardiovascular diseases. In Study IV, based on the longitudinal cohort of 12,875 middle-aged adults with ADHD, we found the use of ADHD medications during the previous two years was associated with a 10% reduction in the risk of long-term unemployment in the following year.Taken together, findings from the thesis highlight the need of future studies with various study designs, to fully understand the aetiology and long-term health outcomes of ADHD across the lifespan.
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| 9. |
- Li, Su-Chen, 1979-
(författare)
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Small Intestinal Neuroendocrine Tumor Analyses : Somatostatin Analog Effects and MicroRNA Profiling
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Small intestinal neuroendocrine tumors (SI-NETs) originate from serotonin-producing enterochromaffin (EC) cells in the intestinal mucosa. Somatostatin analogs (SSAs) are mainly used to control hormonal secretion and tumor growth. However, the molecular mechanisms leading to the control of SI-NETs are unknown. Although microRNAs (miRNAs) are post transcriptional regulators deeply studied in many cancers, are not well-defined in SI-NETs. We adopted a two-pronged strategy to investigate SSAs and miRNAs: first, to provide novel insights into how SSAs control NET cells, and second, to identify an exclusive SI-NET miRNA expression, and investigate the biological functions of miRNA targets.To accomplish the first aim, we treated CNDT2.5 cells with octreotide for 16 months. Affymetrix microarray was performed to study gene variation of CNDT2.5 cells in the presence or absence of octreotide. The study revealed that octreotide induces six genes, ANXA1, ARHGAP18, EMP1, GDF15, TGFBR2 and TNFSF15.To accomplish the second aim, SI-NET tissue specimens were used to run genome-wide Affymetrix miRNA arrays. The expression of five miRNAs (miR-96, -182, -183, -196a and -200a) was significantly upregulated in laser capture microdissected (LCM) tumor cells versus LCM normal EC cells, whereas the expression of four miRNAs (miR-31, -129-5p, -133a and -215) was significantly downregulated in LCM tumor cells. We also detected nine tissue miRNAs in serum samples, showing that the expression of five miRNAs is significantly increased in SSA treated patients versus untreated patients. Conversely, SSAs do not change miRNA expression of four low expressed miRNAs. Silencing miR-196a expression was used to investigate functional activities in NET cells. This experimental approach showed that four miR-196a target genes, HOXA9, HOXB7, LRP4 and RSPO2, are significantly upregulated in silenced miR-196a NET cells.In conclusion, ANXA1, ARHGAP18, EMP1, GDF15, TGFBR2 and TNFSF15 genes might regulate cell growth and differentiation in NET cells, and play a role in an innovative octreotide signaling pathway. The global SI-NET miRNA profiling revealed that nine selected miRNAs might be involved in tumorigenesis, and play a potential role as novel markers for follow-up. Indeed, silencing miR-196a demonstrated that HOXA9, HOXB7, LRP4 and RSPO2 genes are upregulated at both transcriptional and translational levels.
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| 10. |
- Sun, Xiaoyu, 1994-
(författare)
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Thermodynamic investigation of high-temperature coatings : microstructure, oxidation and interdiffusion
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High-temperature coatings, such as metallic MCrAlY (M: Ni and/or Co) coatings, are widely employed to improve the oxidation resistance of superalloys. However, interdiffusion between the coatings and the superalloys under elevated service temperatures leads to microstructural degradation in both. Some of the underlying degradation mechanisms are still elusive, such as the γ′ (Ni3Al) phase depletion in the superalloys, where a large amount of γ′ precipitates dissolve in the γ matrix, even though the incoming Al from the coatings locally increases the Al content. In this research, we investigated the interdiffusion and oxidation behaviors in coating/superalloy systems of Amdry365 (MCrAlY)/IN792, Ta-containing (MCrAlYTa) coatings/IN792 and high-entropy-alloying (AlCrFeCoNi) coatings/IN792 at 1100 °C. Multiple microscopic techniques were employed to study microstructural evolution and chemical compositional changes. Thermodynamic calculations were used to reveal the underlying mechanisms of these changes.The experimental and simulation results reveal a dominant effect of fast-diffusion Al from the coating towards the superalloy in the early stage of phase transitions. This leads to β phase depletion on the coating side and the formation of β as well as and increment of γ′ phase on the superalloy side. We propose an Al-Cr interference (ACI) effect to account for the pile-up behavior of Cr and the reduced Al content observed near the coating/superalloy interface. The incoming Al from the coating enhances the Cr potential in the superalloy, causing Cr diffusion from the interior of the superalloy to the coating/superalloy interface., Meanwhile, the pile-up of Cr increases the Al potential, which accelerates Al diffusion from this region into the interior of the superalloy. Furthermore, local-phase- equilibrium calculations reveal that γ′ phase depletion in the substrate is ascribed to the loss of γ′ forming elements, Ti and Ta. The incoming Al accelerates γ′ phase depletion, since it enhances the potential of Ti and Ta promoting their diffusion from the superalloy side to the coating side. Adding Ta to coatings impedes γ′ depletion in the substrate superalloys and also promotes the phase transition from β (NiAl) to γ′ in the coatings. As for the oxidation behavior, a high Ta content in coatings, such as the coating with 7.2 wt.% Ta reduces the oxidation resistance due to the formation of excessive Ta-rich oxides. It is observed in the high entropy alloy coatings that a lower Al/Cr ratio benefits oxidation resistance as its protective scale consists of larger Al2O3 grains, reducing the number of grain boundaries that function as fast diffusion paths for oxygen.This research clarifies a confusing point: the reason for γ′ phase depletion is the increase of Cr and Co related to interdiffusion. These results answer a long-standing but critical question in the coating/superalloy interdiffusion research, i.e., why the γ′ precipitates dissolve in the γ matrix even though the incoming Al from coatings increases the Al content. The γ′ phase depletion is ascribed to the loss of γ′ forming elements, Ti and Ta, whereas the incoming Al cannot offset their effect. The diffusion simulation performed in this work still requires a long calculation time, as a large number of elements were considered in the diffusion model. According to our findings, γ′ depletion is mainly dependent on the loss of Ta, Ti. Hence, a simplified prediction model for γ′ depletion for fast computation can be established by considering only the diffusion of Ta, Ti and Al.
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