arXiv subjects

Xiangyu Li

Publications and source records attributed to Xiangyu Li.

136 records · Page 8Linked to original sources

arXiv2020

Remarkable Daytime Sub-ambient Radiative Cooling in BaSO4 Nanoparticle Films and Paints

Radiative cooling is a passive cooling technology that offers great promises to reduce space cooling cost, combat the urban island effect and alleviate the global warming. To achieve passive daytime radiative cooling, current state-of-the-art solutions often utilize complicated multilayer structures or a reflective metal layer, limiting their applications in many fields. Attempts have been made to achieve passive daytime radiative cooling with single-layer paints, but they often require a thick coating or show partial daytime cooling. In this work, we experimentally demonstrate remarkable full daytime sub-ambient cooling performance with both BaSO4 nanoparticle films and BaSO4 nanocomposite paints. BaSO4 has a high electron bandgap for low solar absorptance and phonon resonance at 9 um for high sky window emissivity. With an appropriate particle size and a broad particle size distribution, BaSO4 nanoparticle film reaches an ultra-high solar reflectance of 97.6% and high sky window emissivity of 0.96. During field tests, BaSO4 film stays more than 4.5C below ambient temperature or achieves average cooling power of 117 W/m2. BaSO4-acrylic paint is developed with 60% volume concentration to enhance the reliability in outdoor applications, achieving solar reflectance of 98.1% and sky window emissivity of 0.95. Field tests indicate similar cooling performance to the BaSO4 films. Overall, our BaSO4-acrylic paint shows standard figure of merit of 0.77 which is among the highest of radiative cooling solutions, while providing great reliability, the convenient paint form, ease of use and the compatibility with commercial paint fabrication process.

physics.app-ph↗

arXiv2020

Concentrated Radiative Cooling

A fundamental limit of current radiative cooling systems is that only the top surface facing deep-space can provide the radiative cooling effect, while the bottom surface cannot. Here, we propose and experimentally demonstrate a concept of "concentrated radiative cooling" by nesting a radiative cooling system in a mid-infrared reflective trough, so that the lower surface, which does not contribute to radiative cooling in previous systems, can radiate heat to deep-space via the reflective trough. Field experiments show that the temperature drop of a radiative cooling pipe with the trough is more than double that of the standalone radiative cooling pipe. Furthermore, by integrating the concentrated radiative cooling system as a preconditioner in an air conditioning system, we predict electricity savings of $>75\%$ in Phoenix, AZ, and $>80\%$ in Reno, NV, for a single-story commercial building.

physics.app-ph↗

arXiv2020

Full Daytime Sub-ambient Radiative Cooling with High Figure of Merit in Commercial-like Paints

Radiative cooling is a passive cooling technology by reflecting sunlight and emitting radiation in the atmospheric sky window. Although highly desired, full daytime sub-ambient radiative cooling in commercial-like single-layer particle-matrix paints is yet to be achieved. In this work, we have demonstrated full daytime sub-ambient radiative cooling in CaCO3-acrylic paint by adopting large bandgap fillers, a high particle concentration and a broad size distribution. Our paint shows the highest solar reflectance of 95.5% among paints and a high sky-window emissivity of 0.94. Field tests show cooling power exceeding 37 W/m2 and lower surface temperature more than 1.7C below ambient at noon. A figure of merit RC is proposed to compare the cooling performance under different weather conditions. The RC of our cooling paint is 0.62, among the best radiative cooling performance while offering unprecedented benefits of the convenient paint form, low cost, and the compatibility with commercial paint fabrication process.

physics.app-ph↗

arXiv2019

Multi-step Cascaded Networks for Brain Tumor Segmentation

Automatic brain tumor segmentation method plays an extremely important role in the whole process of brain tumor diagnosis and treatment. In this paper, we propose a multi-step cascaded network which takes the hierarchical topology of the brain tumor substructures into consideration and segments the substructures from coarse to fine .During segmentation, the result of the former step is utilized as the prior information for the next step to guide the finer segmentation process. The whole network is trained in an end-to-end fashion. Besides, to alleviate the gradient vanishing issue and reduce overfitting, we added several auxiliary outputs as a kind of deep supervision for each step and introduced several data augmentation strategies, respectively, which proved to be quite efficient for brain tumor segmentation. Lastly, focal loss is utilized to solve the problem of remarkably imbalance of the tumor regions and background. Our model is tested on the BraTS 2019 validation dataset, the preliminary results of mean dice coefficients are 0.886, 0.813, 0.771 for the whole tumor, tumor core and enhancing tumor respectively. Code is available at https://github.com/JohnleeHIT/Brats2019

eess.IV↗

arXiv2019

Iterative Spectral Method for Alternative Clustering

Given a dataset and an existing clustering as input, alternative clustering aims to find an alternative partition. One of the state-of-the-art approaches is Kernel Dimension Alternative Clustering (KDAC). We propose a novel Iterative Spectral Method (ISM) that greatly improves the scalability of KDAC. Our algorithm is intuitive, relies on easily implementable spectral decompositions, and comes with theoretical guarantees. Its computation time improves upon existing implementations of KDAC by as much as 5 orders of magnitude.

stat.ML↗

arXiv2019

Grain boundary effect on nanoindentation: A multiscale discrete dislocation dynamics model

Nanoindentation is a convenient method to investigate the mechanical properties of materials on small scales by utilizing low loads and small indentation depths. However, the effect of grain boundaries (GB) on the nanoindentation response remains unclear and needs to be studied by investigating in detail the interactions between dislocations and GBs during nanoindentation. In the present work, we employ a three-dimensional multiscale modeling framework, which couples three-dimensional discrete dislocation dynamics (DDD) with the Finite Element method (FEM) to investigate GB effects on the nanoindentation behavior of an aluminum bicrystal. The interaction between dislocations and GB is physically modeled in terms of a penetrable GB, where piled-up dislocations can penetrate through the GB and dislocation debris at GBs can emit full dislocations into grains. In the simulation, we confirmed two experimentally observed phenomena, namely, pop-in events and the dependence of indentation hardness on the distance from GB. Two pop-in events were observed, of which the initial pop-in event is correlated with the activation and multiplication of dislocations, while the GB pop-in event results from dislocation transmission through the GB. By changing the distance between the indenter and GB, the simulation shows that the indentation hardness increases with decreasing GB-indenter distance. A quantitative model has been formulated which relates the dependency of indentation hardness on indentation depth and on GB-indenter distance to the back stress created by piled-up geometrically necessary dislocations in the plastic zone and to the additional constraint imposed by the GB on the plastic zone size.

cond-mat.mtrl-sci↗

arXiv2017

A high-resolution programmable Vernier delay generator based on carry chains in FPGA

This paper presents an architecture of high-resolution delay generator implemented in a single field programmable gate array (FPGA) chip by exploiting the method of utilizing dedicated carry chains. It serves as the core component in various physical instruments. The proposed delay generator contains the coarse delay step and the fine delay step to guarantee both large dynamic range and high resolution. The carry chains are organized in the Vernier delay loop style to fulfill the fine delay step with high precision and high linearity. The delay generator was implemented in the EP3SE110F1152I3 Stratix III device from Altera on a self-designed test board. Test results show the obtained resolution is 38.6 ps, and the differential nonlinearity (DNL) and integral nonlinearity (INL) is in the range of (-0.18 least significant bit (LSB),0.24 LSB)\(-0.02 LSB,0.01 LSB) under the nominal supply voltage of 1100 mV and environmental temperature of 20 degrees centigrade. The delay generator is also rather resource cost efficient which uses only 668 LUTs and 146 registers in total.

physics.ins-det↗

arXiv2017

Towards implementing multi-channels, ring-oscillator-based, Vernier time-to-digital converter in FPGAs: key design points and construction method

For TOF positron emission tomography (TOF PET) detectors, time-to-digital converters (TDCs) are essential to resolve the coincidence time of the photon pairs. Recently, an efficient TDC structure called ring-oscillator-based (RO-based) Vernier TDC using carry chains was reported by our team. The method is very promising due to its low linearity error and low resource cost. However, the implementation complexity is rather high especially when moving to multi-channels TDC designs, since this method calls for a manual intervention to the initial fitting results of the compilation software. In this paper, we elaborate the key points toward implementing high performance multi-channels TDCs of this kind while keeping the least implementation complexity. Furthermore, we propose an efficient fine time interpolator construction method called the period difference recording which only needs at most 31 adjustment trials to obtain a targeted TDC resolution. To validate the techniques proposed in this paper, we built a 32-channels TDC on a Stratix III FPGA chip and fully evaluated its performance. Code density tests show that the obtained resolution results lie in the range of (23 ps ~ 37 ps), the differential nonlinearity (DNL) results lie in the range of (-0.4 LSB ~ 0.4 LSB) and the integral nonlinearity (INL) results lie in the range of (-0.7 LSB ~ 0.7 LSB) for each of the 32 TDC channels. This paper greatly eases the designing difficulty of the carry chain RO-based TDCs and can significantly propel their development in practical use.

physics.ins-det↗

arXiv2017

High-performance Thermal Interface Material Based on Few-layer Graphene Composite

We developed high-performance thermal interface materials (TIMs) based on few-layer graphene (FLG) composite, where FLG was prepared by the interlayer catalytic exfoliation (ICE) method. We experimentally demonstrated feasibility of FLG composites as TIMs by investigating their thermal and mechanical properties, and reliability. We measured the thermal interface resistance ($R_{int}$) between FLG composite TIMs (FLGTs) and copper and to be 3.2$\pm$1.7 and 4.3$\pm$1.4 $mm^2$K/W for 5 vol.% and 10 vol.% FLGTs at 330 K, respectively, comparable to or even lower than that of many commercial TIMs. In addition, the thermal conductivity ($κ_{TIM}$) of FLGTs is increased by an enhancement factor ($β$) of ~17 as the FLG concentration increases from 0 to 10 vol.%. We also characterized Vickers hardness and glass transition temperature ($T_g$) of our FLGTs. We find that our FLGTs are thermally and mechanically reliable within practical operating temperature and pressure ranges.

cond-mat.mtrl-sci↗

arXiv2017

Compressive mechanical response of graphene foams and their thermal resistance with copper interfaces

We report compressive mechanical response of graphene foams (GFs) and the thermal resistance ($R_{TIM}$) between copper (Cu) and GFs, where GFs were prepared by the chemical vapor deposition (CVD) method. We observe that Young's modulus ($E_{GF}$) and compressive strength ($σ_{GF}$) of GFs have a power law dependence on increasing density ($ρ_{GF}$) of GFs. The maximum efficiency of absorbed energy ($η_{max}$) for all GFs during the compression is larger than ~0.39. We also find that a GF with a higher $ρ_{GF}$ shows a larger $η_{max}$. In addition, we observe that the measured $R_{TIM}$ of Cu/GFs at room temperature with a contact pressure of 0.25 MP applied increases from ~50 to ~90 $mm^2K/W$ when $ρ_{GF}$ increases from 4.7 to 31.9 $mg/cm^3$.

cond-mat.mtrl-sci↗