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"Li, Jing"
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Area under the ROC Curve has the most consistent evaluation for binary classification
The proper use of model evaluation metrics is important for model evaluation and model selection in binary classification tasks. This study investigates how consistent different metrics are at evaluating models across data of different prevalence while the relationships between different variables and the sample size are kept constant. Analyzing 156 data scenarios, 18 model evaluation metrics and five commonly used machine learning models as well as a naive random guess model, I find that evaluation metrics that are less influenced by prevalence offer more consistent evaluation of individual models and more consistent ranking of a set of models. In particular, Area Under the ROC Curve (AUC) which takes all decision thresholds into account when evaluating models has the smallest variance in evaluating individual models and smallest variance in ranking of a set of models. A close threshold analysis using all possible thresholds for all metrics further supports the hypothesis that considering all decision thresholds helps reduce the variance in model evaluation with respect to prevalence change in data. The results have significant implications for model evaluation and model selection in binary classification tasks.
Journal Article
الاقتصاد الصيني
يتناول كتاب (الاقتصاد الصيني) والذي قام بتأليفه (وو دي لي، سوي فو مين، تشنغ لي) في حوالي (127) صفحة من القطع المتوسط موضوع (اقتصاد الصيني) مستعرضا التالي : يتحدث عن الإنجازات الملحوظة التي حققها الاقتصاد الصيني منذ أكثر من ثلاثين عاما هي مدة الإصلاح والانفتاح، وارتفاع مكانته الدولية بصورة متزايدة. وعن الاتجاه المستقبلي لاقتصاد الصين وإسهامات الاقتصاد الصيني في الاقتصاد العالمي ؟ ويمكن للقارئ أن يحصل على أجوبة لكل هذه الأسئلة من ذلك الكتاب.
Book
Highly stabilized and efficient thermoelectric copper selenide
The liquid-like feature of thermoelectric superionic conductors is a double-edged sword: the long-range migration of ions hinders the phonon transport, but their directional segregation greatly impairs the service stability. We report the synergetic enhancement in figure of merit (ZT) and stability in Cu
1.99
Se-based superionic conductors enabled by ion confinement effects. Guided by density functional theory and nudged elastic band simulations, we elevated the activation energy to restrict ion migrations through a cation–anion co-doping strategy. We reduced the carrier concentration without sacrificing the low thermal conductivity, obtaining a ZT of ∼3.0 at 1,050 K. Notably, the fabricated device module maintained a high conversion efficiency of up to ∼13.4% for a temperature difference of 518 K without obvious degradation after 120 cycles. Our work could be generalized to develop electrically and thermally robust functional materials with ionic migration characteristics.
Cu
2
Se is of interest for thermoelectrics as it is environmentally sustainable and has a high figure of merit ZT; however, copper ion migration impacts device stability. Here a co-doping strategy that combines steric and electrostatic effects is shown to improve device stability as well as improving
ZT
to 3.
Journal Article
Wide-temperature-range thermoelectric n-type Mg3(Sb,Bi)2 with high average and peak zT values
Mg
3
(Sb,Bi)
2
is a promising thermoelectric material suited for electronic cooling, but there is still room to optimize its low-temperature performance. This work realizes >200% enhancement in room-temperature
zT
by incorporating metallic inclusions (Nb or Ta) into the Mg
3
(Sb,Bi)
2
-based matrix. The electrical conductivity is boosted in the range of 300–450 K, whereas the corresponding Seebeck coefficients remain unchanged, leading to an exceptionally high room-temperature power factor >30 μW cm
−1
K
−2
; such an unusual effect originates mainly from the modified interfacial barriers. The reduced interfacial barriers are conducive to carrier transport at low and high temperatures. Furthermore, benefiting from the reduced lattice thermal conductivity, a record-high average
zT
> 1.5 and a maximum
zT
of 2.04 at 798 K are achieved, resulting in a high thermoelectric conversion efficiency of 15%. This work demonstrates an efficient nanocomposite strategy to enhance the wide-temperature-range thermoelectric performance of n-type Mg
3
(Sb,Bi)
2
, broadening their potential for practical applications.
The utilization of Mg
3
(Sb,Bi)
2
in thermoelectric devices is hindered by its low performance near room temperature. Here, authors report thermoelectric performance enhancement of Mg
3
(Sb,Bi)
2
within a wide temperature range by incorporating metallic inclusions at grain boundaries. (279 in total)
Journal Article
Ultralong purely organic aqueous phosphorescence supramolecular polymer for targeted tumor cell imaging
Purely organic room-temperature phosphorescence has attracted attention for bioimaging but can be quenched in aqueous systems. Here we report a water-soluble ultralong organic room-temperature phosphorescent supramolecular polymer by combining cucurbit[n]uril (CB[7], CB[8]) and hyaluronic acid (HA) as a tumor-targeting ligand conjugated to a 4-(4-bromophenyl)pyridin-1-ium bromide (BrBP) phosphor. The result shows that CB[7] mediated pseudorotaxane polymer CB[7]/HA–BrBP changes from small spherical aggregates to a linear array, whereas complexation with CB[8] results in biaxial pseudorotaxane polymer CB[8]/HA–BrBP which transforms to relatively large aggregates. Owing to the more stable 1:2 inclusion complex between CB[8] and BrBP and the multiple hydrogen bonds, this supramolecular polymer has ultralong purely organic RTP lifetime in water up to 4.33 ms with a quantum yield of 7.58%. Benefiting from the targeting property of HA, this supramolecular polymer is successfully applied for cancer cell targeted phosphorescence imaging of mitochondria.
Room-temperature phosphorescence has attracted attention for bioimaging, but quenching in aqueous systems is an issue. Here, the authors report on the synthesis of a water soluble organic phosphorescent polymer and demonstrate application is cancer cell targeting and imaging.
Journal Article
Evolution of defect structures leading to high ZT in GeTe-based thermoelectric materials
GeTe is a promising mid-temperature thermoelectric compound but inevitably contains excessive Ge vacancies hindering its performance maximization. This work reveals that significant enhancement in the dimensionless figure of merit (
ZT
) could be realized by defect structure engineering from point defects to line and plane defects of Ge vacancies. The evolved defects including dislocations and nanodomains enhance phonon scattering to reduce lattice thermal conductivity in GeTe. The accumulation of cationic vacancies toward the formation of dislocations and planar defects weakens the scattering against electronic carriers, securing the carrier mobility and power factor. This synergistic effect on electronic and thermal transport properties remarkably increases the quality factor. As a result, a maximum
ZT
> 2.3 at 648 K and a record-high average
ZT
(300-798 K) were obtained for Bi
0.07
Ge
0.90
Te in lead-free GeTe-based compounds. This work demonstrates an important strategy for maximizing the thermoelectric performance of GeTe-based materials by engineering the defect structures, which could also be applied to other thermoelectric materials.
The intrinsic high-concentration Ge vacancies in GeTe-based thermoelectric materials hinder their performance maximization. Here, the authors find that defect structure engineering strategy is effective for performance enhancement.
Journal Article