Energy-efficient CO2 hydrogenation with fast response using photoexcitation of CO2 adsorbed on metal catalysts.

Many heterogeneous catalytic reactions occur at high temperatures, which may cause large energy costs, poor safety, and thermal degradation of catalysts. Here, we propose a light-assisted surface reaction, which catalyze the surface reaction using both light and heat as an energy source. Conventional metal catalysts such as ruthenium, rhodium, platinum, nickel, and copper were tested for CO2 hydrogenation, and ruthenium showed the most distinct change upon light irradiation. CO2 was strongly adsorbed onto ruthenium surface, forming hybrid orbitals. The band gap energy was reduced significantly upon hybridization, enhancing CO2 dissociation. The light-assisted CO2 hydrogenation used only 37% of the total energy with which the CO2 hydrogenation occurred using only thermal energy. The CO2 conversion could be turned on and off completely with a response time of only 3 min, whereas conventional thermal reaction required hours. These unique features can be potentially used for on-demand fuel production with minimal energy input

Differentially Private Multivariate Statistics with an Application to Contingency Table Analysis

Differential privacy (DP) has become a rigorous central concept in privacy protection for the past decade. Among various notions of DP, $f$-DP is an easily interpretable and informative concept that tightly captures privacy level by comparing trade-off functions obtained from the hypothetical test of how well the mechanism recognizes individual information in the dataset. We adopt the Gaussian differential privacy (GDP), a canonical parametric family of $f$-DP. The Gaussian mechanism is a natural and fundamental mechanism that tightly achieves GDP. However, the ordinary multivariate Gaussian mechanism is not optimal with respect to statistical utility. To improve the utility, we develop the rank-deficient and James-Stein Gaussian mechanisms for releasing private multivariate statistics based on the geometry of multivariate Gaussian distribution. We show that our proposals satisfy GDP and dominate the ordinary Gaussian mechanism with respect to $L_2$-cost. We also show that the Laplace mechanism, a prime mechanism in $\varepsilon$-DP framework, is sub-optimal than Gaussian-type mechanisms under the framework of GDP. For a fair comparison, we calibrate the Laplace mechanism to the global sensitivity of the statistic with the exact approach to the trade-off function. We also develop the optimal parameter for the Laplace mechanism when applied to contingency tables. Indeed, we show that the Gaussian-type mechanisms dominate the Laplace mechanism in contingency table analysis. In addition, we apply our findings to propose differentially private $\chi^2$-tests on contingency tables. Numerical results demonstrate that differentially private parametric bootstrap tests control the type I error rates and show higher power than other natural competitors

Improved Differential Fault Attack on LEA by Algebraic Representation of Modular Addition

Recently, as the number of IoT (Internet of Things) devices has increased, the use of lightweight cryptographic algorithms that are suitable for environments with scarce resources has also increased. Consequently, the safety of such cryptographic algorithms is becoming increasingly important. Among them, side-channel analysis methods are very realistic threats. In this paper, we propose a novel differential fault attack method on the Lightweight Encryption Algorithm (LEA) cipher which became the ISO/IEC international standard lightweight cryptographic algorithm in 2019. Previously proposed differential fault attack methods on the LEA used the Single Bit Flip model, making it difficult to apply to real devices. The proposed attack method uses a more realistic attacker assumption, the Random Word Error model. We demonstrate that the proposed attack method can be implemented on real devices using an electromagnetic fault injection setup. Our attack method has the weakest attacker assumption among attack methods proposed to date. In addition, the number of required fault-injected ciphertexts and the number of key candidates for which exhaustive search is performed are the least among all existing methods. Therefore, when implementing the LEA cipher on IoT deivces, designers must apply appropriate countermeasures against fault injection attacks

Synergistic nanoarchitecture of mesoporous carbon and carbon nanotubes for lithium-oxygen batteries

A rechargeable lithium–oxygen battery (LOB) operates via the electrochemical formation and decomposition of solid-state Li2O2 on the cathode. The rational design of the cathode nanoarchitectures is thus required to realize high-energy-density and long-cycling LOBs. Here, we propose a cathode nanoarchitecture for LOBs, which is composed of mesoporous carbon (MPC) integrated with carbon nanotubes (CNTs). The proposed design has the advantages of the two components. MPC provides sufficient active sites for the electrochemical reactions and free space for Li2O2 storage, while CNT forests serve as conductive pathways for electron and offer additional reaction sites. Results show that the synergistic architecture of MPC and CNTs leads to improvements in the capacity (~ 18,400 mAh g− 1), rate capability, and cyclability (~ 200 cycles) of the CNT-integrated MPC cathode in comparison with MPC. © 2021, The Author(s).1

정적연소실을 이용한 부분 예혼합 조건에서의 저옥탄 연료의 화염 가시화 및 분석 - 연소 컨셉 모델 제시와 스프레이 내부 현상에 대한 이해

학위논문 (박사)-- 서울대학교 대학원 : 기계항공공학부, 2017. 2. 송한호.압축 착화 엔진(CI engine)은 연소 특성상 높은 열효율을 가지고 있지만, 입자상 물질(soot)과 질소산화물(NOx)을 많이 배출하는 단점을 가지고 있다. 현재 많은 국가들에서는 자동차 배기가스에 의한 환경 문제를 줄이기 위해 엔진에서 발생하는 오염물질의 배출을 제한하는 규제를 강화하고 있다. 이러한 추세에 맞추어 압축 착화 엔진에서 입자상 물질을 줄이지 위한 많은 연구가 이루어지고 있는데 그 중 하나가 부분 예혼합 압축 착화(PPCI: Partially premixed compression ignition) 방식이다. 압축 착화 엔진에서 입자상 물질은 실린더로 직접 분사된 연료와 내부 공기가 충분히 혼합되지 못한 상태에서 생성되기 때문에 입자상 물질의 생성을 억제하기 위해서는 연소 전까지 공기와 연료의 충분한 혼합을 유도하는 것이 중요하다. 이를 위한 연소 전략 중에 하나인 PPCI는 물리적인 제어 방식을 통해 당량비가 2 이하인 혼합기의 비율을 높이는 것을 목표로 한다. 다양한 연료들 중에서 가솔린 계열의 연료는 점화 지연이 길기 때문에 연소 시작 전까지 공기와 연료가 혼합될 수 있는 충분한 시간을 보장한다. 이러한 가솔린 계열의 연료들 중에서 옥탄가가 70 전후인 저옥탄 연료의 경우, 낮은 부하, 높은 RPM에서 고옥탄 가솔린보다 연소 안정성이 뛰어나기 때문에 주목을 받고 있다. 현재 저옥탄 연료를 적용한 압축 착화 엔진에 대한 연구들이 이루어지고 있지만, 기존의 디젤 엔진을 그대로 이용한 것이 대부분이다. 압축 착화 엔진에서의 연료 스프레이 특성은 엔진의 효율, 배기 특성, 구조를 결정하는 주요 변수이지만, 저옥탄 연료의 경우, 이러한 핵심 정보들에 대한 연구가 미미하다. 따라서 저옥탄 연료를 이용한 최적화된 PPCI 엔진을 개발하기 위한 선행 단계로서 가솔린의 스프레이에 대한 연구가 필수적이다. 따라서 본 연구에서는 PRF70 연료를 이용하여 정적연소실에서 저옥탄 연료의 스프레이 연소를 모사하고 계측하는 것을 목표로 한다. 우선, 다양한 조건에서의 연소 모드를 비교한 결과, 기존의 연소 모델로 설명할 수 없는 새로운 연소 모드가 존재함을 확인하였고, 이를 Partially premixed combustion zone이라고 명명하였다. 이 조건은 기존의 LTC 연소보다는 분사 기간이 길고, 일반적인 디젤 연소보다 연소 시작 지점이 다운 스트림에 위치하고 있다는 것이 가장 큰 차이점이다. 기존의 디젤 스프레이는 연소는 강한 운동량으로 인해 화염이 앞으로만 전파되는 발달 형상을 보이지만, 이 새로운 연소 모드에서는 저압 분사로 인해 다운 스트림에서의 운동량이 작기 때문에 처음 연소가 시작한 지점에서 모든 방향으로 예혼합 화염 영역의 확장이 발생한다. 화염 영역이 확장이 되면서 스프레이 상류의 특정 지점을 통과하게 되면 최종적으로 강한 발광 신호와 함께 입자상 물질이 생성된다. 반복 실험을 통해 같은 조건에서도 입자상 물질의 발광 세기에 큰 편차가 있음이 확인되었고, 이는 연소 시작 지점의 편차에 기인한 것으로 밝혀졌다. 이러한 확률적인 거동 현상을 이해하기 위해서 시뮬레이션을 통해 스프레이 내부의 열역학적 특성에 대한 정보를 계산하였다. 시뮬레이션 결과, 다운 스트림에서 발생한 화염이 상류의 당량비가 2인 지점을 통과하는지 여부가 입자상 물질의 생성을 결정하며, 인젝터 앞쪽의 온도 편차가 확률적인 거동을 결정하는 중요 변수임을 알 수 있었다. 만약 스프레이 상류의 배경 온도를 일정하게 제어할 수 있다면, 초기 연소 지점 또한 예측, 제어가 가능할 것이며, 이를 통해 입자상 물질의 생성을 줄일 수 있을 것으로 예상된다. 일반적인 LTC 운전에서는 추가적인 입자상 물질의 생성을 막기 위해 분사 기간을 짧게 가져가는 것이 일반적이지만, 본 연구를 통해서 분사 기간이 길더라도 입자상 물질의 생성을 제한하는 것이 가능하다는 것을 알 수 있다. 본 연구에서는 기존에 알려진 디젤 스프레이 연소와는 다른 저옥탄 연료의 새로운 연소 모드를 확인하였고, 광학계측법을 통해 화염의 발달 과정을 이해하였다. 점화 지연이 긴 연료를 저압으로 분사하였을 경우, 다운 스트림에서의 예혼합 화염이 입자상 물질을 생성을 결정하는 주요 메커니즘이며, 인젝터 앞 쪽, 즉, 스프레이 상류의 온도 구배에 큰 영향을 받는다. 온도 제어를 통해 초기 점화 지점을 충분히 다운 스트림 영역으로 제한할 수 있다면, 연소 기간 동안 상류로의 화염 영역의 확장을 예방하여 입자상 물질의 생성을 줄일 수 있다.Compression ignition (CI) engines are widely used in transportation field, especially for high duty vehicles, because of their high thermal efficiency and torque. However, they have emission problems with soot (particulate matter) and NOx (nitrogen oxide). Many countries make effort to reduce the environmental problems resulted by vehicle emissions by strengthening regulation. Among the several attempts to satisfy the new regulation, partially premixed compression ignition (PPCI) is being given attention. In CI type engines, fuel-rich zone during combustion duration is cause of soot production. Therefore, to reduce the soot production, it is important to lead to more mixing of direct injected fuel and ambient air to decrease local equivalence ratio before start of combustion (SOC). In PPCI strategy, by adjusting injection timing or adopting alternative fuels, it is possible to induces enough mixing time, and then makes more premixed charge before SOC. Among the various commercial fuels, gasoline-like fuels having high resistance to auto-ignition have advantages of mixing because they have longer ignition delay than conventional diesel fuel. Especially, low octane fuels having 70 – 80 octane numbers are more effective than high octane gasolines in low load, high RPM and high EGR conditions. To apply low octane fuels and develop optimized CI engines, it is necessary to understand fundamental characteristics of fuel spray and combustion, because they are dominant factors to determine efficiency, emissions, and geometry of CI engines. Recently, although there are several works to demonstrate the superiority of low octane fuel in CI engines, those works are conducted with conventional diesel engine with little optimization. Thus, there is not enough physical information and spray combustion model of low octane fuel yet. In this study, to understand the spray combustion of low octane fuel in PPCI condition as a precedent step to develop new type of CI engine, fundamental researches were conduct on fuel spray. Instead of commercial fuel, spray combustion of primary reference fuel 70 (surrogate fuel for low octane fuel) was analyzed empirically in constant volume chamber by high speed imaging including filtered natural luminosity and shadowgraph. Firstly, from the comparison of combustion modes in operating regime, it is verified that there exists new combustion mode. This new combustion mode is designated as partially premixed combustion zone. This new combustion mode has longer injection duration than conventional low temperature combustion (LTC) model and combustion occurs at more downstream than quasi-steady diesel combustion model. As different with conventional diesel combustion where flame goes forward with high momentum, this mode shows expansion of combustion zone to all radial direction in downstream region where momentum is low enough. When combustion zone expands and finally passes the certain upstream location, soot with intensive luminosity begins to appear. In some cases, there is little soot or no radiation signal during combustion duration even at same operating condition. From the stochastic analysis, it is verified that whether soot intensity is high or low comes from variation in initial combustion location. To understand these stochastic behaviors, validated 1-D computational model was used to calculate the thermodynamic properties of mixture in spray. From the combination of experimental and computational results, it is verified that case by case variation in temperature near the injector tip has significant effects on first ignition location. Namely, as temperature near the tip is high, combustion location becomes close to injector tip and then chance of intensive soot production becomes high. If it is possible to control temperature in upstream region in reasonable range, it is also possible to predict initial combustion. Additionally, if combustion zone is restricted to downstream enough not to expand to certain upstream location, soot production can be reduced. It general, to reduce the soot during combustion duration, short injection duration is recommended, but it is shown that it is possible to limit the soot production even with extreme long injection duration. In this work, it is confirmed that it is possible to achieve distinctive combustion mode with low octane fuel. When low octane fuel is injected with low injection pressure and long injection duration, expansion of initial combustion zone in downstream is dominant mechanism to determine soot production. In this case, temperature in front of injector tip has significant effect on initial combustion location and resultant soot production. If temperature is well regulated and combustion zone can be restricted to downstream, it prevents reaction zone from producing soot in upstream.Chapter 1. Introduction 1 1.1 Research background 1 1.2 PPCI with low octane fuel 11 1.3 Objective and motivation 21 Chapter 2. Experimental set-up and methodology 22 2.1 Introduction 22 2.2 Experimental apparatus 23 2.2.1 Constant volume chamber 23 2.2.2 Fuel pumping and injection system 26 2.2.3 Optical diagnostics devices 29 2.3 Experimental procedure 33 2.3.1 Pre-burn process 33 2.3.2 Determination of target condition 37 2.3.3 Device triggering with pressure signal 42 Chapter 3. Experimental results 1 - Operating regime of PRF70 with various injection conditions 44 3.1 Effects of injection pressure and duration on combustion mode change 44 3.2 Comparison of two different oxygen concentrations 49 3.3 Summary 56 Chapter 4. Experimental results 2 - Understanding of flame development by natural luminosity visualization: Physical properties of flame and resultant soot production 57 4.1 Introduction 57 4.2 Two combustion types in combustion regime 58 4.3 Stochastic behaviors in PRF70 spray combustion 67 4.4 Flame development in PRF70 combustion 70 4.4.1 Radially averaged intensity for analysis 70 4.4.2 Concept A 80 4.4.3 Concept B 85 4.5 Summary 91 Chapter 5. Experimental results 3 – Confirmation of flame structure by shadowgraph and Mie-scattering measurement 93 5.1 Introduction 93 5.2 Cool flame and expansion of initial combustion zone 93 5.3 Summary 100 Chapter 6. Understanding of stochastic combustion behaviors with 1-D spray model 101 6.1 Introduction 101 6.2 Methodology 106 6.3 1-D model for equivalence ratio prediction 111 6.4 Simulation of reacting flow in spray 129 6.5 Summary 153 Chapter 7. Suggestion of combustion model and final conclusions 154 7.1 Suggestion of new conceptual model 154 7.2 Final conclusions and summary 158 References 161 국 문 초 록 173Docto

Syllable pattern-based unknown morpheme estimation for hybrid part-of-speech tagging of Korean

This paper presents a syllable pattern directed generalized unknown morpheme handling method with POSTAG (POStech TAGger)

Side Channel Leakages Against Financial IC Card of the Republic of Korea

Integrated circuit (IC) chip cards are commonly used in payment system applications since they can provide security and convenience simultaneously. More precisely, Europay, MasterCard, and VISA (EMV) are widely known to be well equipped with security frameworks that can defend against malicious attacks. On the other hand, there are other payment system applications at the national level. In the case of the Republic of Korea, standards for financial IC card specifications are established by the Korea Financial Telecommunications and Clearings Institute. Furthermore, security features defending against timing analysis, power analysis, electromagnetic analysis, and TEMPEST are required. This paper identifies side channel leakages in the financial IC cards of the Republic of Korea, although there may be side channel countermeasures. Side channel leakages in the financial IC cards of the Republic of Korea are identified for the first time since the side channel countermeasures were included in the standards. The countermeasure that is applied to the IC card from a black box perspective is estimated to measure security features against power analysis. Then, in order to investigate whether an underlying countermeasure is applied, first-order and second-order power analyses are performed on the main target, e.g., a S-box of the block cipher SEED that is employed in the financial system. Furthermore, the latest proposal in ICISC 2017 is examined to apply block cipher SEED to the financial IC card protocol. As a result, it is possible to identify some side channel leakages while expanding the lemma of the paper accepted in ICISC 2017. Algebraic logic is also constructed to recover the master key from some round keys. Finally, it is found that only 20,000 traces are required to find the master key