Thermal Resolution Research Articles

Eu3+-doped ratiometric optical thermometers: Experiment and Judd-Ofelt modelling

Rare earth-doped materials are successfully utilized as non-contact optical temperature sensors. Thermal sensing is usually based on the ratiometric technique between two separate emission lines originated from thermally-coupled levels. We study Gd 2 O 3 :Eu 3+ 1 at.% and YAG: Eu 3+ 4 at.% phosphors as luminescence thermometers in 298–473 K range using both conventional experimental and Judd-Ofelt based theoretical approaches. Thermometric performances of suggested sensors were quantified through absolute and relative thermal sensitivities and temperature resolution. Experiment and theoretical modelling gave similar results. This proves the theoretical model as a simple and fast method for estimation of the Eu 3+ -doped thermometer perspectives. • Eu 3+ -doped phosphors were synthesized via different modified Pechini methods. • Gd 2 O 3 :Eu 3+ and YAG:Eu 3+ nanopowders were used as contactless luminescence thermometers. • Thermometric performances were estimated through thermal sensitivities and temperature resolution. • Experimental approach and Judd-Ofelt based theoretical model give similar results.

Detection and tracking of cracks based on thermoelastic stress analysis.

Thermoelastic stress analysis using arrays of small, low-cost detectors has the potential to be used in structural health monitoring. However, evaluation of the collected data is challenging using traditional methods, due to the lower resolution of these sensors, and the complex loading conditions experienced. An alternative method has been developed, using image decomposition to generate feature vectors which characterize the uncalibrated map of the magnitude of the thermoelastic effect. Thermal data have been collected using a state-of-the-art photovoltaic effect detector and lower cost, lower thermal resolution microbolometer detectors, during crack propagation induced by both constant amplitude and frequency loading, and by idealized flight cycles. The Euclidean distance calculated between the feature vectors of the initial and current state can be used to indicate the presence of damage. Cracks of the order of 1 mm in length can be detected and tracked, with an increase in the rate of change of the Euclidean distance indicating the onset of critical crack propagation. The differential feature vector method therefore represents a substantial advance in technology for monitoring the initiation and propagation of cracks in structures, both in structural testing and in-service using low-cost sensors.

Nd3+ doped TiO2 nanocrystals as self-referenced optical nanothermometer operating within the biological windows

In this work, we report a potential optical nanothermometer (NTh) based on Nd3+ doped TiO2 nanocrystals, operating within the biological windows. The Nd3+ emissions at around 900, 1060, and 1340 nm were used upon excitation at 808 nm. The fluorescence intensity ratio (FIR) corresponding to the emissions at 1060 nm and 1340 nm (FIR = I1060 nm/I1340 nm) was investigated as a function of the temperature in the range of 300−343 K. In addition, the effect of the Nd3+ ions concentration and influence of the crystalline phase on the luminescence and the relative thermal sensitivity were presented. As a result, it was found that TiO2 nanocrystals doped with 10 wt% of Nd3+ ions and associated with the brookite phase has the highest luminescence emission and relative thermal sensitivity compared to other samples investigated here and in previous literature. The maximum value obtained to the relative thermal sensitivity was 0.86 % K−1, which is high comparatively with other ones of Nd3+ doped systems, and minimum thermal resolution of ∼1 K. This information together with the biocompatibility of TiO2 nanocrystals, and excitation and emissions wavelengths lying within the biological windows, favor potential applications in fluorescence and thermal imaging of biological systems.

Red-Emitting Carbon Nanodot-Based Wide-Range Responsive Nanothermometer for Intracellular Temperature Sensing

Precise sensing of intracellular temperature can provide plenty of information on disease-related cell states and promote the development of a diagnostic method. Fluorescence-based nanothermometers, as the "noncontact" sensors, exhibit great advantages over traditional thermometers due to the dual function of imaging and sensing at the molecular level. Herein, we report a red-emitting carbon nanodots (RCDs)-based nanothermometer for intracellular temperature sensing. Results indicate that RCDs exhibit favorable temperature-responsive fluorescence property with a good linear relationship, reversibility, and reproducibility under heating and cooling treatments in a wide range from 4 to 80 °C. Meanwhile, the RCDs possess satisfactory thermal sensitivity and temperature resolution, which are superior or comparable to the current nanothermometers. The low cytotoxicity and excellent temperature-responsive fluorescence property of RCDs have also been verified in living cell studies. Therefore, the RCDs will be a promising nanothermometer for intracellular temperature sensing in diverse areas.

Graphene on single Ag nanoparticles for nanoscale and quantum applications

Raman spectra of graphene on single Ag nanoparticles (Ag NPs) reveal promising features for quantum and imaging/sensing applications in the nanoscale regime. The active Raman modes of graphene monolayer act as a Raman Thermometry and Ag NPs act as plasmonic nanoheaters excited by the probe laser. The thermal probing resolution can reach the nanometer scale determined by the graphene area attached to the Ag NPs. In addition to the Raman shift and intensity enhancement, the G-peak half width (ΓG) decreases with increasing the laser power inferring the photo-induced electron transfer from AgNPs to the attached graphene area; we are the first to report ΓG reduction by thermal investigations. The proposed graphene/Ag NPs structure is exempt from power dissipation due to substrate effects such as the thermal coupling between graphene and substrate. Graphene plasmons bound to the surface at the nanoscale size driven by the photo-induced electron transfer from single Ag NPs are promising for quantum plasmonic applications that have been difficult to be achieved with conventional metal plasmonics.

Upconversion Nanocrystal Doped Polymer Fiber Thermometer.

In recent years, lanthanide-doped nanothermometers have been mainly used in thin films or dispersed in organic solvents. However, both approaches have disadvantages such as the short interaction lengths of the active material with the pump beam or complicated handling, which can directly affect the achievable temperature resolution. We investigated the usability of a polymer fiber doped with upconversion nanocrystals as a thermometer. The fiber was excited with a wavelength stabilized diode laser at a wavelength of 976 nm. Emission spectra were recorded in a temperature range from 10 to 35 C and the thermal emission changes were measured. Additionally, the pump power was varied to study the effect of self-induced heating on the thermometer specifications. Our fiber sensor shows a maximal thermal sensitivity of 1.45%/K and the minimal thermal resolution is below 20 mK. These results demonstrate that polymer fibers doped with nanocrystals constitute an attractive alternative to conventional fluorescence thermometers, as they add a long pump interaction length while also being insensitive to strong electrical fields or inert to bio-chemical environments.

Thermal behaviour of caesium implanted in UO2: A comparative study with the xenon behaviour

Xenon and caesium are among the most impacting fission products when studying the nuclear fuel: xenon for its role on the fuel rod thermomechanical behaviour during reactor operation and caesium in the case of atmospheric radioactive release during an accident in a nuclear power plant. This paper focuses on the comparison of caesium and xenon thermal behaviour in polycrystalline uranium dioxide (UO2) pellets. Caesium-133 or xenon-136 stable isotopes were introduced in depleted UO2 samples by ion implantation at a maximum concentration of 0.08 at% at a depth of around 140 nm below the sample surface. Annealing under reducing atmosphere (Ar/H2 5 %) was performed at 1000 °C or 1600 °C, which corresponds respectively to a representative temperature during nuclear reactor operation (at the centre of the fuel pellets) and during an accident. The caesium migration in UO2 was investigated by Secondary Ion Mass Spectrometry and compared to the thermal behaviour of xenon in UO2 at 1600 °C. Transmission Electron Microscopy was performed in order to characterise UO2 microstructure before and after annealing. The results indicate that caesium has a different behaviour than xenon with which it is often compared for its release from the nuclear fuel. In particular, we highlight a difference between the growth kinetics of caesium and of xenon bubbles at 1600 °C which can be correlated to the availability of thermal vacancies in UO2 and to the different ability of Xe and Cs atoms for thermal resolution.

Using noise to stochastically enhance the resolution of charge coupled device based thermoreflectance imaging

Charge coupled device (CCD) based, frequency-domain thermoreflectance imaging can be used to characterize the thermophysical properties of solid-state materials, as well as electronic and optoelectronic devices. A four-bucket algorithm is used to obtain the amplitude and phase of the thermoreflectance signal, i.e. the relative change in reflectance of a sample in response to an induced thermal modulation. Prior experiments have shown that thermoreflectance signals smaller than the bit depth of the camera can be accurately measured; this enhanced resolution is posited to be due to stochastic resonance, in which measurement noise dithers the signal over multiple bit levels. Here, we develop an experimentally validated analytical and computational model of the quantization error imposed on the thermoreflectance measurement by the analog-to-digital conversion at the CCD camera and of stochastic resonance in this imaging system, examining how measurement noise, combined with averaging required by the imaging algorithm, can be used to maximize the thermal resolution. We demonstrate analytically and experimentally that noise is required to obtain accurate thermoreflectance measurements; in the absence of noise, the analog-to-digital conversion can lead to large errors in the measured thermoreflectance signal for experimentally reasonable signal levels. Using the model, we derive a close upper bound for the optimal noise amplitude of the thermoreflectance measurement system. Furthermore, we show that, by tuning the experimental parameters, stochastic resonance enhancement can be achieved for any noise level, enabling an order of magnitude or greater improvement in the thermal resolution of this key technique for thermophysical characterization of materials and devices.

Luminescent Nanothermometer Operating at Very High Temperature-Sensing up to 1000 K with Upconverting Nanoparticles (Yb3+/Tm3+).

Lanthanide-based luminescent nanothermometers play a crucial role in optical temperature determination. However, because of the strong thermal quenching of the luminescence, as well as the deterioration of their sensitivity and resolution with temperature elevation, they can operate in a relatively low-temperature range, usually from cryogenic to ≈800 K. In this work, we show how to overcome these limitations and monitor very high-temperature values, with high sensitivity (≈2.1% K–1) and good thermal resolution (≈1.4 K) at around 1000 K. As an optical probe of temperature, we chose upconverting Yb3+–Tm3+ codoped YVO4 nanoparticles. For ratiometric sensing in the low-temperature range, we used the relative intensities of the Tm3+ emissions associated with the 3F2,3 and 3H4 thermally coupled levels, that is, 3F2,3 → 3H6/3H4 → 3H6 (700/800 nm) band intensity ratio. In order to improve sensitivity and resolution in the high-temperature range, we used the 940/800 nm band intensity ratio of the nonthermally coupled levels of Yb3+ (2F5/2 → 2F7/2) and Tm3+ (3H4 → 3H6). These NIR bands are very intense, even at extreme temperature values, and their intensity ratio changes significantly, allowing accurate temperature sensing with high thermal and spatial resolutions. The results presented in this work may be particularly important for industrial applications, such as metallurgy, catalysis, high-temperature synthesis, materials processing and engineering, and so forth, which require rapid, contactless temperature monitoring at extreme conditions.

A fast compact model to simulate the heat exchanges in a bundle of electric wires

The challenges of improving wiring harness designs in automotive, aerospace or transport fields are safety, ecology, weight, and cost. To this end, harnesses require superior thermal management, which involves considering several thermal sources with an uncontrolled layout. To date, the primary methods employed for thermal resolution are based on commercial softwares with meshing capabilities because evaluating temperature evolution influenced by several sources in an uncontrolled wire layout is difficult. This study presents a faster alternative method in which an equation based on the Infinite Line Source (ILS) model is used to create a nodal network. A more efficient matrix mathematical application is adapted to solve faster the ILS model. The ILS model relies on a fully connected node network. The wire bundle is a complex system with a variable environment and an uncontrolled wire layout. This work improves and adapts a fast mathematical model to complex geometries. Thus, the adaptation required by the model involves many assumptions. This new approach was tested on 2-wire and 10-wire layouts and then compared with results obtained by using a commercial software: Ansys Fluent. Some gaps exist between these methods. We propose a corrective model that uses equivalent conductivity to solve that issue.

Pushing the Limit of Boltzmann Distribution in Cr3+-Doped CaHfO3 for Cryogenic Thermometry.

Luminescence Boltzmann thermometry is one of the most reliable techniques used to locally probe temperature in a contactless mode. However, to date, there is no report on cryogenic thermometers based on the highly sensitive and reliable Boltzmann-based 4T2 → 4A2/2E → 4A2 emission ratio of Cr3+. On the basis of structural information of the local HfO6 octahedral site we demonstrated the potential of the CaHfO3:Cr3+ system by combining deep theoretical and experimental investigation. The material exhibits simultaneous emission from both the 2E and 4T2 excited states, following the Boltzmann law in a cryogenic temperature range of 40-150 K. The promising thermometric performance corroborates the potential of CaHfO3:Cr3+ as a Boltzmann cryothermometer, being characterized by a high relative sensitivity (∼ 2%·K-1 at 40 K) and exceptional thermal resolution (0.045-0.77 K in the 40-150 K range). Moreover, by exploiting the flexibility of the 4T2-2E energy gap controlled by the crystal field of the local octahedral site, the design proposed herein could be expanded to develop new Cr3+-doped cryogenic thermometers.

Unprecedented switching endurance affords for high-resolution surface temperature mapping using a spin-crossover film

Temperature measurement at the nanoscale is of paramount importance in the fields of nanoscience and nanotechnology, and calls for the development of versatile, high-resolution thermometry techniques. Here, the working principle and quantitative performance of a cost-effective nanothermometer are experimentally demonstrated, using a molecular spin-crossover thin film as a surface temperature sensor, probed optically. We evidence highly reliable thermometric performance (diffraction-limited sub-µm spatial, µs temporal and 1 °C thermal resolution), which stems to a large extent from the unprecedented quality of the vacuum-deposited thin films of the molecular complex [Fe(HB(1,2,4-triazol-1-yl)3)2] used in this work, in terms of fabrication and switching endurance (>107 thermal cycles in ambient air). As such, our results not only afford for a fully-fledged nanothermometry method, but set also a forthcoming stage in spin-crossover research, which has awaited, since the visionary ideas of Olivier Kahn in the 90’s, a real-world, technological application.

Measurement of the molecular dipole moment using active microwave thermography (AMT)

A method for implementing Active Microwave Thermography (AMT) for use in determining molecular dipole moments is reported. Specifically, the experimental setup along with a mathematical model for determining the dipole moment for deionized water is reported herein. The thermal and spatial resolution of the camera is shown to be of utmost importance in providing statistical and regional relevance, respectfully, of the properties of the material being studied. Deionized water in particular was studied in order to provide foundational knowledge for the veracity of using AMT in determining dipole moment values and representative values are reported herein. In order to extend the technique to more localized regions and composite systems, a more complex model and upgraded hardware are required.

Growth and characterization of carbon nanotubes over CoFe2O4-MgO catalysts at different temperatures

A novel approach for synthesizing carbon nanotubes (CNTs) via chemical vapor deposition (CVD) technique has been elucidated. CNTs are catalytically grown on the CoFe2O4-MgO nano-catalyst at various growth temperatures (700 °C, 800 °C, 900 °C) to clarify the effect of temperature and to achieve the optimum CNT growth temperature. The structural properties of the catalyst and CNTs were studied with thermal analysis (TGA), X-ray diffraction (XRD), FTIR spectroscopy (IR), Raman spectroscopy, and High Resolution Transmission Electron Microscope (HRTEM). X-ray diffraction is used to study changes in the crystal structure of the CoFe2O4+MgO nano-catalyst. The obtained data ratifies the formation of MWCNTs over the nanocatalyst. The intensity of the main peak of CNTs decreases as well as the yield with increasing the growth temperature. The two main Raman modes, G band and the D band, can be identified in the spectra multi-walled carbon nanotubes .The lowest ID/IG ratio is obtained at the highest growth temperature. HRTEM images show that the growth temperature has a significant parameter on CNT quality. The average diameters of grown CNTs are 42, 32 and 16 nm for growth temperatures of 700 ̊C, 800 ̊C and 900 ̊C, respectively. The inner and outer diameters of MWCNTs become thinner and the quality of the tubes is enhanced with increasing the growth temperature. Additionally, it can be concluded from the temperature study that the balance between decomposition of acetylene gas and diffusion rate of manufactured carbon atoms in CoFe2O4-MgO catalyst nanoparticles occurs at the optimal growth temperature of 900 °C.

Securing IoT Space via Hardware Trojan Detection

Hardware Trojan (HT) is a malicious modification in the chip circuitry, which may lead to undesired chip function changing or sensitive information leaking once activated. As recently studied, HT has become one of the main threats for Internet-of-Things (IoT) security, and therefore, protecting IoT against the HT attack attracts growing attention from IoT researchers. In this article, we propose an HT detection technique which makes use of chip temporal thermal information and self-organizing map (SOM) neural network to automatically isolate the Trojan-infected chips with the Trojan-free ones, and meanwhile, confirm the Trojan location at the infected chips. The experimental results reveal that our method is effective. Specifically, for the Trust-hub benchmarks, it can detect HTs which increase only 0.02% power consumption of the original design and localize the Trojan positions precisely without any error. In addition, we demonstrate the advantages of our method over two existing HT detection methods, namely, the thermal and power map (TPM) and ring oscillator net (RON), and make a thorough discussion on how the thermal image resolution, chip technology, and clustering algorithm affect the Trojan detection results.

Photonic and Thermal Modelling of Microrings in Silicon, Diamond and GaN for Temperature Sensing.

Staying in control of delicate processes in the evermore emerging field of micro, nano and quantum-technologies requires suitable devices to measure temperature and temperature flows with high thermal and spatial resolution. In this work, we design optical microring resonators (ORRs) made of different materials (silicon, diamond and gallium nitride) and simulate their temperature behavior using several finite-element methods. We predict the resonance frequencies of the designed devices and their temperature-induced shift (16.8 pm K−1 for diamond, 68.2 pm K−1 for silicon and 30.4 pm K−1 for GaN). In addition, the influence of two-photon-absorption (TPA) and the associated self-heating on the accuracy of the temperature measurement is analysed. The results show that owing to the absence of intrinsic TPA-processes self-heating at resonance is less critical in diamond and GaN than in silicon, with the threshold intensity , and being the linear and quadratic absorption coefficients, respectively.

Applying Infrared Thermography to Soil Surface Temperature Monitoring: Case Study of a High-Resolution 48 h Survey in a Vineyard (Anadia, Portugal).

The soil surface albedo decreases with an increasing biochar application rate as a power decay function, but the net impact of biochar application on soil temperature dynamics remains to be clarified. The objective of this study was to assess the potential of infrared thermography (IRT) sensing by monitoring soil surface temperature (SST) with a high spatiotemporal and thermal resolution in a scalable agricultural application. We monitored soil surface temperature (SST) variations over a 48 h period for three treatments in a vineyard: bare soil (plot S), 100% biochar cover (plot B), and biochar-amended topsoil (plot SB). The SST of all plots was monitored at 30 min intervals with a tripod-mounted IR thermal camera. The soil temperature at 10 cm depth in the S and SB plots was monitored continuously with a 5 min resolution probe. Plot B had greater daily SST variations, reached a higher daily temperature peak relative to the other plots, and showed a faster rate of T increase during the day. However, on both days, the SST of plot B dipped below that of the control treatment (plot S) and biochar-amended soil (plot SB) from about 18:00 onward and throughout the night. The diurnal patterns/variations in the IRT-measured SSTs were closely related to those in the soil temperature at a 10 cm depth, confirming that biochar-amended soils showed lower thermal inertia than the unamended soil. The experiment provided interesting insights into SST variations at a local scale. The case study may be further developed using fully automated SST monitoring protocols at a larger scale for a range of environmental and agricultural applications.

Scanning thermal microscopy method for thermal conductivity measurement of a single SiO2 nanoparticle

Scanning thermal microscopy (SThM) with a thermal resolution of 100 nm can realize the thermal mapping of low-dimensional nanomaterials. We develop an improved quantitative SThM method for the thermal conductivity of a single SiO2 nanoparticle. The calibration of SThM is based on the output voltage as a function of the thermal conductivity of bulk samples. It is found that the thermal conductivity of a single SiO2 nanoparticle is 0.95 ± 0.08 W/mK at 300 K, which is lower than that of SiO2 bulk material. The effect of the contact thermal resistance between the probe and the nanoparticle on the measurement is clarified.

Thermal perturbation of NMR properties in small polar and non-polar molecules

Water is an important constituent in an abundant number of chemical systems; however, its presence complicates the analysis of in situ1H MAS NMR investigations due to water’s ease of solidification and vaporization, the large changes in mobility, affinity for hydrogen bonding interactions, etc., that are reflected by dramatic changes in temperature-dependent chemical shielding. To understand the evolution of the signatures of water and other small molecules in complex environments, this work explores the thermally-perturbed NMR properties of water in detail by in situ MAS NMR over a wide temperature range. Our results substantially extend the previously published temperature-dependent 1H and 17O chemical shifts, linewidths, and spin-lattice relaxation times over a much wider range of temperatures and with significantly enhanced thermal resolution. The following major results are obtained: Hydrogen bonding is clearly shown to weaken at elevated temperatures in both 1H and 17O spectra, reflected by an increase in chemical shielding. At low temperatures, transient tetrahedral domains of H-bonding networks are evidenced and the observation of the transition between solid ice and liquid is made with quantitative considerations to the phase change. The 1H chemical shift properties in other small polar and non-polar molecules have also been described over a range of temperatures, showing the dramatic effect hydrogen bonding perturbation on polar species. Gas phase species are observed and chemical exchange between gas and liquid phases is shown to play an important role on the observed NMR shifts. The results disclosed herein lay the foundation for a clear interpretation of complex systems during the increasingly popular in situ NMR characterization at elevated temperatures and pressures for studying chemical systems.

Increasing the speed of frequency-domain, homodyne thermoreflectance imaging

Charge coupled device (CCD)-based thermoreflectance imaging using a "4-bucket" lock-in imaging algorithm is a well-established, powerful method for obtaining high spatial and thermal resolution two-dimensional thermal maps of optoelectronic, electronic, and micro-electro-mechanical systems devices. However, the technique is relatively slow, limiting broad commercial adoption. In this work, we examine the underlying limit on the image acquisition speed using the conventional "4-bucket" algorithm and show that the straightforward extension to an n-bucket technique by faster sampling does not address the underlying statistical bias in the data analysis and hence does not reduce the image acquisition time. Instead, we develop a modified "enhanced n-bucket" algorithm that halves the image acquisition time for every doubling of the number of buckets. We derive detailed statistical models of the algorithms and confirm both the models and the resulting speed enhancement experimentally, resulting in a practical means of significantly enhancing the speed and utility of CCD-based frequency domain, homodyne thermoreflectance imaging.