High-purity Materials Research Articles

Hollow MoS2-Co3S4 heterostructures derived from ZIF-67 as saturable absorber for generating gigahertz repetition frequency mode-locked fiber lasers

In practical applications, high repetition frequency (HRF) lasers are widely used in the fields of material processing, laser communication and medical therapy. In this work, the passive harmonic mode-locked (HML) technique is used to realize a HRF fiber laser in GHz level, effectively overcoming the limitations of traditional fiber laser structures. In order to maximize the advantages of nanomaterials, we synthesized high-purity hollow MoS2-Co3S4 heterojunction materials by hydrothermal method. Then, a “sandwich” structure saturable absorber (SA) was prepared by photodeposition, and its nonlinear optical properties were investigated. The results demonstrate that the modulation depth and saturation intensity of MoS2-Co3S4 SA are 24 % and 4.53 MW/cm2, respectively. The fiber laser based on MoS2-Co3S4 SA could realize the fundamental frequency mode-locked at 23.11 MHz with a pulse width of 0.99 ps. In addition, various high-order HML are realized with this laser system capable of stable operation for extended periods up to the highest repetition frequency of 1.109 GHz achieved thus far in this field. This research provides strong support to promote the development and commercialization of HRF fiber lasers.

Optimal operation guidelines for direct recovery of high-purity precursor from spent lithium-ion batteries: hybrid operation model of population balance equation and data-driven classifier

The direct resynthesis of precursor from spent lithium-ion batteries (LIBs) via co-precipitation is a crucial step in closed-loop cathode recycling systems. However, design and operation strategies for producing high-purity precursors have not been comprehensively explored or optimized. Herein, we propose the optimization of co-precipitation during the recovery of spent LIBs to achieve impurity-free precursor resynthesis. By incorporating the thermodynamic equilibrium model of the leaching solution of spent LIBs into a population balance equation (PBE) model, we identified the operating ranges that prevented the formation of impurities. Bayesian optimization was employed within the screened operating ranges to determine the optimal operating conditions for minimizing both operation time and maximum particle size. This optimization was performed for both unseeded batch and semi-batch systems. The results demonstrate that the selection of an optimal semi-batch operation can reduce the operation time by 23.33% and increase the particle size by 54.75%, owing to the high nucleation and particle growth rate during the initial time step. By employing an optimization approach based on the PBE model, this study provides detailed operational guidelines for batch and semi-batch co-precipitation, enabling the production of high-purity precursor materials from spent LIBs, while minimizing both operating time and maximum particle size.

Preparation of High-Purity Nano-Iron Phosphate from Titanium-Extraction Tailings by Co-Leaching Synergetic Ultrasonic-Enhanced Precipitation Process.

The preparation of high-performance electrode materials from metallurgical solid waste is an effective strategy to address current energy and environmental challenges. This study utilizes a mixed acid leaching and ultrasound-assisted precipitation process to extract valuable metallic iron from titanium-extraction tailings (TET) to produce high-purity nano-FePO4 electrode material precursors with unique crystal structures. A leaching efficiency of 95.2% for Fe was attained by using the optimized process parameters, which included a mixed acid concentration of 4 mol/L, a liquid-to-solid ratio of 4:1, and a leaching temperature of 70 °C for 1 h. The optimal precipitation conditions were a pH of 2.0, a temperature of 60 °C, an aging time of 30 min, and a stirring speed of 600 rpm, resulting in FePO4 purity up to 99.6% and fine particle size. Thermodynamic calculations, combined with various characterizations, elucidated the leaching and precipitation mechanisms, highlighting the synergistic effect of phosphoric acid and hydrochloric acid in enhancing the leaching reaction. The thermogravimetric analysis indicated that the decomposition of residual ammonium chloride impurities requires calcination above 360 °C. This research not only provides new insights into the high-value, clean utilization of metallurgical solid waste but also supports sustainable resource recovery and environmental protection by transforming waste into valuable products.

Influence of green solvents on the recovery of cathode active materials from electrode scraps: A comparative study

Direct recycling of cathode materials from spent Li-ion batteries (LIBs) or electrode scraps necessitates the efficient recovery of active materials from the aluminum foil. The variability in electrode types and recovery processes across previous studies complicates the comparative assessment of the recovery performance of green solvents. In this study, we evaluated the performance of three green solvents—triethyl phosphate (TEP), dihydrolevoglucosenone (Cyrene), and propylene carbonate (PC)—in recovering valuable active materials from industrial-grade cathode scraps. Using ultrasonication, we developed a standardized, energy-efficient recovery process that eliminated the need for conventional stirring and achieved complete cathode delamination from the aluminum foil. Furthermore, we successfully recovered the used green solvents after the process, ensuring their reuse and supporting a circular economy. The recovered materials retained their original morphology, chemical composition, and crystalline structure; however, the presence of surface impurities varied significantly depending on the green solvent used. These impurities had a considerable impact on the electrochemical performance of the recovered materials. TEP and PC yielded high-purity active materials and aluminum foils suitable for reuse or direct recycling, while Cyrene resulted in substantial residues of PVDF/solvent, requiring additional post-processing. Additionally, the recyclability of these green solvents was influenced by their solubility power for PVDF. This study provides valuable insights into the green solvent-based recycling process, laying the groundwork for future sustainable practices in LIB recycling.

Effect of pH, acid catalyst, and aging time on pore characteristics of dried silica xerogel

ABSTRACT This study investigates the impact of pH and ageing time on silica xerogel pore characteristics prepared via the sol-gel method. Using tetraethyl orthosilicate (TEOS) and HCl as a precursor and acid catalyst, respectively, tailored pore size distributions in the meso- and macroporous range were achieved. Field emission scanning electron microscopy (FESEM) revealed interconnected porous structures, with pore size increasing linearly with ageing time. pH variation yielded average pore sizes ranging from ~ 31 nm (mesoporous) to ~ 59 nm (macroporous). Higher acid catalyst concentration accelerated gelation, resulting in smaller pores. Elemental analysis confirmed silicon and oxygen as primary constituents, while X-ray diffraction (XRD) confirmed amorphous nature. Sol-gel synthesis provides versatile control over microstructure and properties, enabling customisation for applications such as liquid filters, batteries, sensors, and bio-scaffolds. It offers a cost-effective, direct synthesis of high-purity multi-component materials, emphasising its potential for functional material development.

Methodological Review of Methods and Technology for Utilization of Spent Carbon Cathode in Aluminum Electrolysis

Spent carbon cathode (SCC) is one of the major hazardous solid wastes generated during the overhaul of electrolysis cells in the aluminum production process. SCC is not only rich in carbon resources but also contains soluble fluoride and cyanide, which gives it both recycling value and significant leaching toxicity. In this study, we explore the properties, emissions, and disposal strategies for SCC. Pyrometallurgy involves processes such as vacuum distillation, molten salt roasting, and high-temperature roasting. Hydrometallurgy describes various methods used to separate valuable components from leachate and prepare products. Collaborative disposal plays a positive role in treating SCC alongside other solid wastes. High-value utilization provides an approach to make full use of high-purity carbon-based materials. Finally, we analyze and summarize future prospects for the disposal of SCC. This study aims to contribute to the large-scale treatment and resource utilization of SCC while promoting circular economy principles and green development initiatives.

Preparation of high-purity magnesia spinel refractory raw materials with spinel-wrapped periclase structures using bischofite from salt lake

A large amount of bischofite is produced in the process of potassium extraction from salt lake, which seriously affects the ionic balance of brine system. In this study, a high-purity magnesia spinel refractory raw material with a spinel-wrapped periclase structure was directly prepared using bischofite by a precipitation-sintering approach. A coupling process of one-time crude magnesium chloride solution recrystallization and three-time precipitates washing was employed to remove crucial impurities (sodium, potassium, boron, etc.) and prepare the magnesium hydroxide precipitates with a high purity of 99.37 %. The lightly calcined magnesia gained from the high-purity magnesium hydroxide precipitates and white corundum were then employed for preparing the refractory raw materials. The effects of particle size and dosage of white corundum on the phase distribution, microstructure, and physical properties of the materials were thoroughly studied. The results illustrated that the prepared refractory raw materials were mainly composed of periclase and spinel phases, showing a distinct spinel-wrapped periclase structure that could enhance the physical properties. Therefore, the prepared refractory raw materials showed a high bulk density of 3.46 g·cm−3, a low apparent porosity of 2.46 %, and a linear shrinkage rate of 12.33 %, under the optimum conditions of white corundum particle size of 3.00 μm and alumina/magnesia mass ratio of 3:10.

Formation of Fully Stoichiometric, Oxidation-State Pure Neptunium and Plutonium Dioxides from Molecular Precursors.

Amidate-based ligands (N-(tert-butyl)isobutyramide, ITA) bind κ2 to form homoleptic, 8-coordinate complexes with tetravalent 237Np (Np(ITA)4, 1-Np) and 242Pu (Pu(ITA)4, 1-Pu). These compounds complete an isostructural series from Th, U-Pu and allow for the direct comparison between many of the early actinides with stable tetravalent oxidation states by nuclear magnetic resonance (NMR) spectroscopy and single crystal X-ray diffraction (SCXRD). The molecular precursors are subjected to controlled thermolysis under mild conditions with the exclusion of exogenous air and moisture, facilitating the removal of the volatile organic ligands and ligand byproducts. The preformed metal-oxygen bond in the precursor, as well as the metal oxidation state, are maintained through the decomposition, forming fully stoichiometric, oxidation-state pure NpO2 and PuO2. Powder X-ray diffraction (PXRD), scanning transmission electron microscopy (STEM), and energy dispersive X-ray spectroscopy (EDS) elemental mapping supported the evaluation of these high-purity materials. This chemistry is applicable to a wide range of metals, including actinides, with accessible tetravalent oxidation states, and provides a consistent route to analytical standards of importance to the field of nuclear nonproliferation, forensics, and fundamental studies.

Structural, optical, and detailed photoluminescence characterization of solvothermal synthesized V2O5, ZrO2, and ZrV2O7 nanoparticles

It is indisputable that structural defects are pivotal in modulating the properties of materials. This study provides a comprehensive analysis of the structural, optical, and photoluminescence characteristics of V2O5, ZrO2, and ZrV2O7.These materials were chosen for their potential applications in various key technological domains. The XRD results revealed the formation of high-purity materials with no secondary phases. It was determined that ZrO2exhibits the most significant quantity of structural defects among the investigated materials. The variation in crystallite size as determined by XRD aligns with the variation in grain size observed through Scanning Electron Microscopy (SEM).The optical band gaps of V2O5, ZrO2, and ZrV2O7 were determined to be 2.27 eV, 5.19 eV, and 2.38 eV, respectively. X-ray Photoelectron Spectroscopy (XPS) analysis indicated the presence of constituent elements without any contaminants. A detailed examination of the photoluminescence (PL) emission characteristics in both UV–Vis and near-infrared (NIR) regions for all materials presented broad visible luminescence in the [450–650] nm range under UV excitation. Structural defects are crucial in determining the physico-chemical properties of materials. This is thoroughly examined for V2O5, ZrO2, and ZrV2O7.The infrared (IR) emission spectra, introduced for the first time in this study for V2O5, ZrO2, and ZrV2O7, are employed to elucidate the localization of structural defects within the band gap.

Identification by HSQC and quantification by qHNMR innovate pharmaceutical amino acid analysis

This study introduces a new NMR-based methodology for identification (ID) and quantification (purity, strength) assays of widely used amino acids. A detailed analysis of four amino acids and their available salts was performed with both a high-field (600 MHz) and a benchtop (60 MHz) NMR instrument. To assess sensitivity constraints, samples for 1H NMR analysis were initially prepared using only 10 mg of analyte and 1 mg of maleic acid (MA) as an internal calibrant (IC) and secondary chemical shift reference. The characteristic dispersion of the peak patterns indicating the presence or absence of a counterion (mostly chloride) was conserved at both high and low-field strength instruments, showing that the underlying NMR spectroscopic parameters, i.e., chemical shifts and coupling constants, are independent of the magnetic field strength. However, as the verbal descriptions of 1H NMR spectra are challenging in the context of reference materials and pharmaceutical monographs, an alternative method for the identification (ID) of amino acids is proposed that uses 13C NMR patterns from multiplicity-edited HSQC (ed-HSQC), which are both compound-specific and straightforward to document. For ed-HSQC measurements, the sample amount was increased to 30 mg of the analyte and several acquisition parameters were tested, including t1 increments used in the pulse program, number of scans, and repetition time. Excellent congruence with deviations <0.1 ppm was achieved for the 13C chemical shifts from 1D 13C NMR spectra (150 MHz) vs. those extracted from ed-HSQC (15 MHz traces). Finally, all samples of amino acid candidate reference materials were quantified by 1H qNMR (abs-qHNMR) at both 600 and 60 MHz. At high field, both IC and relative quantitations were performed, however, with the low-field instrument, only the IC method was used. The results showed that the analyzed reference material candidates were generally highly pure compounds. To achieve adequately low levels of uncertainty for such high-purity materials, the sample amounts were increased to 100 mg of analytes and 10 mg of the IC and replicates were analyzed for selected amino acids.

PHASE TRANSFORMATIONS OF SILICA IN VEIN QUARTZ OF THE TULAKUL DEPOSIT

The results of the study of vein quartz from the Tulakul deposit, which are a potential source of high-purity quartz raw materials, are presented. A brief description of the most promising deposits of vein quartz is given. Thermal studies of vein quartz of the Tulakul birthplace have been carried out. Stability temperatures and phase transition temperatures are established.

Deep separation of Bi and Pb from leaching solution of molybdenite-bismuthinite mixed ore by solvent extraction method

A novel lead and bismuth deep-separation extraction-stripping system is proposed for the leaching solution of molybdenite-bismuthinite mixed ore, which creates perfect conditions for the preparation of high-purity bismuth-based materials. Based on the activity and ionic strength theories, the forms of bismuth and lead present in the solution were predicted according to the thermodynamic models of different systems. The results showed that N235 + 2-octyl alcohol + kerosene formed a synergistic extraction system, which realized the synergistic extraction of bismuth, iron and lead with solvent extraction ratios of 98.74 %, 9.34 % and 46.24 %, respectively. In addition, the FTIR spectra of bismuth and lead before and after extraction were recorded to better understand the extraction and vaporization mechanisms. When stripping with water, the stripping ratios of iron and lead were 99.87 % and 99.74 %, respectively, while the loss ratio of bismuth was only 0.01 %. In conclusion, this study will be a key link in the comprehensive recycling process of bismuth resources.

Expressed Protein Ligation in Flow.

The development of a flow chemistry platform for the generation of modified protein targets via expressed protein ligation (EPL) is described. The flow EPL platform enables efficient ligation reactions with high recoveries of target protein products and superior reaction rates compared to corresponding batch processes. The utility of the flow EPL technology was first demonstrated through the semisynthesis of the tick-derived chemokine-binding protein ACA-01 containing two tyrosine sulfate modifications. Full-length, sulfated ACA-01 could be efficiently assembled by ligating a recombinantly expressed C-terminal protein fragment and a synthetic sulfopeptide thioester in flow. Following folding, the semisynthetic sulfoprotein was shown to exhibit potent binding to a variety of pro-inflammatory chemokines. In a second modified protein target, we employed an in-line flow EPL-photodesulfurization strategy to generate both unmodified and phosphorylated forms of human β-synuclein by fusing a recombinant protein thioester, generated through cleavage of an intein fusion protein, and a synthetic (phospho)peptide. The semisynthetic proteins were assembled in 90 min in flow, a significant improvement over corresponding batch protein assembly, and enabled access to tens of milligrams of high purity material. Flow EPL has the potential to serve as a robust technology to streamline access to homogeneously modified proteins for a variety of applications in both academia, as well as in the pharmaceutical and biotechnology sector.

High-purity La3-xTe4 Material with Superior Thermoelectric Performance Synthesized via Te-vapor Transport and Solid-State Diffusion.

La3-xTe4 is a very promising high-temperature candidate applied in next-generation Radioisotope Thermoelectric Generators (RTGs). Conventional synthesis of such materials is based on the mechanochemical method, which makes the sample difficult to purify due to the high-energy ball milling. In this report, a novel synthetic method is developed, which utilizes Te-vapor transport and solid-phase diffusion to efficiently produce the RE3-xTe4 phases (RE = La, Ce, Pr, Nd). Notably, this method obviates the requirement for high-energy ball-milling instruments, conventionally indispensable in the mechanochemical syntheses. For as-synthesized La2.74Te4 material, a high figure of merit of 1.5 is achieved at 1073K, owning to the reduced electronic thermal conductivity with metal impurities well eliminated.

Loading of bacterial cellulose dressing with frutalin, a lectin from Artocarpus incisa L.

Bacterial cellulose (BC), produced by bacterial fermentation, is a high-purity material. BC can be oxidized (BCOXI), providing aldehyde groups for covalent bonds with drugs. Frutalin (FTL) is a lectin capable of modulating cell proliferation and remodeling, which accelerates wound healing. This study aimed to develop an FTL-incorporated dressing based on BC, and to evaluate its physicochemical properties and biological activity in vitro. An experimental design was employed to maximize FTL loading yield onto the BC and BCOXI, where independent variables were FTL concentration, temperature and immobilization time. BCOXI-FTL 1 (44.96 % ± 1.34) had the highest incorporation yield (IY) at the experimental conditions: 6 h, 5 °C, 20 μg mL−1. The second highest yield was BCOXI-FTL 6 (23.28 % ± 1.43) using 24 h, 5 °C, 100 μg mL−1. Similarly, the same reaction parameters provided higher immobilization yields for native bacterial cellulose: BC-FTL 6 (16.91 % ± 1.05) and BC-FTL 1 (21.71 % ± 1.57). Purified FTL displayed no cytotoxicity to fibroblast cells (<50 μg mL−1 concentration) during 24 h. Furthermore, BCOXI-FTL and BC-FTL were non-cytotoxic during 24 h and stimulated fibroblast migration. BCOXI-FTL demonstrated neutrophil activation in vitro similar to FTL. These promising results indicate that the bacterial cellulose matrices containing FTL at low concentrations, could be used as an innovative biomaterial for developing wound dressings.

Study on structural luminescence characteristics of a new Dy3+ activated white luminescent phosphor

A series of Y1-xGa1.5Al1.5(BO3)4: xDy3+ were prepared by a high-temperature solid-phase method. Rietveld refinement characterized the phase purity of the samples obtained, suggesting that the prepared phosphors were high-purity material phases. After synthesizing high purity phase, the luminescence mechanism is revealed, under 348 nm excitation in Y0.97Ga1.5Al1.5(BO3)4: 0.03Dy3+ phosphor, the samples have two characteristic peaks at 480 nm and 572 nm and the burst mechanism is dipole-dipole mechanism. The mechanism of luminescence and quenching is analyzed by analyzing the band structure, and the relationship between the band gap and thermal stability is explained. The prepared phosphor has a quantum efficiency of 51.22 % and exhibits intense white light emission. The thermal stability of the samples was characterized, and the phosphor could still achieve 88.20 % thermal stability at 423 K. Finally, the chromaticity coordinates of the Y1-xGa1.5Al1.5(BO3)4: xDy3+ phosphors all fall in the white-light region, with the color coordinates around (0.33, 0.33). By combining with the 365 nm chip and Y0.97Ga1.5Al1.5(BO3)4: 0.03Dy3+ phosphor, the W-led device with Ra = 68.3 and CCT = 6670 K was synthesized. Therefore, the Y0.97Ga1.5Al1.5(BO3)4: 0.03Dy3+ phosphor prepared in this experiment has some application value in the field of solid-state lighting.

The Simpler the Better: Ultrafast Air-Plasma Synthesis of 3D Crosslinked Graphene Nanoscroll-Nanosheet Aerogels at Room Temperature for Capacitive Deionization.

Graphene nanoscroll (GNS) is an important 1D tubular form of graphene-derivative materials, which has garnered widely attention. However, conventional fabrication methods commonly suffer from complex processing and time-consuming. Herein, with graphene oxide (GO) as a precursor, the study puts forward a facile air-plasma synthesis strategy to fabricate 3D graphene nanoscroll-nanosheet aerogels (GSSA). It is demonstrated that without using any chemical additives, a highly efficient reduction-exfoliation-scrolling process can be achieved all-in-one at room temperature within 1 s. The GNSs "grew" from 2D graphene sheets and firmly cross-linked them together, and they not only provide a shortcut path for electron transport but also act as intrinsic spacers to prevent restacking of graphene sheets. When using as an electrode material for capacitive deionization (CDI), GSSA exhibits excellent merits of salt-removal performance. These findings open a new pathway to large-scale synthesis of high-quality and high-purity GNS-based materials with promising applications in CDI and beyond.

Manufacturing of High Purity Cr2AlC MAX Phase Material and Its Characterization

AbstractPresent study discusses about a technique for producing high-purity Cr2AlC MAX phase materials and gaining insight into their thermal behavior for high-temperature applications. The research conducted involved synthesizing a pure layered ternary carbide Cr2AlC MAX phase material by mixing powders of Chromium, Aluminum, and Carbon and then subjecting them to two-step pressureless sintering process in argon atmosphere. First step involves the annealing of ball-milled mixture at 750 °C for 2 h followed by the second step in which the annealed mixture is subjected to heat-treatment at 1350 °C for 2 h. Analysis using XRD and Raman techniques revealed that the synthesized product consists of Cr2AlC phase, without any impurities. SEM studies confirmed that the Cr2AlC had a layered topography, while EPMA analysis indicated that the atomic percentage of Cr, Al, and C was consistent with the XRD phase analysis. XPS investigations confirmed the presence of Cr-C bonds representing Mn+1Xn of the MAX phase material. TG-DSC results showed an approximately 2% increase in weight. The Cr2AlC phase exhibited an endothermic pattern below 725 °C, an exothermic pattern above it, and did not decompose up to 1400 °C in vacuum environment. High-temperature XRD analysis at various temperatures also confirmed no formation of Al2O3 or CrO impurity compounds.

Superconducting heat switch made from a transition metal

High sensitivity astrophysics detectors such as transition edge sensor (TES) bolometers or microwave kinetic inductance detectors (MKIDs) require sub-Kelvin operating temperatures for observing low energy photons in infrared and x-ray ranges. Continuous Adiabatic Demagnetization Refrigerators (CADRs) are presently the state of the art solution for providing cooling to these detectors below as low as 50 mK. The CADR shuttles heat through a series of paramagnetic stages from a 50 mK heat load to a 1-6+ K heat sink, using heat switches between stages to “switch” between thermal isolation and thermal communication. A superconducting heat switch is required between the two lowest temperature stages and is considered a critical system component as it determines recycling time and cooling power of the lowest temperature stage. A figure of merit for this technology is the “switching ratio”: the ratio of the thermal conductivity in the on and off states. This ratio is a function of temperature, fixed material properties, and the purity and anneal state of the metal. Lead is most commonly used in this type of thermal switch because it is readily available as a high purity material. However, an analysis of the material properties of other superconducting material suggests that transition metals such as tantalum and vanadium could achieve a significantly higher switching ratio – as high as 5-10 times that of lead. Increasing the switching ratio could allow for shorter recycling times and reduce the parasitic load on the lowest temperature stage. This work discusses relevant properties of various potential heat switch material and compares predicted performance in the CADR against the presently used lead switch.

Advancing respirable coal mine dust source apportionment: a preliminary laboratory exploration of optical microscopy as a novel monitoring tool

Exposure to respirable coal mine dust (RCMD) can cause chronic and debilitating lung diseases. Real-time monitoring capabilities are sought which can enable a better understanding of dust components and sources. In many underground mines, RCMD includes three primary components which can be loosely associated with three major dust sources: coal dust from the coal seam itself, silicates from the surrounding rock strata, and carbonates from the inert ‘rock dust’ products that are applied to mitigate explosion hazards. A monitor which can reliably partition RCMD between these three components could thus allow source apportionment. And tracking silicates, specifically, could be valuable since the most serious health risks are typically associated with this component-particularly if abundant in crystalline silica. Envisioning a monitoring concept based on field microscopy, and following up on prior research using polarized light, the aim of the current study was to build and test a model to classify respirable-sized particles as either coal, silicates, or carbonates. For model development, composite dust samples were generated in the laboratory by successively depositing dust from high-purity materials onto a sticky transparent substrate, and imaging after each deposition event such that the identity of each particle was known a priori. Model testing followed a similar approach, except that real geologic materials were used as the source for each dust component. Results showed that the model had an overall accuracy of 86.5%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$86.5\\%$$\\end{document}, indicating that a field-microscopy based monitor could support RCMD source apportionment and silicates tracking in some coal mines.