Presence Of LIF Research Articles

Synergistic Effects of Bisalt Additives in High-Voltage Rechargeable Lithium Batteries.

The stability of high-energy-density lithium metal batteries (LMBs) heavily relies on the composition of the solid electrolyte interphase (SEI) formed on lithium metal anodes. In this study, the inorganic-rich SEI layer was achieved by incorporating bisalts additives into carbonate-based electrolytes. Within this SEI layer, the presence of LiF, polythionate, and Li3N was observed, generated by combining 1.0 м lithium bis(trifluoromethanesulfonyl)imide in ethylene carbonate: ethyl methyl carbonate:dimethyl carbonate in a 1 : 1 : 1 volume ratio, with the addition of 2 wt% lithium difluorophosphate and 2 wt% lithium difluoro(oxalato)borate additives (EL-DO). Furthermore, this formulation effectively mitigated corrosion of aluminum current collectors. EL-DO exhibited outstanding performance, including an average coulombic efficiency of 98.2 % in Li||Cu cells and a stable discharge capacity of approximately 162 mAh g-1 after 200 cycles in a Li||LiNi0.8Co0.1Mn0.1O2 (NCM811) configuration. Moreover, EL-DO displayed the potential to enhance the performance not only of LMBs but also of lithium-ion batteries. In the case of Gr||NCM811 cell using EL-DO, it consistently maintained high discharge capacities, even achieving around 135 mAh g-1 after the 100th cycle, surpassing the performance of other electrolytes. This study underscores the synergistic impact of bisalts additives in elevating the performance of lithium batteries.

Directing Fluorinated Solid Electrolyte Interphase by Solubilizing Crystal Lithium Fluoride in Aprotic Electrolyte for Lithium Metal Batteries

AbstractLithium fluoride (LiF) facilitates robust and fast‐ion‐transport solid electrolyte interphase (SEI) in lithium metal batteries. Fluorinated solvents/salts are ubiquitously employed to introduce LiF into SEI through electrochemical decomposition, but this approach is usually at the expense of their continuous consumption. A direct approach to fluorinate SEI that employs crystal LiF is limited by its poor solubility in the current battery electrolyte formulation. Dissolving crystal LiF in high‐dielectric‐constant solvents, like ethylene carbonate (EC) is nearly neglected. Herein, the feasibility of directly fluorinating SEI by the addition of crystal LiF in aprotic electrolyte with the assistance of EC is verified, and its mechanisms in fluorination of SEI and anti‐acidification of electrolyte are explored. The dissolved LiF is encapsulated by solvent‐/salt‐derived organic skins to promote the fluorinated SEI. Meanwhile, the presence of LiF in electrolyte alters hazardous thermodynamic equilibrium, suppressing the production of acid species to mitigate electrolyte acidification and SEI degradation. Such collective benefits yield a capacity retention ratio of ≈88% after 150 cycles at a high areal capacity (4.5 mAh cm−2) in Li||NCM622 cells. This facile and effective fluorination of SEI contributes to an in‐depth understanding of SEI formation and rational design of well‐performing lithium metal batteries.

Lithium Fluoride Embedded Prelithiated Graphite Interface Layer Enables Stable All‐solid‐state Lithium Ion Batteries

AbstractAll‐solid‐state rechargeable batteries are regarded as one of the most promising next‐generation energy storage devices, while their cycling stability is still a great challenge due to the nonuniform lithium ion transportation and the loss of active lithium during cycling. Herein, a LiF embedded prelithiated graphite interface layer is designed and inserted between Li6PS5Cl solid electrolyte layer and graphite anode layer. The presence of LiF, C−F bonds and prelithiated graphite in this unique interface layer can facilitate uniform lithium ion migration and compensate the loss of active lithium, thus significantly improving the cyclic performances of both monopolar and bipolar all‐solid‐state lithium ion batteries. After 100 cycles at 0.1 C, the capacity retention increases from 58 % to 78.5 % for the monopolar LiNi0.5Co0.2Mn0.3O2|Li6PS5Cl|LiF@2.5 %Li@G|graphite all‐solid‐state lithium ion battery. Besides, the bipolar all‐solid‐state lithium ion batteries show a high discharge plateau of ~7.6 V with a capacity retention of 60.2 % after 190 cycles at 0.1 C. This work demonstrates the effectiveness of LiF embedded prelithiated graphite interface layer for improving the electrochemical performances of all‐solid‐state lithium ion batteries.

Interfacial modification of LiF-incorporated MnO2 mixed P3HT:PC60BM-based organic photoactive layer

Interfacial charge transfer in the photoactive layer plays an essential role in determining device performance in organic photovoltaics. Recently, numerous studies have reported interfacial modification by incorporating inorganic nanostructures into the bulk heterojunction photoactive layer, with the aim of increasing the interfacial area to improve charge transfer efficiency. However, there is limited understanding of in-depth defect analysis and the charge transfer mechanism. In this report, different weight percentages of LiF were incorporated into MnO2 (LMO) mixed photoactive layers. The robust relationship between charge transport dynamics and defect density of the device in the presence of LiF is elucidated. The results show that the photoactive layer with 10.0 wt% LMO exhibited a 41% improvement in device performance compared to that of pristine P3HT:PC60BM, with a power conversion efficiency (PCE), open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF) of 2.43%, 0.52 V, 9.77 mA/cm2, and 0.48, respectively. The photovoltaic performance improvement can be attributed to (i) the efficient charge transfer properties in the 10.0 wt% LMO, as evidenced by the suppressed S 2p and intensified O=C in X-ray photoelectron spectroscopy (XPS) analysis, and (ii) the low carrier recombination due to the success of LiF as a surface passivation agent, as indicated by 53% photoluminescence quenching. This report provides a fundamental understanding and offers an alternative solution for designing high-efficiency organic photovoltaics in the near future.

Boosting capacitive deionization of monovalent and hardness ions using Ti3C2Tx MXene as an intercalation-type pseudocapacitive electrode

MXene is a promising electrode material for energy storage and capacitive deionization (CDI) applications. In this study, a viable approach was developed to synthesize Ti3C2Tx at different LiF/Ti3AlC2 mass ratios of 0.5 – 2.0 for the enhanced CDI to remove different cations including hardness (Mg2+ and Ca2+), salt (Na+), and ammonium (NH4+) ions. The Ti3C2Tx was fabricated using the hydrothermal method in the presence of LiF and HCl. The optimal mass ratio of 1.0 results in the formation of hydrophilic and well-delaminated nanosheets. Moreover, the Ti3C2Tx-1 shows an interconnected meso-macroporous structure with a specific surface area of 48.4 m2/g. The Ti3C2Tx MXene exhibits an exceptional electrochemical performance with a specific capacitance of 540 F g−1 at 5 mV s−1 in the presence of 1 M H2SO4. A remarkable desalination performance with the salt adsorption capacity (SAC) of 39.7 mg g−1 is obtained. The well-suited lattice fringe and inter-connected porous structure of the Ti3C2Tx nanosheets accelerate the electron transport, which can maintain the cyclability over 50 cycles. This enhanced electron transport leads to an improved deionization capability during the CDI process. Moreover, the Ti3C2Tx-based CDI device exhibits prominent selectivity for hardness ions with SACs of 43.8 and 39.5 mg g−1 for Ca2+ and Mg2+, respectively. The SAC of NH4+ can also be up of 39.7 mg g−1 at 1000 mg L−1. Results in this study clearly elaborate the promising potentiality for the utilization of MXene as an effective electrode material for desalination and other electrochemical applications.

Impact of the Stimulation and Inhibition of NAD<sup>+</sup> Biosynthesis on the Maintenance of Pluripotency in Mouse Embryonic Stem Cells

Nicotinamide adenine dinucleotide (NAD+) plays a key role in cellular metabolism and signaling. In recent years, evidence has accumulated that NAD+-dependent processes are involved in the regulation of pluripotency and differentiation of mammalian embryonic stem cells. The major means to maintain NAD+ levels in mammalian cells is through its biosynthesis from various forms of vitamin B3. In this study, we examined how stimulation and inhibition of NAD+ biosynthesis affect the maintenance of the pluripotency of mouse embryonic stem cells E14 Tg2a (E14 cells). The pluripotency status of E14 cells was assessed by immunocytochemical and immunoblotting analysis using antibodies to the pluripotency factor Oct4, as well as by staining for alkaline phosphatase. Using NMR spectroscopy, we have found that the concentration of NAD+ in pluripotent E14 cells cultured in the presence of LIF is about 4 nmol/mg, and it remains unchanged after induction of differentiation with retinoic acid. We have also demonstrated that pharmacological stimulation of NAD+ biosynthesis by nicotinamide riboside increases the level of intracellular NAD+ by 20%, but it does not affect the maintenance of pluripotency in E14 cells. Moreover, under conditions of critical depletion of NAD+ pool by Nampt inhibition with FK866 E14 cells maintained pluripotency, though the expression level of Oct4 was decreased.

Highly Reactive Palladium-Catalyzed and Acetonitrile-Mediated Three-Component Reactions for Arylsulfone Synthesis.

Arylsulfone groups play an important role in the synthesis of functionalized molecules. The acetonitrile-mediated three-component reactions for arylsulfone synthesis were developed in the presence of a 0.00025 mol % palladium catalyst. Arylboronic acids reacted with potassium metabisulfite (K2S2O5) and benzyl bromide in the presence of LiF and a very low concentration of PdCl2 in acetonitrile solvents to produce the corresponding benzyl arylsulfones in moderate to good yields. Various arylboronic acids reacted with K2S2O5 and carbon electrophiles to produce the desired arylsulfones under the optimized conditions. It was proposed that acetonitrile accelerated the generation of aryl anion species from the reaction of arylboronic acids and LiF.

Co2+:MgAl2O4 saturable absorber transparent ceramics fabricated by high-pressure spark plasma sintering

Single-stage processing of high-quality transparent functional polycrystalline ceramics is desirable but challenging. In the present work, spark plasma sintering (SPS) was employed for fabrication of Co2+:MgAl2O4 saturable absorbers for laser passive Q-switching. Densification of commercial MgAl2O4 powders, doped via co-precipitation, was carried out by conventional SPS and high-pressure SPS (HPSPS) under pressures of 60 and 400 MPa, respectively. The presence of LiF, a common sintering additive, was detrimental to optical properties as it promoted reaction of cobalt with sulfur impurities and the formation of Co9S8 inclusions. Densification by HPSPS without LiF allowed to obtain highly transparent Co2+:MgAl2O4. The optical properties of samples, with doping concentrations varying between 0.01 and 0.1 at.% Co2+, were assessed and saturable absorption was demonstrated at ~1.5 µm wavelength, exhibiting ground-state (σgs) and excited (σes) cross-sections of 3.5×10-19 and 0.8×10-19 cm2, respectively. Thus, it was established that HPSPS is an effective method to fabricate transparent Co2+:MgAl2O4 ceramics.

Interaction of Cysteine with Li+ and LiF in the Presence of (H2O) n (n = 0-6) Clusters.

The interaction between cysteine with Li+ and LiF in the microcosmic water environment was investigated to elucidate how ions interact with amino acids and the cation–anion correlation effect involved. The structures of Cys·Li+(H2O)n and Cys·LiF(H2O)n (n = 0–6) were characterized using ab initio calculations. Our studies show that the water preferentially interacts with Li+/LiF. In Cys·Li+(H2O)0–6, Li+ interacts with amino nitrogen, carbonyl oxygen, and hydrophobic sulfur of Cys to form a tridentate mode, whereas in Cys·LiF(H2O)n, Li+ and F– work in cooperation and interact with carbonyl oxygen and hydroxyl hydrogen of Cys to form a bidentate type. The neutral and zwitterionic forms are essentially isoenergetic when the water number reaches three in the presence of Li+, whereas this occurs at four water molecules in the presence of LiF. Further research revealed that the interaction between Li+/LiF and Cys was mainly electrostatic, followed by dispersion, and the weakest interaction occurs at the transition from the neutral form to zwitterionic form. Natural population analysis charge analyses show that for Cys·Li+(H2O)n, the positive charge is mostly concentrated on Li+ except for the system containing three water molecules. For Cys·LiF(H2O)n, the positive charge is centered on the LiF unit in the range n = 0–6, and at n = 5, electron transfer from Cys to water occurs. Our study shows that the contribution of anions in zwitterionic state stabilization should be addressed more generally along with cations.

Examining CO2 as an Additive for Solid Electrolyte Interphase Formation on Silicon Anodes

We demonstrate that the addition of CO2 to a standard 1.0 M LiPF6 3:7 wt% ethylene carbonate:ethyl methyl carbonate electrolyte results in the formation of a thinner insoluble solid electrolyte interphase (SEI) that is dominated by the presence of LiF. In contrast, cells without CO2 result in a thicker insoluble SEI layer containing more organic constituents. The CO2 is incorporated in the dimethyl carbonate soluble part of the SEI composed primarily of polymeric poly(ethylene oxide) (PEO) on the surface of a thin inorganic layer. This combination of properties from CO2 addition provides an improved cycling performance through the reduction of irreversible side reactions, leading to higher coulombic efficiency. The results indicate that CO2 incorporates into the SEI and plays a role similar to additives like fluorinated ethylene carbonate and vinylene carbonate with respect to polymeric components.

X‐ray Powder Diffraction Study of Molybdenum Oxides Formed From the Thermal Reactions of MoS2, MoS2/LiF, and MoS2/Ag in Air

MoS2 can generate useful molybdenum oxides by thermally reacting with LiF or Ag nanoparticles (NPs). MoS2 easily loses sulfide and reacts with oxygen gas upon heating in air to give orthorhombic molybdenum trioxide, α‐MoO3. Morphological changes of α‐MoO3 from 500 to 900°C were investigated using X‐ray powder diffraction and scanning electron microscope. The strong diffraction peaks of the (020), (040), and (060) planes at 800°C revealed highly anisotropic growth of the oxides with a layered crystal structure. In the presence of LiF, MoS2 reacts with oxygen gas to generate Li2MoO4 and MoO2, depending on the LiF concentration. In the presence of Ag NPs, the thermal reactions of MoS2 at 800 and 900°C give silver molybdates Ag2Mo2O7 and Ag2MoO4, respectively. These results demonstrate that MoS2 is an important precursor for generating useful molybdenum oxides on the surface of inert substrates without requiring sophisticated, expensive equipment.

AICAR Stimulates the Pluripotency Transcriptional Complex in Embryonic Stem Cells Mediated by PI3K, GSK3β, and β-Catenin.

Pluripotent stem cells maintain the property of self-renewal and differentiate into all cell types under clear environments. Though the gene regulatory mechanism for pluripotency has been investigated in recent years, it is still not completely understood. Here, we show several signaling pathways involved in the maintenance of pluripotency. To investigate whether AMPK is involved in maintaining the pluripotency in mouse embryonic stem cells (mESCs) and elucidating the possible molecular mechanisms, implicated D3 and R1/E mESC lines were used in this study. Cells were cultured in the absence or presence of LIF and treated with 1 mM and 0.5 mM 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR), 2 mM metformin, compound C, and the PI3K inhibitor LY294002 for 24, 72, and 120 h. The levels of Nanog, Oct3/4, and REX1 and Brachyury, Notch2, and Gata4 mRNAs and Nanog or OCT3/4 protein levels were analyzed. Alkaline phosphatase and the cellular cycle were determined. The pGSK3β, GSK3β, p-β-catenin, and β-catenin protein levels were also investigated. We found that AMPK activators such as AICAR and metformin increase mRNA expression of pluripotency markers and decrease mRNA expression of differentiation markers in R1/E and D3 ES cells. AICAR increases phosphatase activity and arrests the cellular cycle in the G1 phase in these cells. We describe that AICAR effects were mediated by AMPK activation using a chemical inhibitor or by silencing this gene. AICAR effects were also mediated by PI3K, GSK3β, and β-catenin in R1/E ES cells. According to our findings, we provide a mechanism by which AICAR increases and maintains a pluripotency state through enhanced Nanog expression, involving AMPK/PI3K and p-GSK3β Ser21/9 pathways backing up the AICAR function as a potential target for this drug controlling pluripotency. The highlights of this study are that AICAR (5-aminoimidazole-4-carboxamied-1-b-riboside), an AMP protein kinase (AMPK) activator, blocks the ESC differentiation and AMPK is a key enzyme for pluripotency and shows valuable data to clarify the molecular pluripotency mechanism.

Formation of Li3B7O12 and O2BF4 phases from glass system of 0.5LiF-0.5B2O3 containing P2O5 and their structural properties

Borate glass containing lithium fluoride doped with different ratios of phosphate units was produced by conventional melting–annealing technique. Moreover, the produced samples were subjected to two steps of controlled heat treatment in order to achieve the glass–ceramic derivatives (nucleation and crystal growth regimes). Thermal expansion technique revealed that the nucleation temperature for the studied glasses is around 340 °C and the softening temperature (crystal growth) is around 432 °C to form glass–ceramic samples. Li3B7O12 phase was observed as primary phase for the glass–ceramic samples. Also, other crystalline phases such as LiPO3 and O2BF4 were formed. Structural changes of both of the glass and the glass–ceramics were investigated through the Fourier transform infrared technique (FTIR). The active range in the FTIR which distinguishes the glass and the crystalized sample is lying from 400 to 1250 cm−1. The presence of LiF in B2O3 network changes the characteristic units of BO3 and BO4 units into BO2F and BO3F units, respectively.

The molecular logic of Nanog-induced self-renewal in mouse embryonic stem cells

Transcription factor networks, together with histone modifications and signalling pathways, underlie the establishment and maintenance of gene regulatory architectures associated with the molecular identity of each cell type. However, how master transcription factors individually impact the epigenomic landscape and orchestrate the behaviour of regulatory networks under different environmental constraints is only partially understood. Here, we show that the transcription factor Nanog deploys multiple distinct mechanisms to enhance embryonic stem cell self-renewal. In the presence of LIF, which fosters self-renewal, Nanog rewires the pluripotency network by promoting chromatin accessibility and binding of other pluripotency factors to thousands of enhancers. In the absence of LIF, Nanog blocks differentiation by sustaining H3K27me3, a repressive histone mark, at developmental regulators. Among those, we show that the repression of Otx2 plays a preponderant role. Our results underscore the versatility of master transcription factors, such as Nanog, to globally influence gene regulation during developmental processes.

Dual effects of LiF doping on the size evolution and luminescence properties of Ag nanoclusters in borosilicate glass

In this research, the Ag nanoclusters (Ag NCs) dispersed borosilicate glasses with the absence and presence of LiF were prepared by the melt-quenching method. The absorption, photoluminescence, FTIR results indicated that the doping of LiF plays dual effects on the size evaluation of Ag species: the introduction of F− and Li+ can promote and restrain the formation of Ag NCs, respectively. It was also indicated that the promotion effect of F− is overwhelming at lower LiF doping concentration, while the restrain effect of Li+ becomes dominated when increase LiF concentration to 8 mol%. Besides, our research further proved that the formation of Ag NCs is very sensitive to the presence of F content, even the introduction of rare earth ions in the form of fluoride or oxide can obviously modify the amount and size distribution of Ag NCs and thus affect the luminescence properties of the codoped glasses.

Role of LiF additive on spark plasma sintered transparent YAG ceramics

Abstract It is well known that the presence of LiF as sintering aid to process transparent polycrystalline Y 3 Al 5 O 12 (YAG) ceramics by the Spark Plasma Sintering (SPS) technique induces three positive effects: a better particle rearrangement, a higher densification rate and the elimination of carbon contamination leading to the improvement of the material optical properties. However, the reaction mechanisms responsible of these effects still need to be clarified. Two YAG powders presenting different characteristics were selected to highlight the possible interaction between impurities and the sintering aid. In order to facilitate the determination of the LiF action during processing and reveal the involved phenomena, an excessive quantity of 5 wt% LiF was added. Samples were sintered at different temperatures between 800 °C and 1450 °C under 50 MPa. XRD, SEM and EDX techniques were used to characterise the microstructural development of ceramics and to establish the mode of action of LiF. The results highlighted that the role of LiF is mainly dependent on the purity and the phase composition of starting powders and that the SPS cycle must be adapted to the sintering aid characteristic temperatures.

Analysis of the structural evolution of the SEI layer in FeF2 thin-film lithium-ion batteries upon cycling using HRTEM and EELS

Conversion compound electrodes such as FeF2 allow a reversible change of more than one Li ion per 3d metal cation, which provides a pathway for overcoming the intrinsic capacity bottleneck associated with traditional intercalation electrodes. However, many of the basic mechanisms involving structural changes to the electrode with cycling, and the correlation with capacity changes must be better understood for future implementation in batteries. Understanding the SEI layer is of particular importance as it is where ion exchange occurs, and the kinetics of the battery is largely determined. We note in this context that thin-films are binder free, and therefore provide a model platform to obtain insight to the structural and electrochemical behavior of uncontaminated phases of the FeF2 and other conversion materials. FeF2/LiPON thin-film batteries were fabricated and subjected to Galvanostatic cycling, and the evolution of the SEI analyzed using HRTEM and EELS spectroscopy. Samples were subjected to single and 15cycles, and compared in their lithiated and delithiated states to understand the relationship between microstructural evolution and capacity. The SEI formation is essentially complete after the first cycle, and is characterized by incomplete reconversion of LiF which is resistive to both electron and ion transport. With subsequent cycling the L3/L2 ratios (compared to the pristine and singly cycled films) show that the phase fractions of unconverted Fe and LiF increase, making the SEI more resistive to ion exchange. The large capacity drop measured after the first cycle is due to the formation of the SEI and the presence of LiF which degrades Li and electron mobility, and in turn adversely impacts capacity.

SMARCAD1 Contributes to the Regulation of Naive Pluripotency by Interacting with Histone Citrullination

Histone citrullination regulates diverse cellular processes. Here, we report that SMARCAD1 preferentially associates with H3 arginine 26 citrullination (H3R26Cit) peptides present on arrays composed of 384 histone peptides harboring distinct post-transcriptional modifications. Among ten histone modifications assayed by ChIP-seq, H3R26Cit exhibited the most extensive genomewide co-localization with SMARCAD1 binding. Increased Smarcad1 expression correlated with naive pluripotency in pre-implantation embryos. In the presence of LIF, Smarcad1 knockdown (KD) embryonic stem cells lost naive state phenotypes but remained pluripotent, as suggested by morphology, gene expression, histone modifications, alkaline phosphatase activity, energy metabolism, embryoid bodies, teratoma, and chimeras. The majority of H3R26Cit ChIP-seq peaks occupied by SMARCAD1 were associated with increased levels of H3K9me3 in Smarcad1 KD cells. Inhibition of H3Cit induced H3K9me3 at the overlapping regions of H3R26Cit peaks and SMARCAD1 peaks. These data suggest a model in which SMARCAD1 regulates naive pluripotency by interacting with H3R26Cit and suppressing heterochromatin formation.

Morphological and ultrastructural studies of human spermatogonial stem cells from patients with maturation arrest.

Destruction of spermatogonial stem cells (SSCs) along the chemotherapy and radiotherapy is one of the side effects of cancer treatments that lead to infertility. In vitro propagation of hSSCs is necessary to obtain an adequate number of cells for successful transplantation. In this study, hSSCs were isolated from testis biopsies of the patients with maturation arrest and proliferated in DMEM in the presence of LIF and bFGF for 5 weeks. The various types of human spermatogonia were identified in culture system and compared with testis tissue using morphological criteria at the ultrastructural level. The results showed that although many various types of spermatogonia were identified, but no remarkable difference was observed between spermatogonial cells in culture system and testis tissue. Electron and light microscopic studies of hSSC colonies did not show differentiated SSCs in the culture system. The results also showed that probably the suitable time for transplanting of SSCs in recipient testis is 2-3 weeks after culture. Because apoptosis which may affect the development of germ cells has not started in colony cells at this time and the population of apoptotic cells are low.

Transparent MgAl2O4/LiF ceramics by hot-pressing: Host–additive interaction mechanisms issue revisited

Hot-pressing schedules – able to ensure a proper balance between positive and negative effects of LiF – allow fabrication of highly transparent MgAl2O4 parts under low-pressure (≤50MPa); peak temperatures in excess of 1550°C are needed, with best results obtained at ∼1650°C. At the concentration-levels and conditions present during hot-pressing, LiF does not react with spinel; it is inert also toward graphite. The lubricant and solvent abilities of liquid LiF make possible achievement of densification levels >90%TD under 1200°C. All LiF leaves the specimens as such at temperatures ≤1550°C. LiF's ability to close porosity at temperatures lower than those (>1350°C) – which allow significant carbon penetration into spinel – is essential in achievement of transparency. Light absorption – occurring when carbon is present – is thus prevented. The main light-scattering defect produced by HPing, in the presence of LiF, is micro and macro-cracking; opaque spots, due to un-complete densification, are rare.