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Articles 271 - 300 of 427

Full-Text Articles in Materials Science and Engineering

Optical And Electrical Characterization Of Bulk Grown Indium-Gallium-Arsenide Alloys, Austin C. Bergstrom Mar 2010

Optical And Electrical Characterization Of Bulk Grown Indium-Gallium-Arsenide Alloys, Austin C. Bergstrom

Theses and Dissertations

Advances in crystal growth techniques have allowed increased quality in growth of bulk ternary InxGa1-xAs. Here, the optical and electrical properties of samples grown through the vertical Bridgman (or multi-component zone melting growth) method have been investigated through photoluminescence spectroscopy and Hall effect measurements. Indium mole fractions varied from 0.75 for 1. Hall effect measurements at temperatures ranging from 10 to 300 K revealed moderate n-type doping with carrier concentrations ranging from 1.5 to 9.6×1016 cm-3 at 10 to 15 K. Carriers from deep donor levels became appreciable between 50 and 100 K. Hall …


Afm-Patterned 2-D Thin-Film Photonic Crystal Analyzed By Complete Angle Scatter, Nicholas C. Herr Mar 2010

Afm-Patterned 2-D Thin-Film Photonic Crystal Analyzed By Complete Angle Scatter, Nicholas C. Herr

Theses and Dissertations

The purpose of this research was to use an atomic force microscope (AFM) to generate a 2-D square array of sub-wavelength surface features from a single material over a region large enough to permit optical characterization. This work is an extension of previous AFIT nano-patterning work and is in response to the small subunit sizes demanded for the production of optical metamaterials and photonic crystals. A diamond nano-indentation AFM probe was used to produce a 325-μm by 200-μm array of indentations in a 120-nm thick polystyrene film deposited on silicon. Indentation spacing of 400 nm produced well-defined surface features with …


Hard Collisions In Rubidium Using Sub-Doppler Spectroscopy, Douglas E. Thornton Mar 2010

Hard Collisions In Rubidium Using Sub-Doppler Spectroscopy, Douglas E. Thornton

Theses and Dissertations

To better understand the laser kinetics of an alkali gain medium, hard collisions, or velocity-changing collisions, has been studied and a velocity-changing collisional rate has been calculated. Previous works have studied these collisions, but no rate has been calculated. Using the precise tool of sub-Doppler spectroscopy, atomic hard collisions can be observed. The collected spectra are fitted with two different line shapes to demonstrate the accuracy of this method. From the fits, the number of hard collisions can be extracted. The time scale of the hard collisions in rubidium is interpolated by varying the chopping frequency of the pump beam, …


Characterization And Measurement Of Passive And Active Metamaterial Devices, Christopher A. Lundell Mar 2010

Characterization And Measurement Of Passive And Active Metamaterial Devices, Christopher A. Lundell

Theses and Dissertations

This document addresses two major obstacles facing metamaterial development: uncertainty in the characterization of electromagnetic field behavior in metamaterial structures and the relatively small operational bandwidth of metamaterial structures. To address the first obstacle, a new method to characterize electromagnetic field behavior in a metamaterial is presented. This new method is a bistatic radar cross section (RCS) measurement technique. RCS measurements are well-suited to measuring bulk metamaterial samples because they show frequency dependence of scattering angles and offer common postprocessing techniques that can be useful for visualizing results. To address the second obstacle, this document characterizes the effectiveness of an …


Thermo-Mechanical Characterization Of Silicon Carbide-Silicon Carbide Composites At Elevated Temperatures Using A Unique Combustion Facility, Ted T. Kim Sep 2009

Thermo-Mechanical Characterization Of Silicon Carbide-Silicon Carbide Composites At Elevated Temperatures Using A Unique Combustion Facility, Ted T. Kim

Theses and Dissertations

This research investigated four different types of CMCs in a simulated gas turbine engine hot section condition that involved the simultaneous application of a combustion environment and mechanical fatigue loading using a unique burner rig facility developed for this study at AFIT. Three of the materials were woven Melt-Infiltrated (MI) BN/SiC reinforced by Hi-Nicalon Type S (Hi-Nic-S), Sylramic (Syl) and Sylramic with insitu BN coating (Syl-iBN) fibers, respectively. They were made by the slurry cast method. The other CMC was MI Hi-Nic-S/BN/SiC made using the prepreg MI processing. Prepreg MI CMC and Sylramic-iBN fiber reinforced CMC showed better resistance to …


Investigation Of Electrical And Optical Properties Of Bulk Iii-V Ternary Semiconductors, Travis C. Gomez Mar 2009

Investigation Of Electrical And Optical Properties Of Bulk Iii-V Ternary Semiconductors, Travis C. Gomez

Theses and Dissertations

Bulk grown III-V ternary semiconductors of In0.08Ga0.92Sb and In0.15Ga0.85As were investigated through Hall-effect and photoluminescence measurements to determine carrier concentration, mobility, sheet resistivity, and luminescence spectrum. In the past, epitaxial layers of ternary compounds have been grown on binary compound substrates, and thus very limited lattice matched ternary alloys were available. Recently, bulk grown ternary substrates have been developed, and it has presented a renewed interest in using these substrates to grow high quality ternary compounds for use in many next generation optoelectronic devices. The results of photoluminescence (PL) study for the …


Non-Destructive Evaluation Of Aerospace Composites, Jeremy D. Johnson Mar 2009

Non-Destructive Evaluation Of Aerospace Composites, Jeremy D. Johnson

Theses and Dissertations

Five methods of non-destructive material evaluation (NDE) were used to inspect various forms of damage commonly found in aerospace fiberglass composites: voids, edge and sub-surface delaminations, surface burning, and cracking. The images produced by X-ray, X-ray Computed Tomography, terahertz (THz) imaging, ultrasound, and flash IR thermography were analyzed for the detection of defects. Test results and analysis of each NDE method's capabilities provide a comparison study of conventional techniques versus the emerging technology of THz imaging for the non-destructive evaluation of aerospace composite materials. A comparison guide to the five methods' damage detection effectiveness for fiberglass composites is provided by …


Effects Of Environment On Creep Behavior Of Nextel720/Alumina-Mullite Ceramic Composite With ±45º Fiber Orientation At 1200ºc, Muzaffer Ozer Mar 2009

Effects Of Environment On Creep Behavior Of Nextel720/Alumina-Mullite Ceramic Composite With ±45º Fiber Orientation At 1200ºc, Muzaffer Ozer

Theses and Dissertations

The present effort will investigate the off-axis creep behavior of the Nexte720/Alumina-Mullite CMC, a material system which relies on a porous alumina/mullite matrix for damage tolerance as compared to the pure alumina matrix in earlier studies. This research evaluated the creep behavior of N720/AM with a ± 45° fiber orientation at 1200°C in: laboratory air, steam, and argon environments. Creep-rupture tests at the creep stress levels of: 32, 30, 26, and 20 MPa were conducted in each environment. Additionally 35 MPa stress level examined in air and 13 MPa stress level examined both in air and steam. The N720/AM with …


Creep And Fatigue Interaction Characteristics Of Pwa1484, Christian J. Pierce Mar 2009

Creep And Fatigue Interaction Characteristics Of Pwa1484, Christian J. Pierce

Theses and Dissertations

One of the main methods for predicting the service life of jet engine turbine blades is through creep analysis. A sample of the turbine blade material will be mechanically tested to characterize its creep performance. This characterization will be used to determine safe operating conditions for the material. While methods for modeling creep behavior are generally well developed, this constant load creep testing does not fully represent the loading conditions present in a jet engine due to cyclic loading caused by the mission profile and throttle movements. As the industry seeks to become more accurate in physics based modeling of …


Analysis And Application Of The Bi-Directional Scatter Distribution Function Of Photonic Crystals, Robert B. Lamott Mar 2009

Analysis And Application Of The Bi-Directional Scatter Distribution Function Of Photonic Crystals, Robert B. Lamott

Theses and Dissertations

Photonic crystals (PCs) are periodic structures built from materials with different refractive indices repeated at sub-wavelength intervals, which results in unusual optical characteristics, including narrowband laser protection, and zero reflectance and high absorption anomalies. Most of the research into the optical properties of PCs has concentrated only on the small range of wavelengths and angles where these effects occur. To better understand where all light leaving a PC is scattered, a Complete Angle Scatter Instrument was used to analyze the scatter from three Guided Mode Resonance Filters designed for laser protection. In the plane of incidence, measurements of the scatter …


Effects Of Contact Load On The Fretting Fatigue Behavior Of In-100 At Elevated Temperature, Alfred G. Traylor Ii Mar 2009

Effects Of Contact Load On The Fretting Fatigue Behavior Of In-100 At Elevated Temperature, Alfred G. Traylor Ii

Theses and Dissertations

This thesis studied the effects of contact load of 8006 N on the fretting fatigue behavior of IN-100 at an elevated temperature of 600°C. First, S-N curves were created for a range of applied axial stresses at an identical stress ratio of 0.03 at 8006 N. A condition of partial slip condition necessary to fretting fatigue was verified by generating hysteresis loops, plotting shear force against axial stress. The half-contact width and the crack initiation location were determined for all of the fretting specimens using optical and scanning electron microscopes. The contact widths were consistent with expected analytical values and …


Ceramic Matrix Composite Characterization Under A Combustion And Loading Environment, Andrew R. Nye Mar 2009

Ceramic Matrix Composite Characterization Under A Combustion And Loading Environment, Andrew R. Nye

Theses and Dissertations

Lightweight materials that can withstand high temperatures and corrosive environments are constantly sought after in the aerospace industry, typically for Gas Turbine Engine (GTE) application. These materials need to retain their strength throughout the long service period they would see in the combustor and turbine components of a GTE. One material that is ideal for these types of applications is an oxide/oxide Ceramic Matrix Composite (CMC). The fatigue behavior of the oxide/oxide CMC NextelTM 720/Alumina (N720/A) was investigated in a unique high temperature environment. N720/A consisted of an 8-harness satin weave of NextelTM aluminum oxide/silicon oxide fibers bound …


Production And Characterization Of High Repetition Rate Terahertz Radiation In Femtosecond-Laser-Induced Air Plasma, Michael L. Dexter Mar 2009

Production And Characterization Of High Repetition Rate Terahertz Radiation In Femtosecond-Laser-Induced Air Plasma, Michael L. Dexter

Theses and Dissertations

The purpose of this research was to produce and characterize high repetition rate terahertz radiation in ionized air plasma. An 800 nanometer, 50 femtosecond, 0.35 Watt, 40 KHz, pulsed Ti:Sapphire laser system was used as the source infrared beam. This beam was focused onto a second harmonic generation crystal to produce a collinear, perpendicularly polarized secondary beam at 400 nm. After realigning the polarization of the fundamental to the second harmonic and compensating for group velocity dispersion introduced by the optics, both beams were recombined and focused by a 3.75 cm focal length mirror to form an air plasma. An …


A Comparative Study Of The Bidirectional Reflectance Distribution Function Of Several Surfaces As A Mid-Wave Infrared Diffuse Reflectance Standard, Bradley Balling Mar 2009

A Comparative Study Of The Bidirectional Reflectance Distribution Function Of Several Surfaces As A Mid-Wave Infrared Diffuse Reflectance Standard, Bradley Balling

Theses and Dissertations

The Bi-Directional Reflectance Distribution Function (BRDF) has a well defined diffuse measurement standard in the ultraviolet, visible, and near infrared (NIR), Spectralon(trade name). It is predictable, stable, repeatable, and has low surface variation because it is a bulk scatterer. In the mid-wave IR (MWIR) and long-wave IR (LWIR), there is not such a well-defined standard. There are well-defined directional hemispherical reflectance (DHR) standards, but the process of integrating BRDF measurements into DHR for the purpose of calibration is problematic, at best. Direct BRDF measurement standards are needed. This study use current calibration techniques to ensure valid measurements and then systematically …


Wave Optics Simulation Of Optically Augmented Retroreflections For Monostatic/Bistatic Detection, John J. Tatar Iii Mar 2009

Wave Optics Simulation Of Optically Augmented Retroreflections For Monostatic/Bistatic Detection, John J. Tatar Iii

Theses and Dissertations

Optical devices interrogated with a laser in the appropriate band can exhibit strong, deterministic reflections of the incident beam. This characteristic could be exploited for optical target detection and identification. The distribution of reflected power is strongly dependent on the geometry of the interrogation scenario, atmospheric conditions, and the cross section of the target optical device. Previous work on laser interrogation systems in this area has focused on analytic models or testing. To the best of my knowledge, I am presenting for the first time an approach to predict reflected power for a variety of interrogation configurations, targets, and propagation …


Nondestructive Evaluation Of Aircraft Composites Using Terahertz Time Domain Spectroscopy, Christopher D. Stoik Dec 2008

Nondestructive Evaluation Of Aircraft Composites Using Terahertz Time Domain Spectroscopy, Christopher D. Stoik

Theses and Dissertations

Terahertz (THz) time domain spectroscopy (TDS) was assessed as a nondestructive evaluation technique for aircraft composites. Material properties of glass fiber composite were measured using both transmission and reflection configuration. The interaction of THz with a glass fiber composite was then analyzed, including the effects of scattering, absorption, and the index of refraction, as well as effective medium approximations. THz TDS, in both transmission and reflection configuration, was used to study composite damage, including voids, delaminations, mechanical damage, and heat damage. Measurement of the material properties on samples with localized heat damage showed that burning did not change the refractive …


The Effect Of Radiation On The Electrical Properties Of Aluminum Gallium Nitride/Gallium Nitride Heterostructures, John W. Mcclory Jun 2008

The Effect Of Radiation On The Electrical Properties Of Aluminum Gallium Nitride/Gallium Nitride Heterostructures, John W. Mcclory

Theses and Dissertations

AlGaN/GaN Heterojunction Field Effect Transistors (HFETs) were irradiated at low temperature and the temperature dependent changes to drain current, gate current, capacitance, and transconductance were measured. The results were compared to the charge control model of the drain current and trap-assisted tunneling model of the gate current to determine the source of the radiation-induced changes. AlGaN/GaN HFETs demonstrated threshold voltage shifts and drain current changes after irradiation. After electron and neutron irradiation applied at ~80 K, measurement of the drain current at this temperature showed an increase that saturated after 1013 electrons/cm2 or 1010 neutrons/cm2 due …


Analysis Of Multi-Layered Materials Under High Velocity Impact Using Cth, Jason K. Lee Mar 2008

Analysis Of Multi-Layered Materials Under High Velocity Impact Using Cth, Jason K. Lee

Theses and Dissertations

Multi-layer armor containing ceramic and metallic layers has become more common in the past two decades. Typically, ceramics have high compressive strength which combined with their low density make them highly desirable for armor applications. This research effort numerically simulates high velocity impact of cylindrical projectiles on multi-layer metallic and ceramic targets of finite thickness. The impact of the projectile occurs normal to the surface of the target. The projectiles used are made of either S7 tool steel or tungsten. The targets consist of either rolled homogeneous armor, 4340 steel and boron carbide ceramic or rolled homogeneous armor and boron …


Effects Of Environment On Creep Behavior Of Nextel 720/Alumina-Mullite Ceramic Composite At 1200°C, Christopher L. Genelin Mar 2008

Effects Of Environment On Creep Behavior Of Nextel 720/Alumina-Mullite Ceramic Composite At 1200°C, Christopher L. Genelin

Theses and Dissertations

The creep behavior of an oxide-oxide ceramic matrix composite (CMC) was investigated at 1200°C in laboratory air, in steam and in argon. The composite consisted of a porous alumina-mullite matrix reinforced with laminated, woven mullite/alumina (Nextel/720) fibers. The composite had no fiber coating and relied on its porous alumina/mullite matrix for flaw tolerance. Tensile stress-strain behavior was investigated and the tensile properties were measured at 1200°C in laboratory air. Tensile creep behavior of the CMC was examined for creep stress levels of 73, 91, 114 and 136 MPa. Creep run-out, set to 100 h, was achieved for stress levels ≤ …


Electrical Activation Studies Of Silicon Implanted Aluminum Gallium Nitride With High Aluminum Mole Fraction, Elizabeth A. Moore Dec 2007

Electrical Activation Studies Of Silicon Implanted Aluminum Gallium Nitride With High Aluminum Mole Fraction, Elizabeth A. Moore

Theses and Dissertations

This research demonstrates a method for producing highly conductive Si-implanted n-type aluminum gallium nitride (AlxGa1-xN) alloys, and represents a comprehensive analysis of the resulting material's electrical and optical properties as a function of Al mole fraction, anneal temperature, anneal time, and implantation dose. Highly conductive alloys are critical to the fabrication of devices operating in deep UV, high-temperature, high-power, and high-frequency environments, and thus this research is significant in regard to the application of such devices. The AlxGa1-xN wafers of this study, with Al concentrations of 10 to 50%, were implanted at …


Effect Of Prior Aging On Fatigue Behavior Of Im7/Bmi 5250-4 Composite At 191°C, Christine G. Ladrido Jun 2007

Effect Of Prior Aging On Fatigue Behavior Of Im7/Bmi 5250-4 Composite At 191°C, Christine G. Ladrido

Theses and Dissertations

IM7/BMI 5250-4 with carbon fiber orientations [± 45] and [0/90] were aged in air at 191 °C for up to 1000 hours. The weight loss was analyzed during the aging process. Tension to failure tests were performed on both the unaged and aged specimens to establish a baseline for the Ultimate Tensile Strengths and Young's Modulus. Tension-tension cyclic load fatigue testing was conducted on the specimen to determine the effect of prior aging on the fatigue response of the composite. Cyclic strain accumulation was not influenced by prior aging, but it was influenced by the stress levels applied. [± 45] …


High Temperature Degradation Of 5250-4 Polymer Resin, Patrick E. Link Jun 2007

High Temperature Degradation Of 5250-4 Polymer Resin, Patrick E. Link

Theses and Dissertations

5250-4 bismaleimide resin is used in high performance polymer matrix composites with high temperature aeronautical applications. This thesis investigated the thermal and oxidative degradation of 5250-4 neat resin powder in argon, air, and oxygen environments. The powder was aged at 163 deg. C, 177 deg. C, and 190 deg. C in all environments for at least 250 hours. Isothermal thermo-gravimetric analysis demonstrated that weight loss was negligible for aging in the argon environment, indicating weight loss is the result of an oxidative process at these temperatures. The 5250-4 powder exhibited an initial period of weight gain before eventually losing weight …


Optical Parametric Oscillation In Orientation-Patterned Gallium Arsenide, Scott A. Shell Mar 2007

Optical Parametric Oscillation In Orientation-Patterned Gallium Arsenide, Scott A. Shell

Theses and Dissertations

Tunable laser sources in the mid-infrared (MIR) spectral range are required for several Air Force applications. Existing lasers with output in the near-infrared can be converted to more desirable MIR by using nonlinear effects. Orientation patterned gallium arsenide (OPGaAs) is a promising nonlinear conversion material because it has broad transparency and can be engineered for specific pump laser and output wavelengths using quasi-phase matching techniques. This research examines optical parametric oscillation (OPO) of several OPGaAs samples using a 2.052 micrometers wavelength Tm, Ho:YLF pump laser. Of the seven samples available the five that were capable of getting OPO output with …


Cobalt Doping Of Semiconducting Boron Carbide Using Cobaltocene, Lonnie Carlson Mar 2007

Cobalt Doping Of Semiconducting Boron Carbide Using Cobaltocene, Lonnie Carlson

Theses and Dissertations

The decomposition of cobaltocene and metacarborane (closo-1,7-dicarba-decaborane) under low energy electron irradiation at about 200 K results in a material with the Fermi level closer to the valence band than the material resulting from the decomposition of metacarborane alone. This indicates that cobaltocene provides a relatively p-type dopant as seen in ultraviolet photoemission spectroscopy/inverse photoemission spectroscopy. Upon warming to room temperature, however, the Fermi level shifts towards the conduction band, suggesting an n-type dopant. This temperature dependent surface photovoltage effect is not compelling evidence for the majority carrier type but does suggest an increase in the carrier concentration …


Characterization Of Intercalated Graphite Fibers For Microelectromechanical Systems (Mems) Applications, Bryan W. Winningham Mar 2007

Characterization Of Intercalated Graphite Fibers For Microelectromechanical Systems (Mems) Applications, Bryan W. Winningham

Theses and Dissertations

Research was accomplished to characterize the electrical and physical characteristic changes of the Thornel® P-100 carbon fiber and five variants when intercalated with 96% sulfuric acid and incorporated the use of Microelectromechanical Systems (MEMS) structures for testing purposes. The five fiber variants were oxidized in 1 M nitric acid at 0.5 A for 30 seconds, 1 and 2 minutes, the last two samples were detreated at 1150 °C for one hour prior to the nitric acid treatment. The fibers were mounted onto a MEMS die, placed into a chip carrier, sulfuric acid added, the chip carrier sealed and testing accomplished. …


An Exploration Of Several Structural Measurement Techniques For Usage With Functionally Graded Materials, Robert A. Reuter Dec 2006

An Exploration Of Several Structural Measurement Techniques For Usage With Functionally Graded Materials, Robert A. Reuter

Theses and Dissertations

Titanium / titanium boride functionally graded 6"x 1"x1" beams were subjected to a four-point beam test in order to critique the value of several measurement techniques. Also, finite element analysis results were compared with experimental values and general observations about the experiment were recorded. Uniform 85% TiB /15% Ti and uniform commercially pure titanium specimens were also subjected to the same loading conditions as a control. Techniques used include digital image correlation, fiber optic strain gauging, strain gauging, and differential infrared thermography techniques. The strain data results were compared with one another and to linear finite element models. It was …


Modeling Fracture In Z-Pinned Composite Co-Cured Laminates Using Smeared Properties And Cohesive Elements In Dyna3d, Jason K. Freels Sep 2006

Modeling Fracture In Z-Pinned Composite Co-Cured Laminates Using Smeared Properties And Cohesive Elements In Dyna3d, Jason K. Freels

Theses and Dissertations

The purpose of the present research was three-fold: 1) gain a more sophisticated understanding of the response of co-cured composite joints with and without through-thickness reinforcement (TTR), 2) compare the behavior of specimens reinforced with various sizes and densities of reinforcement, and 3) use experimental data to verify the existing DYNA3D smeared property model. Double cantilever beam, end-notch flexure and T-section specimens reinforced with 0.011" diameter z-pins at 2% and 4% volume densities were tested to determine the mode I, mode II and mixed mode (I and II) behavior. Results were added to preliminary research in which tests were conducted …


Effects Of Environment On Creep Behavior Of Two Oxide-Oxide Ceramic Matrix Composites At 1200°C, Pavlos Koutsoukos Sep 2006

Effects Of Environment On Creep Behavior Of Two Oxide-Oxide Ceramic Matrix Composites At 1200°C, Pavlos Koutsoukos

Theses and Dissertations

Previous studies by the advisor and graduate students examined creep behavior of the Nextel720/Alumina CMC in air and in 100% steam environments at 1200 and 1330°C. Results showed that while this oxide/oxide system exhibits an exceptionally high fatigue limit at 1200°C it also experiences substantial strain accumulation under sustained loading conditions. Furthermore, these earlier investigations revealed a significant degrading effect of 100% steam environment on material performance under both static and cyclic loadings. The present effort will investigate creep rupture behavior of Nextel720/Alumina composite in the inert gas environment. In addition, creep rupture behavior of Nextel720/Aluminosilicate CMC will be investigated …


Effects Of Polishing Shot-Peened Surfaces On Fretting Fatigue Behavior Of Ti-6al-4v, Kasey S. Scheel Sep 2006

Effects Of Polishing Shot-Peened Surfaces On Fretting Fatigue Behavior Of Ti-6al-4v, Kasey S. Scheel

Theses and Dissertations

The research of this thesis was done to investigate the effects of polishing a shot-peened specimen of Ti-6Al-4V on the fretting fatigue life of that specimen. The shot-peening process, though one of the most beneficial techniques in prolonging fretting fatigue life, creates a textured surface that may lead to problems on the micro level. This research was done in an attempt to further improve the peening process by examining the effects of another surface treatment to be used in conjunction, surface polishing. The rough peened surface may contain abrupt changes in surface geometry that act as stress risers, which are …


Characterization Of Passivated Indium Antimonide, Catherine Ann Taylor Jun 2006

Characterization Of Passivated Indium Antimonide, Catherine Ann Taylor

Theses and Dissertations

Infrared absorption and photoluminescence measurements have been used to optically characterize bulk-grown, 680-μm thick, indium antimonide (InSb), both as-grown and after passivation by either anodization or a 700-Å layer of silicon oxide (SiOx). Spectra were obtained using Fourier transform infrared (FT-IR) spectroscopy. Results include the effects of sample temperature in the range of 10 to 300 K and 4.636 μm laser pump power in the range of 28 mW to 1.43 W for the photoluminescence spectrum.