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D J Larson

Publications and source records attributed to D J Larson.

18 recordsLinked to original sources
PubMed2006

In situ site-specific specimen preparation for atom probe tomography.

Techniques for the rapid preparation of atom-probe samples extracted directly from a Si wafer are presented and discussed. A systematic mounting process to a standardized microtip array allows approximately 12 samples to be extracted from a near-surface region and mounted for subsequent focused-ion-beam sharpening in a short period of time, about 2h. In addition, site-specific annular mill extraction techniques are demonstrated that allow specific devices or structures to be removed from a Si wafer and analyzed in the atom-probe. The challenges presented by Ga-induced implantation and damage, particularly at a standard ion-beam accelerating voltage of 30 keV, are shown and discussed. A significant reduction in the extent of the damaged regions through the application of a low-energy "clean-up" ion beam is confirmed by atom-probe analysis of the damaged regions. The Ga+ penetration depth into {100} Si at 30 keV is approximately 40 nm. Clean-up with either a 5 or 2 keV beam reduces the depth of damaged Si to approximately 5 nm and <1 nm, respectively. Finally, a NiSi sample was extracted from a Si wafer, mounted to a microtip array, sharpened, cleaned up with a 5 keV beam and analyzed in the atom probe. The current results demonstrate that specific regions of interest can be accessed and preserved throughout the sample-preparation process and that this preparation method leads to high-quality atom probe analysis of such nano-structures.

Journal Article↗
PubMed2004

Modeling image distortions in 3DAP.

A numerical model has been developed to simulate images obtained from the three-dimensional atom probe. This model was used to simulate the artefacts commonly observed in two-phase materials. This model takes into account the dynamic evolution of the atomic-scale shape of the specimen during field evaporation. This article reviews the model and its applications to some specific cases. Local magnification effects were studied as a function of the size, the shape, and the orientation of precipitated phases embedded in the matrix. Small precipitates produce large aberrations in good agreement with experiments. The magnification from such precipitates, as measured from the simulation, is only found to match the theoretical value for mesoscopic scale precipitates (size similar to the specimen size). Orientation effects are also observed in excellent agreement with experiments. The measured thickness of a grain-boundary-segregated film in the simulation is found to decrease with the angle between the normal to the grain boundary and the tip axis. Depth scaling artefacts caused by variation in the evaporation field of atoms in multilayer structures were successfully simulated and again showed good agreement with effects observed experimentally.

Alloys↗
PubMed2004

The role of atomic scale investigation in the development of nanoscale materials for information storage applications.

It is well established that the response of devices based on the giant magnetoresistance (GMR) effect depends critically on film microstructure, with parameters such as interfacial abruptness, the roughness and waviness of the layers, and grain size being crucial. Such devices have applications in information storage systems, and are therefore of great technological interest as well as being of fundamental scientific interest. The layers must be studied at high spatial resolution if the microstructural parameters are to be characterized with sufficient detail to enable the effects of fabrication conditions on properties to be understood, and the techniques of high resolution electron microscopy, transmission electron microscopy chemical mapping, and atom probe microanalysis are ideally suited. This article describes the application of these techniques to a range of materials including spin valves, spin tunnel junctions, and GMR multilayers.

Alloys↗
PubMed2002

Porcine circovirus type 2 (PCV-2) coinfections in US field cases of postweaning multisystemic wasting syndrome (PMWS).

The prevalence of different pathogens detected in combination with porcine circovirus type 2 (PCV-2) was studied retrospectively in field cases of postweaning multisystemic wasting syndrome (PMWS) diagnosed at the Iowa State University Veterinary Diagnostic Laboratory, Ames, Iowa, between January 2000, and September 2001. The presence of PCV-2 antigen in lymphoid tissues and/or lung, demonstrated by immunohistochemistry, together with moderate to severe lymphoid depletion and/or granulomatous lymphadenitis, was used as the criteria for the diagnosis of PMWS. A total of 484 cases fulfilled these criteria. Most of the cases (294/369) of PMWS occurred in pigs between the ages of 8 and 18 weeks, with a peak at 10 weeks of age. Porcine reproductive and respiratory syndrome virus was detected in 51.9% of the cases, Mycoplasma hyopneumoniae in 35.5%, bacterial septicemia in 14.0%, bacterial pneumonia in 7.6%, swine influenza virus in 5.4%, and PCV-2 alone in 1.9%. In cases with bacterial septicemia the most frequently isolated pathogen was Streptococcus suis. In cases with bacterial pneumonia, Pasteurella multocida was the most prevalent.

Animals↗
PubMed2001

Advances in Atom Probe Specimen Fabrication from Planar Multilayer Thin Film Structures.

A sample preparation method has been developed whereby sharp needle-shaped specimens for atom probe analysis are fabricated from multilayer thin films deposited onto silicon substrates. The specimens are fabricated in an orientation such that atom probe composition profiles across the layer interfaces can be determined with atomic-layer spatial resolution, i.e., the layer normals are parallel to the needle axis. The method uses standard silicon etching techniques and focused ion-beam milling. The feasibility and utility of this technique are shown through its application to a NiFe/CoFe/Cu/CoFe-based thin film structure.

Journal Article↗
PubMed1997

Detection of Arcobacter species in gastric samples from swine.

Swine stomachs were surveyed for evidence of Arcobacter spp. and Helicobacter spp. infections associated with gastric ulceration. A nested PCR test targeted to the 16S rRNA was developed to detect many Arcobacter spp. and Helicobacter spp. An internal oligonucleotide probe was used for differentiation and confirmation of the PCR product. Tissue samples were obtained from the nonglandular and glandular regions of 86 swine stomachs. Evidence of infection with these microbes was detected in 51%, with 77% of the positive samples being identified as A. butzleri using a highly specific probe. Nonglandular stomach samples (44%) were more likely to be positive by PCR than samples from the glandular (23%) region. Gross lesions of any stage of gastric ulceration, ranging from parakeratosis, erosions and ulceration, were observed in 24% of stomachs examined. Of 21 samples with lesions, 52% were positive by the broadly reactive PCR assay for Arcobacter spp. and Helicobacter spp. The majority of PCR-positive samples (75%) had no gross lesions. When a single step PCR assay that was more specific for Arcobacter spp. was used on the nonglandular stomach samples, 10.4% of the 86 samples were positive. Arcobacter spp. were cultured from four of the sample stomachs. Partial sequencing of the 16S rRNA gene identified the isolates as A. butzleri (n = 2), A. cryaerophilus, (n = 1), and a mixed culture of A. butzleri and another Arcobacter spp. (n = 1). A single step PCR assay targeted to the urease gene and culturing methods were used to screen for H. pylori or other closely related urease positive bacteria, but none were found.

Animals↗
PubMed1997

Application of multiplex polymerase chain reaction for rapid identification of Campylobacter jejuni and C coli associated with reproductive failure.

OBJECTIVE: To evaluate a multiplex polymerase chain reaction (PCR) to distinguish Campylobacter jejuni from C coli as causes of reproductive failure. PROCEDURE: Review of clinical cases of reproductive failure attributed to C jejuni or C coli. RESULTS: A case of swine abortion was attributable to infection with C coli. The porcine abortion isolates were verified as C coli by restriction fragment length polymorphism and multiplex PCR. Cases of endometritis in a fox and in mink caused by C jejuni were reviewed, and isolates were confirmed as C jejuni by results of the multiplex PCR. CONCLUSION: Multiplex PCR was useful in identifying C coli and C jejuni recovered from atypical cases of reproductive failure. Multiplex PCR in conjunction with conventional assays may be useful for verifying other unusual instances of campylobacteriosis.

Abortion, Septic↗
PubMed1996

Infection of cesarean-derived colostrum-deprived 1-day-old piglets with Arcobacter butzleri, Arcobacter cryaerophilus, and Arcobacter skirrowii.

Neonatal piglets have been used as models to study human campylobacteriosis and helicobacteriosis. The purpose of this study was to determine the relative pathogenicities, on the basis of the duration of fecal shedding and colonization of tissues, of three Arcobacter species in 1-day-old cesarean-derived colostrum-deprived piglets. Two experiments were conducted. In experiment 1, two piglets each were infected per os with either Arcobacter butzleri ATCC 49616, Arcobacter cryaerophilus 1B ATCC 43159, Arcobacter skirrowii CCUG 10374, or the three field strains of A. butzleri (approximately 5 X 10(9) CFU per piglet). Rectal swab samples were taken prior to infection and daily thereafter for up to 7 days. Arcobacter spp. were detected at least once in rectal swab samples of all but one of the experimentally infected piglets but not in the control. At necropsy, A. butzleri was recovered from the lung, kidney, ileum, or brain tissues of the four infected piglets which had received either the field strain or the ATCC type strain of A. butzleri. A. cryaerophilus 1B was detected in rectal swab samples for up to 7 days postinfection but was not cultured from tissues at necropsy. Arcobacters were detected in the rectal swab sample of the A. skirrowii-infected piglet only on day 3 postinfection; no isolates were obtained from tissues at necropsy. No gross pathological lesions were consistently noted in the experimentally infected piglets. In experiment 2, two piglets each were infected per os with A. butzleri ATCC 49616, A. cryaerophilus 1A ATCC 43158, A. skirrowii CCUG 10374, or the single A. butzleri field strain Yard J/c (approximately 5 X 10(9) CFU per piglet). Arcobacter spp. were cultured from rectal swab samples of all but one of the experimentally infected piglets at least once. At necropsy Arcobacter spp. were cultured from the liver, kidney, ileum, or brain tissues of two of the four A. butzleri-infected piglets. However, no severe gross pathology was noted. These data suggest that Arcobacter spp., especially A. butzleri, can colonize neonatal pigs.

Animals↗
PubMed1996

Phenotypic and ribosomal RNA characterization of Arcobacter species isolated from porcine aborted fetuses.

Aerotolerant organisms resembling Campylobacter, now designated as Arcobacter, have been described from aborted farm animals and from cases of human enteritis worldwide. The goals of this study were 1) to attempt to recover Arcobacter spp. from cases of porcine abortion, 2) to characterize these isolates by phenotype and ribotype, and 3) to compare the usefulness of ribotype and phenotype patterns for identifying Arcobacter butzleri and the DNA hybridization groups 1A and 1B of A. cryaerophilus. Isolates of Arcobacter spp. from North Carolina and Iowa were recovered from porcine tissues. In Iowa, Arcobacter spp. were recovered from 43% (13/30) of porcine abortion cases evaluated. Isolations were made from placenta (44%), kidney (44%), and stomach contents (12%), which were the only tissues examined. The most reliable biochemical tests for A. butzleri included growth in 1% glycine and in 1.5% NaCl, weak catalase activity, and resistance to cadmium chloride. Arcobacter cryaerophilus strains were characterized by strong catalase activity and sensitivity to cadmium chloride. The DNA hybridization groups 1A and 1B of A. cryaerophilus could not be distinguished by biochemical tests. This represents the first description of A. cryaerophilus DNA group 1A in animals within the United States.

Abortion, Veterinary↗
PubMed1986

Diagnostic necropsy of suckling swine.

A diagnosis of many problems affecting the baby pig can be made by careful necropsy of several freshly killed, acutely affected, untreated pigs and the application of one or more of the examinations outlined in this article. The selection and preservation of specimens to be forwarded to diagnostic laboratories for assistance in confirming the tentative diagnosis are of critical importance in gaining the maximum information from a necropsy examination.

Animals↗
PubMed1980

Mild transmissible gastroenteritis in pigs suckling vaccinated sows.

A strain of transmissible gastroenteritis (TGE) virus of low virulence was isolated from 14-day-old pigs suckling sows vaccinated with an attenuated TGE vaccine. Diarrhea developed in suckling pigs approximately 14 days after farrowing in 4 farrowings; however, none of these pigs died from diarrhea. Diarrhea ceased after the 4th farrowing, when vaccination of sows was discontinued. Experimentally, both the field isolate and the vaccine strain were infective and in some instances lethal for 2-day-old pigs exposed orally; however, neither strain was as virulent as the Purdue strain.

Animal Population Groups↗
PubMed1979

Transmissible gastroenteritis in neonatal dogs: experimental intestinal infection with transmissible gastroenteritis virus.

Fourteen neonatal dogs (4 through 11 days of age) were exposed orally to the Purdue strain of transmissible gastroenteritis (TGE) virus, and six dogs of similar age were noninoculated controls. Clinical signs of enteric disease did not develop. Both exposed and control dogs had normal fecal passages and appetite throughout the experiment. Jejunal epithelium from dogs euthanatized at 12, 24, 48, and 96 hours and at 10 days after exposure did not exhibit morphologic alterations detectable by light microscopy. Electron microscopic examination indicated that jejunal epithelial cells contained TGE viral particles as early as 12 hours after dogs were exposed. There were no apparent morphologic alterations or signs of desquamation of virus-infected cells, however. Results of pig transmission studies indicated that viable TGE virus was in jejunal tissue of the dogs as early as 12 hours and as late as 10 days after exposure to the virus.

Animals↗